ﺑﺴﻢ ﺍﷲ ﺍﻟﺮﺣﻤﻦ ﺍﻟﺮﺣﻴﻢ effect of frying process on physicochemical
TRANSCRIPT
بسم اهللا الرحمن الرحيم
EFFECT OF FRYING PROCESS ON PHYSICOCHEMICAL CHARACTERISTICS OF CORN AND SUNFLOWER OILS
By Mohammed Salaheldin Mustafa Elhassan
B.Sc. Agric. (Honours) Faculty of Agriculture
University of Khartoum (2004)
Supervisor Dr. Hassan Ali Mudawi
A thesis submitted to the Graduate College, University of Khartoum for partial fulfillment for requirements for Master Degree
of Food Science and Technology
Department of Food Science and Technology
Faculty of Agriculture University of Khartoum
June 2008
أ
DEDICATION
To my Parents, my brother, my sisters and All those who taught me a letter
ب
ACKNOWLEDGEMENTS
I am indebted to "Alla" who granted me every thing including the
mind, health and patience to accomplish this work.
I wish to express my deepest gratitude to my supervisor
Dr. Hassan Ali Mudawi for his supervision, encouragement and
support, through the duration of this work.
I express my thanks to Dr. Ahlam Ahmed, Food Research Centre,
Dr. Asha Fagir, Faculty of Agriculture, University of Khartoum.
due Thanks to my cousin Rania Mustafa, Chemical Engineer, Mona
Tag elSir and Mr. Omer Carbose, Arabian Company for Plant Oils
Production, my friend Mr. Police 2nd lieutenant, Osman Abdel Rahman,
all my colleagues and my friends specially the eighth batch in the Faculty
of Agriculture and fourth batch of M.Sc. in Food Science, University of
Khartoum. Also I express my thanks to all staff of agricultural engineers
in administration of agriculture in shendi town.
My thanks also due to all those who helped me to achieve this
study.
ت
LIST OF CONTENTS
Page DEDICATION i ACKNOWLEDGEMENTS ii LIST OF CONTENTS iii LIST OF TABLES v LIST OF CURVES vi ABSTRACT vii ARABIC ABSTRACT viii CHAPTER ONE: INTRODUCTION 1 CHAPTER TWO: LIETERATURE REVIEW 4 2.1 Quality of frying oils 4 2.2 Oxidation of oils during frying 6 2.3 Measurement parameters of oils oxidation 9
2.3.1 peroxide value 9 2.3.2 Iodine value 11
2.4 Hydrolysis of oils 13 2.5 Measurement parameters of oils hydrolysis 14
2.5.1 Free Fatty Acids 14 2.6 Polymerization of oils 16 2.7 Measurement parameters of oils polymerization 18
2.7.1 The effectiveness on the oil polymer content during frying 18 2.8 The effectiveness on the oil fatty acid composition during frying 20 2.9 The effectiveness on the oil viscosity during frying 21 2.10 The effectiveness on the oil refractive index during frying 23 2.11 The effectiveness on the oil colour during frying 24 2.12 Factors affecting the quality of oil during frying 26
2.12a Replenishment of fresh oil 26 2.12b Frying time and temperature 27 2.12c Quality of frying oil 27 2.12d Compositions of foods 30 2.12e Types of fryer 31 2.12f Antioxidants 31 2.12g Dissolved oxygen contents in oil 34
CHAPTER THREE: MATERIALS AND METHODS 34 3.1 Materials 34
3.2.1 Raw materials 34 3.2.2 Chemicals and reagents and instruments 34
3.2 Preparation of raw materials 34 3.3 Frying processes 34 3.4 Analytical procedures 35
3.4.1 Oil characteristics 35 3.4.1.1 Refractive index 35
ث
3.4.1.2 Colour of oil 35 Page 3.4.1.3 Viscosity 36 3.4.1.4 Peroxide value (PV) 36 3.4.1.5 Free fatty acids (FFA) 37
3.5 Statistical analysis 38 CHAPTER FOUR: RESULTS AND DISCUSSIONS 39 4.1 Physicochemical properties of corn and sunflower oils 39 4.2 Changes in physical parameters of oils during frying 39
4.2.1 The effectiveness on the oil refractive index during frying 39
4.2.2 The effectiveness on the oil viscosity during frying 39 4.2.3 The effectiveness on the oil colour during frying 45
4.3 Changes in chemical parameters of oils during frying 45 4.3.1 The effectiveness on the oil peroxide value during
frying 45 4.3.2 The effectiveness on the oil free fatty acids value
during frying 52 CHAPTER FIVE: CONCLUSIONS AND RECOMMENDATIONS 53 5.1 Conclusions 53 5.2 Recommendations 54 References 55 Appendices
ج
LIST OF TABLES
Table Page
1 Physicochemical properties of corn and sunflower oils
40
2 The effectiveness on the oil refractive index during frying
41
3 The effectiveness on the oil viscosity during frying
43
4 The effectiveness on the oil colour during frying 46
5 The effectiveness on the oil peroxide value during frying
48
6 The effectiveness on the oil free fatty acids value during frying
50
ح
LIST OF FIGURES
Figure Page
1 The effectiveness on the oil refractive index during frying 42
2 The effectiveness on the oil viscosity during frying 44
3 The effectiveness on the oil colour during frying 47
4 The effectiveness on the oil peroxide value during frying 49 5 The effectiveness on the oil free fatty acids value during
frying 51
خ
ABSTRACT
This study has been conducted to explore the effect of frying on
the physical and chemical characteristics of corn and sunflower oils.
For the purpose of this study, corn and sunflower oils and potatoes
were obtained from the local market, the oils was reused for frying five
days consecutively
After each frying process a sample of oil was taken daily after it
cools. These samples were physically and chemically tested, all the
physicochemical characteristics were changed.
The colour of corn oil increased from 0.9 to 6.4, and the colour of
sunflower oil increased from 0.4 to 5.1. The refractive index of corn oil
Increased from 1.4750 to 1.4820, and in sunflower oil it increased from
1.4750 to 1.4810. Viscosity of corn oil Increased from 28.7 to 31.0,
Viscosity of sunflower oil Increased from 28.3 to 31.0.
The free fatty acids increased in corn oil, from 0.125 to 200.0
whereas in sunflower oil they increased from 0.110 to 0.150
In case of the peroxide value in the both oils the study showed that
it was not constant. It worth mentioning here that the frying process
followed in this study is the same that is usually used at homes.
د
لدراسةخالصة ا
فيزيائية على الخواص الكيميائية وال)التحمير( أجريت هذه الدراسة لمعرفة أثر القلي
.لزيت الذرة وزهرة الشمس
تم الحصول على زيت الذرة وزيت زهرة الشمس من السوق المحلي، وأجري تحمير
بطاطس لخمس مرات بمعدل مرة كل يوم، ومن ثم تم سحب عينة العلى كل منهما لشرائح
.بعد كل تحميرة بعد أن يبرد الزيت وأجريت التحاليل الفيزيائية والكيميائية عليها
0.4 اما لون زيت زهرة الشمس ازداد من 6.4 الي 0.9ره ازداد من لون زيت الذ
ومعامل انكسار زيت 1.4820 الي 1.4750ره ازداد من زيت الذاما معامل انكسار. 5.1الي
ره ازدادت من اللزوجه بالنسبه لزيت الذ. 1.4810 الي 1.4750زهرة الشمس ازداد من
.31.1 الي 28.3بالنسبه لزيت زهرة الشمس ازدادت اللزوجه من و31.1 الي 28.7
في زيت زهرة و0.200 الي 0.125ره من الحموض الدهنيه الحره في زيت الذازدادت
ت الدراسه انه غير بالنسبه لرقم البيروكسيد اوضح .0.150 الي 0.110لشمس ازدادت من ا
.ثابت اثناء التحمير
.والجدير بالذكر أن هذه التجربة أجريت على حسب ما يحدث في القلي المنزلي .
1
CHAPTER ONE
INTRODUCTION
Fat frying is one of the oldest and popular food preparations. Fried
foods have desirable flavor, colour, and crispy texture, which make deep-
fat fried foods very popular to consumers (Boskou et al., 2006). Frying
is a process of immersing food in hot oil with a contact among oil, air,
and food at a high temperature of 150 to 190°C. The simultaneous heat
and mass transfer of oil, food, and air during fat frying produces the
desirable and unique quality of fried foods. Frying oil acts as a heat
transfer medium and contributes to the texture and flavor of fried food.
Fat frying produces desirable or undesirable flavor compounds and
changes the flavor stability and quality of the oil by hydrolysis,
oxidation, and polymerization. Tocopherols, essential amino acids, and
fatty acids in foods are degraded during deep-fat frying. The reactions in
deep-fat frying depend on factors such as replenishment of fresh oil,
frying conditions, original quality of frying oil, food materials, type of
fryer, antioxidants, and oxygen concentration. High frying temperature,
the number of fryings, the contents of free fatty acids, polyvalent metals,
and unsaturated fatty acids of oil decrease the oxidative stability and
flavor quality of oil. Antioxidant decreases the frying oil oxidation, but
the effectiveness of antioxidant decreases with high frying temperature.
Lignan compounds in sesame oil are effective antioxidants in deep-fat
frying.
Frying time, food surface area, moisture content of food, types of
breading or battering materials, and frying oil influence the amount of
2
absorbed oil to foods (Moreira et al., 1997). The oil contents of potato
chips, corn chips, tortilla chips, doughnuts, French fries, and fried noodle
(ramyon) are 33 to 38%, 30 to 38%, 23 to 30%, 20 to 25%, 10 to 15%
(Moreira and others 1999a), and 14% (Choe et al., 1993), respectively.
The absorbed oil tends to accumulate on the surface of fried food during
frying in most cases and moves into the interior of foods during cooling
(Moreira and others 1997).
Foods fried at the optimum temperature and time have golden
brown colour, are properly cooked, and crispy, and have optimal oil
absorption (Blumenthal, 1991). However, underfried foods at lower
temperature or shorter fying time than the optimum have white or slightly
brown color at the edge, and have ungelatinized or partially cooked
starch at the center. The underfried foods do not have desirable deep-fat
fried flavor, good color, and crispy texture. Overfried foods at higher
temperature and longer frying time than the optimum frying have
darkened and hardened surfaces and a greasy texture due to the excessive
oil absorption.
Deep-fat frying produces desirable or undesirable flavor
compounds, changes the flavor stability and quality, color, and texture of
fried foods, and nutritional quality of foods. The hydrolysis, oxidation,
and polymerization of oil are common chemical reactions in frying oil
and produce volatile or nonvolatile compounds. Most of volatile
compounds evaporate in the atmosphere with steam and the remaining
volatile compounds in oil undergo further chemical reactions or are
absorbed in fried foods. The nonvolatile compounds in the oil change the
physical and chemical properties of oil and fried foods. Nonvolatile
3
compounds affect flavor stability and quality and texture of fried foods
during storage. Deep-fat frying decreases the unsaturated fatty acids of
oil and increases foaming, color, viscosity, density, specific heat, and
contents of free fatty acids, polar materials, and polymeric compounds.
Frying temperature and time, frying oil, antioxidants, and the type
of fryer affect the hydrolysis, oxidation, and polymerization of the oil
during frying. This review focuses on the chemical reactions of frying
oil and improvement of the oxidative stability and flavor quality of frying
oil during deep-fat frying (Choe and Min, 2007).
The Objective of this Study is:
- To explore the effect of Frying on the Physical and Chemical
Characteristics of Corn and Sunflower oils.
4
CHAPTER TWO
LITERATURE REVIEW
2.1 Quality of frying oils:
Free fatty acids increase the thermal oxidation of oils, and their
unsaturation rather than chain length led to significant effects on
thermooxidative degeneration; the addition of 0.53 mmol of tridecanoic,
palmitic, and oleic acids to virgin olive oil showed 15.0, 14.3, and 10.1 h
of induction period with a Rancimat (Metrohm) (Frega et al., 1999).
Stevenson and others (1984) and Warner and others (1994) reported that
the oxidation rate of oil increased as the content of unsaturated fatty acids
of frying oil increased. This explains why corn oil with less unsaturated
fatty acid is a better frying oil than soybean or canola oils with more
unsaturated fatty acids (Warner and Nelsen, 1996). The content of
linolenic acid is critical to the frying performance, the stability of oil, and
the flavor quality of fried food. Total polar compounds contents of
sunflower oil (C16:0, 5.3%; C18:0, 4.8%; C18:1, 55.4%; C18:2, 32.4%)
and high linolenic canola oil (C16:0, 4.1%; C18:0, 2.2%; C18:1, 69.3%;
C18:2, 13.8%; C18:3, 6.8%) after 80-h frying at 190°C were 44.6% and
47.5%, respectively. The oil containing linolenic acid at 8.5% produced
extremely undesirable acrid and fishy odors when heated above 190°C.
Low linolenic acid (2.5%) canola oil produced less free fatty acids and
less polar compounds during deep-fat frying of potato chips at 190°C
(Xu et al., 1999). Warner et al. (1997) reported that polar compounds
formation in cottonseed oil during deep-fat frying of potato chips
5
increased proportionally with the linoleic acid content in the oil (Choe
and Min, 2007).
Hydrogenation and genetic modification are two of the processes
to decrease the unsaturated fatty acids of frying oil. Hydrogenation
increases the frying stability of oil (Choe and Min, 2007). However,
hydrogenation produces trans fatty acid or metallic flavor, and it does not
additionally improve the quality of oil with low linolenic acid.
Hydrogenated soybean oil with 0.1% linolenic acid had more hydrolytic
degradation, but lower p-anisidine values and polymer formation, than
the soybean oil with 2.3% linolenic acid. Genetically modified high oleic
corn oil improved frying stability over normal corn oil. Therefore, low
linolenic acid oil by genetic modification was suggested to be a potential
alternative to hydrogenated frying oil. Blending of several oils can
change the fatty acid compositions of oils, and can decrease the oxidation
of oils during deep-fat frying.
The esterified acylglycerol does not show any prooxidant activity
in the oil oxidation during deep-fat frying. Free fatty acids in frying oil
accelerated thermal oxidation of the oil (Choe and Min, 2007). The
filtering of oil with adsorbents lowered free fatty acids and improved the
frying quality of oil. Used frying oil was filtered with a mixture of 2%
pekmez earth, 3% bentonite, and 3% magnesium silicate, and the filtering
process decreased the contents of free fatty acids and conjugated dienoic
acids of sunflower oil during deep-fat frying of potato samples at 170°C
and increased the formation of aldehyde compounds (Maskan and Bagci,
2003). Daily filtering treatments of canola oil with a mixture of calcium
6
silicate-based Hubesorb 600, magnesium silicate-based magnesol, and
rhyolite and citric acid-based fry powder decreased free fatty acids and
polar compounds formation and improved the frying quality of oil
(Bheemreddy and others 2002). The treatment of shortening with
bleaching clay, charcoal, celite, or MgO improved the oil quality for
French fries. Daily addition of ascorbyl palmitate to fresh oil decreased
free fatty acid formation, but increased dielectric constant and color
changes (Choe and Min, 2007). Stevenson et al. (1984) reported that oils
with less than 0.05% free fatty acids and 1.0 meq peroxides in 1 kg of oil
are desirable for deep-fat frying.
2.2 Oxidation of oils during frying:
The oxygen in deep-fat frying reacts with oil (Choe and Min,
2007). The chemical mechanism of thermal oxidation is principally the
same as the autoxidation mechanism. The thermal oxidation rate is faster
than the autoxidation, but specific and detailed scientific information and
comparisons of oxidation rates between thermal oxidation and autoxida-
tion are not available. The mechanism of thermal oxidation involves the
initiation, propagation, and termination of the reaction (Fig. 3).
Nonradical singlet state oil does not react with triplet state
diradical oxygen due to the spin barrier. Ordinary oxygen in air is a
diradical compound. Radical oxygen requires radical oil for the oxidation
of oil. Oil should be in a radical state to react with radical oxygen for oil
oxidation reaction. The hydrogen with the weakest bond on the carbon of
oil will be removed first to become radical. The energy required to break
carbon-hydrogen bond on the carbon 11 of linoleic acid is 50 kcal/mole
7
(Min and Boff, 2002). The double bonds at carbon 9 and carbon 12
decrease the carbon-hydrogen bond at carbon 11 by withdrawing
electrons. The carbon-hydrogen bond on carbon 8 or 11, which is α to the
double bond of oleic acid, is about 75 kcal/mole. The carbon-hydrogen
bond on the saturated carbon without any double bond next to it is
approximately 100 kcal/mol (Min and Boff, 2002). The various strengths
of hydrogen-carbon bond of fatty acids explain the differences of
oxidation rates of stearic, oleic, linoleic, and linoleic acids during thermal
oxidation or autoxidation. The weakest carbon-hydrogen bond of linoleic
acid is the one on carbon 11 and the hydrogen on carbon 11 will be
removed first to form a radical at carbon 11 (Figure 3). The radical at
carbon 11 will be rearranged to form conjugated pentadienyl radical at
carbon 9 or carbon 13 with trans double bond as shown in Figure 3. Heat,
light, metals, and reactive oxygen species facilitate the radical formation
of oil. The polyvalent metals such as Fe3+ and Cu2+ remove hydrogen
protons from oil to form alkyl radicals by oxidation-reduction mechanism
of metals even at low temperatures. The site of radical formation in
saturated fatty acids is different from those of unsaturated oleic or
linoleic acids. The alkyl radical of saturated fatty acids is formed at α
position of the carboxyl group having electron-withdrawing property.
The formation of the alkyl radical from an oil molecule by removing
hydrogen is called the initiation step in the oxidation reaction of oil. The
alkyl radical can also react with alkyl radicals, alkoxy radicals, and
peroxy radicals to form dimers and polymers.
8
Alkyl radicals with a reduction potential of 600 mV react rapidly
with diradical triplet oxygen and produce peroxy radicals at the rate of
109/M/s. The peroxy radical with a reduction potential of 1000 mV
abstracts hydrogen from oleic acid and linoleic acid and produces
hydroperoxide at a rate of approximately 1 × 100/M/s and 1 × 10/M/s,
respectively. The peroxy radical abstracts a hydrogen atom from another
oil molecule and forms new hydroperoxide and another alkyl radical.
This chain reaction is called free radical chain reaction in foods and
propagation step. Peroxy radicals also react with other radicals at the rate
of about 1.1 × 106/M/s to form dimers or polymers (Choe and Min 2005).
The chain reactions of free alkyl radicals and peroxy radicals accelerate
the thermal oxidation of oil.
The oxygen-oxygen bond strength of R-O-O-H is about 44
kcal/mole, which is a relatively weak covalent bond (Choe and Min,
2007). The hydroperoxides are not generally stable during the deep-fat
frying (Nawar 1984). Hydroperoxides are decomposed to alkoxy radicals
and hydroxy radicals by homolysis of the peroxide bond. The
hydroperoxide is decomposed to produce oxy- and hydroxy radicals.
Toschi and others (1997) identified mono-oxygenated products such as 9-
keto-10, 12-, and 13-keto-9-, 11-octadecadienoate and hydroxyl
derivatives such as 9-hydroxy-10, 12-, and 13-hydroxy-9-, and 11-
octadecadienoate from hydroperoxides of thermally oxidized methyl
linoleate at 150°C. The alkoxy radical reacts with other alkoxy radicals
or is decomposed to form nonradical products. The formation of
9
nonradical volatile and nonvolatile compounds at the end of oxidation is
called the termination step as shown in Figure 3.
Most volatiles are removed from frying oil by steam during deep-
fat frying (Choe and Min, 2007). The addition of water to a frying system
decreases the volatile compounds in oil. The amount of volatile
compounds present in the oil varies widely depending on the kind of oil,
food, and frying conditions. The disappearance or loss of volatile
compounds in frying oil is due to the combination of evaporation and
decomposition, and the reaction of volatile compounds with other food
components. The volatile compounds in frying oil undergo further
reactions such as oxidation, dimerization, and polymerization. Volatile
compounds significantly contribute to the flavor quality of the frying oil
and the fried foods. The rate of oxidative degradation reactions increases
as the concentration of oxygen and free radicals increases. The amount of
mono- and diacylglycerols is small at the beginning of frying. The high
interfacial tension in the frying system breaks steam bubbles and forms a
steam blanket over the oil surface. The steam blanket reduces the contact
between the oil and oxygen, and lowers the oil oxidation (Choe and Min,
2007). Refined oil usually contains less than 1 ppm of alkaline materials
such as sodium or potassium salt of fatty acids. Fresh frying oil should
have less than 10 ppm alkaline materials (Choe and Min, 2007).
2.3 Measurement parameters of oils oxidation:
2.3.1 Peroxide value:
Oxidation of lipids to hydroperoxides, referred to as the peroxide
value (PV) of a fat represents the reactive oxygen content and is
10
estimated through the liberation of iodine from potassium iodide in
glacial acetic acid and expressed as milliequivalent of peroxides/kg
sample (Hartley, 1967). Changes in the PV can often be used to monitor
the potential shelf-life and is a good guide to the quality of a fat. It has
well been found that PV not to be used to measuring frying oil
deterioration since they are very unstable at frying temperature.
However, the extent of oxidation which has occurred during frying is
clearly reflected by the rate of peroxide formation after the sample is
been taken and allowed to cool and settle before analysis (Fritisch et al.,
1979). It has been stated that, at high temperatures, the rate of
decomposition of the peroxide is higher than the rate of their formation
(Taha et al., 1988).
Khattab et al. (1974) studied the stability of extracted and
commercial samples of cottonseed, groundnut, sesame oil and butter fat;
the oils and fats were subjected to a temperature of 195oC for 3 hours.
The result of peroxide and carbonyl values showed the following order of
stability cottonseed oil > sesame oil > groundnut oil. Moreover, the PV
increased for the three oils on exposure to deep frying temperature. The
result showed a higher increase in the PV in heated cottonseed oil than in
oil used for frying (Sonia et al., 1983). Chemical changes in samples of
corn oil thermally oxidized for 24 hours at 120, 160 and 200oC were
studied in order to see what effect temperature had on the types of
changes produced. Generally, the observed changes of the PV showed
low reading for the oils which had been heated to 160 and 200oC
compared with those heated at 120oC which indicated that former
peroxides were not stable (Johnson and Kummerous, 1957).
11
Despite these drawbacks, good correlations have been reported
between PV and sensory scores for a number of commercial fats (Gray,
1978).
2.3.2 Iodine value:
Iodine value (IV) is the number of grammes of iodine absorbed per
100 g fat. It is a measure of the proportion of unsaturated constituents
present in a fat. Hence, it is the halogen addition to the double bonds of
the unsaturated fatty acids and the quantity of halogen taken up is
expressed in terms of iodine as the IV (Hartley, 1967).
The measurement of IV is found to be one of the most convenient
and simple methods to determine fat and oil deterioration. Although the
IV gives indication of the number of double bonds in any particular oil or
fat it, however, also indicates the total amount of unsaturation (Lea et al.,
1960). In addition, since the mean unsaturation of oils changes to a
greater or lesser during storage, there will be a corresponding change in
IV. The progress of the oxidative rancidity can be followed in a number
of ways which include measurement of iodine absorption and uptake of
oxygen by oil. Hence, it has been reported that IV is probably an
excellent indication of oxidation, or at least oxidation plus polymeriza-
tion as it occur in herring meal, where the changes in the oil will be the
mainly oxidation followed by oxidation-induced polymerization and
degradation. Furthermore, it has been suggested that IV change is a
satisfactory measure of chemical oxidation and subsequent polymeriza-
tion in a system such as fish meal (Lea et al., 1960). Various reports on
cottonseed, sesame and groundnut oils, respectively evaluate that the
12
initial IV were 106.7, 103.5 and 97.5 (mg/iodine/100g fat). When these
oils were subjected to a temperature of 195oC, the iodine values
decreased gradually in case of cottonseed and groundnut oils but rather
sharply in case of sesame oil particularly after 2 hrs of exposure (Khattab
et al., 1974).
It has been concluded that, the IV of cottonseed and sunflower oils
decreased when these oils are subjected to heat (Robertson, 1972).
A viable data on reactions taking place during frying indicate that
they are probably different from those produced by heating the oil in the
presence of air. Still, it has been found that the content of oxidation
products was somewhat higher in case of potato frying than in frying of
fish or the leguminous Tammiya paste (Sonia and Badereldeen, 1983).
Studies on frying refined deodorized palm olein and hydrogenated
sunflower, cottonseed and soybean oils clearly considered that palm olein
compared favourably with sunflower, cottonseed and soybean in terms of
its resistance to oxidative changes during frying (Augustin et al., 1989).
A comprehensive study on soybean oil and vanaspati (blends of
vegetable oil of cotton, palm, rapeseed, mustard, soybean, sunflower and
maize in different proportions) believed that IV of soybean oil decrease
from their initial level of 129.8 to 106.7, 96.2 and 94.4 in 70 hrs at 170,
180 and 190oC, respectively where IV for vanaspati under similar frying
conditions decreased from 74.7 for fresh to 63.0, 61.0 and 56.0 From
these results, it is evident that the smaller decrease of IV of vanaspati
showed its relative higher stability at the frying temperature (Tyagi and
Vasishtha, 1996).
13
Data for frying performance of palm olein, corn oil and soya oil
which were elaborately studied reported that the decrease in IV of three
batches of the oil after four days of frying were in the order of 4.4 – 4.9
for palm olein, 6.3 – 10.1 for corn oil and 7.1 – 10.8 for soya oil. The
smaller change observed in IV in palm olein compared to the iodine
value of corn and soya oils indicated that less oxidation of unsaturated
fatty acids had taken place in palm olein (Augustin et al., 1987). Heating
operations with palm olein and groundnut oils reflected a significant drop
in IV over 8 days of frying which obviously indicated irreversible
deterioration of he oil that has been exposed to static heating at 180oC
(Augustin et al., 1987).
2.4 Hydrolysis of oil:
When food is fried in heated oil, the moisture forms steam, which
evaporates with a bubbling action and gradually subsides as the foods are
fried. Water, steam, and oxygen initiate the chemical reactions in the
frying oil and food. Water, a weak nucleophile, attacks the ester linkage
of triacylglycerols and produces di- and monoacylglycerols, glycerol, and
free fatty acids. Free fatty acids contents in frying oil increase with the
number of fryings (Chung and others 2004) as shown in Figure 2. Free
fatty acid value is used to monitor the quality of frying oil. Thermal
hydrolysis takes place mainly within the oil phase rather than water-oil
interface (Choe and Min, 2007). Hydrolysis is more preferable in oil with
short and unsaturated fatty acids than oil with long and saturated fatty
acids because short and unsaturated fatty acids are more soluble in water
14
than long and saturated fatty acids. Water from foods is easily accessible
to short-chain fats and oils for hydrolysis.
Large amounts of water hydrolyze the oil rapidly (Dana et al.,
2003). Water hydrolyzes the oil faster than steam (Choe and Min, 2007).
Large contact between the oil and the aqueous phase of food increases
hydrolysis of oil. Mono- and diacylglycerols in cottonseed oil during
frying of potato chips at 155 to 195°C increased initially and then
reached a plateau. Frequent replacement of frying oil with fresh oil slows
down the hydrolysis of frying oil. Sodium hydroxide and other alkalis
used for cleaning a fryer increase the oil hydrolysis. The duration of
frying did not affect the hydrolysis of oil (Naz et al., 2005).
Free fatty acids and their oxidized compounds produce off-flavor
and make the oil less acceptable for deep-fat frying. Di- and
monoacylglycerols, glycerol, and free fatty acids accelerate the further
hydrolysis reaction of oil. Glycerol evaporates at 150°C and the
remaining glycerol in oil promotes the production of free fatty acids by
hydrolysis (Naz et al., 2005). Stevenson et al. (1984) suggested that the
maximum free fatty acid content for frying oil is 0.05 to 0.08%.
2.5 Measurement parameters of oils hydrolysis:
2.5.1 Free Fatty Acids:
Chemically, the most prestigious reaction, of the fats from the
procedure's point of view is hydrolysis; i.e. the formation of free glycerol
and free fatty acids through a splitting of the fat molecule and the
addition of the elements of water which may be partly represented by the
equation:
15
C3H5 (OOCR)3 + 3H2O ----------- C3H5 (OH)3 + 3R CooH
Where C3H5 represents the glycerol radical, and R represents the
hydrocarbon chain of the fatty acid radical (Hartley, 1967). The
development of free fatty acids in an oils is usually considered to be one
of the main parameters to use in evaluating the quality of oil, especially
the state of storage and heat. In the frying procedures, the reaction of
water and fats to form free fatty acids and glycerol is conditioned by the
usage of hot fats. Subsequently, additional heating causes smoking as
free glycerol breaks down to a volatile eye-irritating compound known
as a crolein. This phenomenon is intimately associated with darkening of
colour and gradual development of strong flavour. A crolein the first
member of the ethrylene series is the primary oxidation product of alkyl
alcohol. It is, however, conveniently prepared by heating glycerol with
dehydrating agents such as KHSO4 to about 200oC (Fritisch, 1981).
Indeed, it has long been admitted that acids produced during frying
are largely deposited by both hydrolysis and oxidations. The rate of
hydrolysis can vary between different lots of the same shortening and is
influenced by the initial free fatty acids of the shortening (Fritisch et al.,
1971). Moreover, it was shown that the free fatty acids extracted from
heated oils gave a typical profile originally present in the oils. This is
indicative to the increase in the content of the free fatty acids of heated
oils due to hydrolytic cleavage of triglycerides (Peled et al., 1975).
Several studies on the selectivity of hydrolysis frying oils and
model systems of triglycerides have consistently indicated that shorter
chains and unsaturated fatty acids are hydrolyzed at faster rates than
16
other fatty acids and apparently attributed to the greater water solubility
of these acids (Nobel et al., 1967; Buziassy and Nowar, 1968). From
frying practices with sunflower seed oils, it has been reported that after
running 75 batches of potato chips an observable increase in acid values
has been recorded after the first 30 runs and eventually plateaued
thereafter (Lopez et al., 1995). Further, it has been found that palm olein
is more susceptible to acidity and to smoking as compared to sunflower,
cottonseed and soybean oils (Augustin et al., 1989).
2.6 Polymerization of oils:
Volatile compounds are extremely important to the flavor qualities
of frying oil and fried foods, but volatile contents in total decomposition
products of frying oil are present at the concentration of part per million
levels The major decomposition products of frying oil are nonvolatile
polar compounds and triacylglycerol dimers and polymers. The amounts
of cyclic compounds are relatively small compared to the nonvolatile
polar compounds, dimers, and polymers (Choe and Min, 2007). Dimers
and polymers are large molecules with a molecular weight range of 692
to 1600 Daltons and formed by a combination of -C-C-, -C-O-C-, and -C-
O-O-C- bonds (Stevenson and others 1984). Dehydroxydimer,
ketodehydrodimer, monohydrodimer, dehydrodimer of linoleate, and
dehydrodimer of oleate are dimers found in soybean oil during frying at
195°C. Dimers and polymers have hydroperoxy, epoxy, hydroxy, and
carbonyl groups, and -C-O-C- and -C-O-O-C- linkages.
Dimers or polymers are either acyclic or cyclic depending on the
reaction process and kinds of fatty acids consisting of the oil (Choe and
17
Min, 2007). Dimerization and polymerization in deep-fat frying are
radical reactions. Allyl radicals are formed preferably at methylene
carbons α to the double bonds. Dimers are formed from the reactions of
allyl radicals by C-C linkage. Triacylglycerols react with oxygen and
produce alkyl hydroperoxides (ROOH) or dialkyl peroxides (ROOR).
They are readily decomposed to alkoxy and peroxy radicals by RO-OH
and ROO-R scission, respectively. Alkoxy radicals can abstract hydrogen
from oil molecule to produce hydroxy compounds, or combine with other
alkyl radicals to produce oxydimers. Peroxy radicals can combine with
alkyl radicals and produce peroxy dimers (4). Formation of dimers and
polymers depends on the oil type, frying temperature, and number of
fryings. As the number of fryings and the frying temperature increase, the
amounts of polymers increased (Choe and Min, 2007). The oil rich in
linoleic acid is more easily polymerized during deep-fat frying than the
oil rich in oleic acid (Choe and Min, 2007).
Cyclic polymers are produced within or between triacylglycerols
by radical reactions (Figure 6) and the Diels-Alder reaction (Figure 7).
The formation of cyclic compounds in frying oil depends on the degree
of unsaturation and the frying temperature. The formation of cyclic
monomers and polymers increased as the amount of linolenic acid
increased (Choe and Min, 2007)The formation of cyclic monomers was
negligible until the linolenic acid content exceeds about 20%. Cyclic
compounds are not formed to a significant extent until the oil temperature
reaches 200 to 300°C. Soybean oil produced tricyclic dimers and bicyclic
dimers of linoleate as well as cyclic monomers during deep-fat frying.
18
Polymers formed in deep-fat frying are rich in oxygen. Yoon et al.
(1988) reported that oxidized polymer compounds accelerated the
oxidation of oil. Polymers accelerate further degradation of the oil,
increase the oil viscosity (Tseng et al., 1996), reduce the heat transfer,
produce foam during deep-fat frying, and develop undesirable color in
the food. Polymers also cause the high oil absorption to foods.
Polymers are highly conjugated dienes and produce a brown, resin-
like residue along the sides of the fryer, where the oil and metals come in
contact with oxygen from the air. Resin-like residue is often produced
when the oil does not release the moisture but keeps it trapped while also
incorporating air (Choe and Min, 2007).
2.7 Measurement parameters of oils polymerization:
2.7.1 The effectiveness on the oil polymer content during frying:
Polymers are substances made of recurring structural units, each of
which can be regarded as derived from a specific compound called a
monomer. The properties of a polymer, both physical and chemical, are
in many ways as sensitive to changes in the structure of the monomer as
are the properties of the monomer itself. Polymers can be classified in
several different ways according to their structure, the types of reactions
by which they are prepared, their physical properties or their
technological uses. However, these classifications are not all mutually
exclusive (Lillard, 1983).
Generally, it has been found that, oils containing relatively small
quantities of the oxidative polymers would possess an objectionable
19
flavour. It has also been reported that considerable concentrations of
thermal polymers may be present in some oils without organoleptic
indication, where the formation of toxic cyclized monomeric acid is
possible at frying temperature (Melnick, 1957).
Available data on changes in iodine value (IV) are indicative of
thermal polymers, where failure to find a drop in the iodine value clearly
indicates that polymer formation is obstructed (Melnick, 1957).
In a comparatively elaborate study on changes in cottonseed oil
during deep-fat frying of foods, it has been revealed that the amount of
polymeric and oxidized glycerides increased by increasing the frying or
heating time (Sonia and Badereldeen, 1983). It is yet recognized that the
losses in the content of fatty acid were ultimately accompanied by a
considerable augmentation in the amount of polymers. Such polymers
whenever isolated are dark in appearance and their weight remained
constant even after heating for 5 hours at 140oC and 25 torr pressure
(Peled et al., 1975). A similar comparative study on high oleic sunflower
oil used for 75 deep fat frying runs of potatoes with fast turn over of fresh
oil during frying, had found that triglycerol polymers progressively
increased until the 60th run, but did not increase further with continued
frying (Romero et al., 1995). An extensive series of studies by Cramptom
et al. (1953) had answered many of the questions concerning the toxicity
of thermally polymerized oils. This group had demonstrated that
unsaturated oils if heated to 250-300oC for 6 to 24 hr, would contain
polymeric material corresponding to cyclic-monomeric, dimeric, trimeric
and higher polymers. Nutritional studies on these oils have shown that
20
the toxicity or growth depressing effects of such oils is caused by
polymeric materials formed in the oils. In the Netherlands, frying fats can
be rejected for consumption on the basis of sensory evaluation as well as
when levels of polymeric and dimeric triglycerides exceed 10% or
dimeric triglycerides exceed 6% (Robern and Gray, 1981). Melnick
(1957) concluded that problem of concern in frying oils (if any) is related
to the presence of thermal polymers of the non-oxidative type,
particularly since these polymers even enhance the flavour stability of an
oil.
2.8 The effectiveness on the oil fatty acid composition during frying:
Since oleic, linoleic and linolenic acids are the major unsaturated
fatty acids in edible fats and oils, they are all capable of forming bound
C18:0 from their 9-hydroperoxide. However, bound heptanoatic (C7:0) is
generated exclusively from the 8-hydroperoxide of oleic acid.
Effectively, soybean oil and vanaspati were for long been used for deep-
fat frying potato chips at 170, 180 and 190oC. The results there upon
have found that changes in frying fatty acid profile of all contributing oils
during frying were basically unsaturated whereas saturated fatty acids
remained constant. Moreover, polyunsaturated fatty acid consistently
decrease proportionally to frying time and temperature e.g. Hexa-
decenoic acid (C16:1) originally present in fresh soybean oil in trace
concentration disappeared absolutely after deep-fat frying for 16 hours at
170 and 180oC and after 10 hrs at 190oC. Octadecenoic acid (C18:1) as
well showed a gradual decrease within 10 hrs of frying period and
decrease to 23.3, 20.7 and 19.7% at 170, 180 and 190oC in 70 hrs
21
respectively (Tyagi and Vasishtha, 1996). It has also been stated that the
higher the degree of un-saturation of and oil or fat, the more susceptible
it would be towards oxidative deterioration and that since cottonseed oil
which has a lower content of unsaturated sites, would likely be more
stable to heating than sunflower oil relative to its lower percent decrease
in linoleic acid content (Celegg, 1973). As for comparing the frying
performance of market samples of palm olein corn and soy oil, the results
have found that lesser amounts of unsaturated fatty acids were exposed to
oxidation in palm olein than in corn and soya oils (Augustin et al., 1987).
An exclusive study on extracted and commercial samples of
cottonseed and sesame has reported that one of the major factors in the
degradation process of these oils is the amount of saturated fatty acids
which are present in contact with unsaturated and peroxidized fatty acids.
Such an interpretation has been widely discussed in relation to the degree
of unsaturation of the oil used and their vitamin E content (Khattab et al.,
1974) which is fat-soluble and occurs abundantly in seed oils and green
vegetables where it is used therapeutically in case of recurring
miscarriages.
2.9 The effectiveness on the oil viscosity during frying :
Viscosity is a measure of internal friction in oil and is an essential
index in the study of oils and their intermolecular forces. Viscosity is
also a useful criterion on desegregation or de-polymerization such as that
which occurs in initial stages of hydrolysis of fats and oils during storage
(Joslyn, 1970). It was found that, unsaturation reduced viscosity of fats
and oils, e.g. linoleic acid is less viscous than oleic and oleic is less
22
viscous than stearic, etc (Kaufmann, 1954; Rock and Roth, 1966). Also,
it has been accepted that hydrogenation of oil increase its viscosity as it
decreases its unsaturation. Viscosity was recognized quite early as a
significant property of oils and here is a considerable investigation on
this subject, that found a relationship between viscosity and temperature,
and concluded that, increasing rate of viscosity was proportional to
increasing temperature in oils and fats. Viscosity is considered an
important parameter in the overall assessment of the quality of edible oils
since it can have a significant effect on frying performance and on fried
food acceptability. It controls the amount of oil absorbed by fried
materials and thus the total sum of oxidative products within the food.
Moreover, it is looked upon as a major determinant of heat transfer or
cooking rate of the oil and it is believed that the heat transfer rate
decrease with increasing viscosity. Various studies carried out on
sunflower seed oil varieties and cottonseed oils heated at 180oC indicated
that observed small change in viscosity of cottonseed compared to that of
sunflower seed which increased rapidly after 60 hours of heating, this
concept is most probably due to a greater level of polymerization
(Robertson, and Morrison, 1977).
In a study of a commercially refined corn oil heated at 200oC
with/without aeration using variation in the heating time, it has been
reported that the rate of aeration plays an important role in the changes
which eventually occur during thermal oxidation. The viscosity of the
samples increased proportionally with the rate of aeration. On the other
hand, the viscosity of thermally oxidized corn oils without aeration
23
increased rapidly after 12 to 16 hrs of heating. In the case of continuation
of the heat treatment to 48 hours a very marked increase in viscosity has
been observed. In addition, it has been found that changes in the oil
stability during the initial hours of heating may effectively produce
materials which will slowly polymerize after 12 hrs of heating (Elgaili,
2000). A separate study on fats and oils commercially used for deep-fat
frying clearly stated that the degree of deterioration is independent of
their degree of unsaturation but rather depends upon how they are
manipulated. Out of two analyzed batches of cottonseed oil samples, the
used sample had 1.05 centistokes increase in viscosity while the abused
sample had a minor increase in viscosity (Thompson et al., 1967). When
triolein has been heated under stimulated deep-fat frying conditions at
185oC for 74 hours the result found that there was a noticeable
augmentation in viscosity (Elgaili, 2000).
2.10 The effectiveness on the oil refractive index during frying:
Refractive index (RI) is defined as the ratio of the velocity of light
in a vacuum to the velocity of light in the medium being measured, needs
to be modified or supplemental. It is among the most rapid and sensitive
methods available for determining identity, purity and as control for
changes in fats and oils during processes and storage. It has been
recognized that, the RI of a fat is related to its molecular structure and
degree of unsaturation, in addition it has been suggested that, for the
same type of oil, variation due to unsaturation are greater than those due
to other sources (Mehlenbacher, 1960).
24
Generally, it has been reported that, oxidation and polymerization
reactions tend to raise the RI of linseed oil. Furthermore, it was
concluded that, increase in temperature of oils and fats causes the RI to
decrease, along with decrease in density. Also, the changes for most fatty
oils uses found to be between 0.00035 and 0.0004 per degree. The data
on physicochemical changes of hydrogenated cottonseed oil during deep-
fat frying and continuous heating reported that RI increased in case of
frying from 1.403 to 1.4609 and in case of heating to 1.4620; this
indicated that chemical reactions which take place during deep-fat frying
were different from those due to continuous heating (Stephen et al.,
1978). The RI of fats and oils have been reported to increase on elaborate
autoxidation (Joslyn, 1970). Soybean oil and vanaspati were used for
deep-fat frying of potato chips at 170, 180 and 190oC the results found
that vanaspati has the lowest changes of RI irrespective of frying
temperature or duration of frying (Tyagi and Vasishtha, 1996).
2.11 The effectiveness on the oil colour during frying:
Oils are always more or less coloured, as they contain true or
colloidal in solution varying quantities of different liposoluble pigments.
These may be lipochromes originated from oleiferous tissues, or artifacts
caused by degradation, most usually thermal, during processing-
treatments (Maurice and Sambuce, 1968). Most vegetable oils and fats
have a specific colour due to the presence of natural vegetable pigments,
products of their decomposition and their accompanying substances, and
santhophyll and other colour substances related to them. Investigation of
the green pigments which appear in oils have established that there are
25
two chlorophyll components-A and B-and the products of their
decomposition.
Pheophytins A and B-present in oils as well as in other products
from vegetable sources. Chlorophyll and pheophytin are present in crude
and refined vegetable oils with the amounts of pheophytin usually found
higher. Chlorophyll and pheophytin are considered to act as prooxidant in
fats and oils. While pheophytin shows higher prooxidant activity and
higher stability in the autoxidation of triglycerides, chloropohyll is
reported to decompose by hdyroperoxide and free fatty acid (Taha et al.,
1988). Robertson (1972) who subjected sunflower seed oil from two
varieties (Alabama and Minnesota) and cottonseed oil to high
temperature (180 ± 4oC) observed that the change in colour in cottonseed
oil was the least compared with the sunflower seed oil, and he also
reported a change in colour of both varieties from a pale yellow to a dark
brown colour during heating. The oil colour of the Minnesota sunflower
variety increased sharply after 48 hrs of heating and the Alabama oil after
72 hrs. It has also been found that change in colour of cottonseed oil
upon deep-frying at 18oC was deeper than that of heated oil (Sonia and
Badereledeen, 1983). Moreover, the colour of the oil was deeper in case
of fish or Taamiya (traditional food paste of legume) frying than in
potato frying. There are many visual signs in the development of
degradation of the oil. The most obvious of which are darkening in
colour, the development of acid odour during heating and a rancid
flavour often "Tallowy" odour when the oil has cooled down (Khalil,
1983). It has been reported that edible oils become darker as heating
26
proceeds. Finally, reports of the decrease in the oil extinction at 460 nm
at the initial period of heating (one hr.) confirm that such phenomenon is
consistently observed when the oil was heated in the presence of either
air or nitrogen, and this could be attributed to thermal decomposition of
cartenoids (Peled et al., 1975).
2.12 Factors affecting the quality of oil during frying:
The turnover rate of oil, frying time and temperature, type of
heating, frying oil composition, initial oil quality, composition of food to
be fried, type of fryer, antioxidants, and oxygen content affect the
deterioration of oil during deep-fat frying. The effects of frying factors
on the quality of frying oil are sometimes reported differently or
oppositely, due to the use of different analytical methods for quality
determination and different experimental conditions.
2.12.a Replenishment of fresh oil:
A high ratio of fresh oil to total oil provides better frying oil
quality ,Frequent replenishment of fresh oil decreases the formation of
polar compounds, diacylglycerols, and free fatty acids and increases the
frying life and quality of oils (Choe and Min, 2007). Replenishment with
fresh oil improved the quality of frying oil only after the 30th frying
(Choe and Min, 2007). Cuesta and others (1993) reported that frequent
turnover of oil caused more oxidative reaction than hydrolytic reaction
during deep-fat frying of potatoes. A recommended daily turnover is 15%
to 25% of the capacity of the fryer, and the high turnover can decrease
the use of antifoaming agent such as silicones (Choe and Min, 2007).
27
2.12.b Frying time and temperature:
`Frying time increases the contents of free fatty acids ,polar
compounds such as triacylglycerol dimers and oxidized triacylglycerols,
dimers and polymers (Tompkins and Perkins, 2000). The 1st 20 fryings
increase the formation of polar compounds rapidly. There was no
significant increase of polar compounds after the 30th frying at (P> 0.05
(Choe and Min, 2007).
High frying temperature accelerates thermal oxidation and
polymerization of oils (Choe and Min, 2007). Soybean oil showed 3.09%
and 1.68% of conjugated dienes and trans acids, respectively, after 70-h
frying of potato chips at 170oC. However, soybean oil that performed the
same frying at 190oC showed 4.39% and 2.60% of the respective values
(Tyagi and Vasishtha, 1996). High frying temperature decreased
polymers with peroxide linkage and increased the polymers with ether
linkage or carbon to carbon linkage (Choe and Min, 2007).
The intermittent heating and cooling of oils causes higher
deterioration of oils than continuous heating due to the oxygen solubility
increase in the oil when the oil cools down from the frying temperature
(Clark and Serbia, 1991). The 25% of linoleic acid of the sunflower oil
was destroyed in the intermittent frying, whereas only the 5% was
destroyed in continuous frying (Choe and Min, 2007).
2.12c Quality of frying oil:
Free fatty acids increase the thermal oxidation of oils, and their
unsaturation rather than chain length led to significant effects on
thermooxidative degeneration; the addition of 0.53 mmol of tridecanoic,
28
palmitic, and oleic acids to virgin olive oil showed 15.0, 14.3, and 10.1 h
of induction period with a Rancimat (Metrohm) (Choe and Min, 2007).
Stevenson and others (1984) and Warner and others (1994) reported that
the oxidation rate of oil increased as the content of unsaturated fatty acids
of frying oil increased. This explains why corn oil with less unsaturated
fatty acid is a better frying oil than soybean or canola oils with more
unsaturated fatty acids (Warner and Nelsen 1996). The content of
linolenic acid is critical to the frying performance, the stability of oil, and
the flavor quality of fried food (Choe and Min, 2007). Total polar
compounds contents of sunflower oil (C16:0, 5.3%; C18:0, 4.8%; C18:1,
55.4%; C18:2, 32.4%) and high linolenic canola oil (C16:0, 4.1%; C18:0,
2.2%; C18:1, 69.3%; C18:2, 13.8%; C18:3, 6.8%) after 80-h frying at
190 °C were 44.6% and 47.5%, respectively (Xu et al., 1999). The oil
containing linolenic acid at 8.5% produced extremely undesirable acrid
and fishy odors when heated above 190°C (Frankel and others 1985).
Low linolenic acid (2.5%) canola oil produced less free fatty acids and
less polar compounds during deep-fat frying of potato chips at 190°C
(Xu et al., 1999). Warner and others (1997) reported that polar
compounds formation in cottonseed oil during deep-fat frying of potato
chips increased proportionally with the linoleic acid content in the oil.
Hydrogenation and genetic modification are two of the processes
to decrease the unsaturated fatty acids of frying oil. Hydrogenation
increases the frying stability of oil (Choe and Min, 2007).
However, hydrogenation produces trans fatty acid or metallic
flavor, and it does not additionally improve the quality of oil with low
29
linolenic acid. Hydrogenated soybean oil with 0.1% linolenic acid had
more hydrolytic degradation, but lower p-anisidine values and polymer
formation, than the soybean oil with 2.3% linolenic acid. Genetically
modified high oleic corn oil improved frying stability over normal corn
oil. Therefore, low linolenic acid oil by genetic modification was
suggested to be a potential alternative to hydrogenated frying. Blending
of several oils can change the fatty acid compositions of oils and can
decrease the oxidation of oils during deep-fat frying (Choe and Min,
2007).
The esterified acylglycerol does not show any prooxidant activity
in the oil oxidation during deep-fat frying. Free fatty acids in frying oil
accelerated thermal oxidation of the oil. The filtering of oil with
adsorbents lowered free fatty acids and improved the frying quality of
oil. Used frying oil was filtered with a mixture of 2% pekmez earth, 3%
bentonite, and 3% magnesium silicate, and the filtering process decreased
the contents of free fatty acids and conjugated dienoic acids of sunflower
oil during deep-fat frying of potato samples at 170°C and increased the
formation of aldehyde compounds (Maskan and Bagci, 2003). Daily
filtering treatments of canola oil with a mixture of calcium silicate-based
Hubesorb 600, magnesium silicate-based magnesol, and rhyolite and
citric acid-based fry powder decreased free fatty acids and polar
compounds formation and improved the frying quality of oil. The
treatment of shortening with bleaching clay, charcoal, celite, or MgO
improved the oil quality for French fries. Daily addition of ascorbyl
palmitate to fresh oil decreased free fatty acid formation, but increased
30
dielectric constant and color changes. Stevenson et al. (1984) reported
that oils with less than 0.05% free fatty acids and 1.0 meq peroxides in 1
kg of oil are desirable for deep-fat frying.
2.12d Compositions of foods:
Moisture in foods creates a steam blanket over the fryer and
reduces contact with air (Choe and Min, 2007). A large amount of
moisture in foods increases the oil hydrolysis during deep-fat frying. The
more the moisture content of food, the higher the hydrolysis of oils.
Lecithin from frying foods caused foam formation at the initial stage of
deep-fat frying. Phosphatidylcholine decreased the oxidation of salmon
oil at 180°C for 3 h. Starch increases the degradation of oil and amino
acids protect the oil from degradation during deep-fat frying (Choe and
Min, 2007).
Transition metals such as iron, which is present in meat, were
accumulated in the oil during frying and this increased the rates of
oxidation and thermal degradation of oil spinach powder at 5, 15, or 25%
in flour dough decreased the formation of polar compounds in soybean
oil (Lee et al., 2002). Addition of red ginseng extract to the flour dough
at 1 and 3% decreased the formations of free fatty acids, conjugated
dienoic acids, and aldehydes in palm oil during deep-fat frying of the
dough at 160°C (Kim and Choe, 2003). Beef nuggets battered with
glandless cottonseed flour decreased free fatty acids, conjugated diene
compounds, and thiobarbituric acid-reactive substances. Addition of
carrot powder to the dough at 10, 20, or 30% decreased the oxidative
stability of soybean oil at (P< 0.05) during frying (Lee et al., 2003).
31
Sodium pyrophosphate inhibited after-cooking darkening in
French fries and decreased free fatty acid formation in the hydrogenated
canola oil during 12 to 72 h frying. Calcium acetate shows little effects
on the formation of free fatty acids in the oil during deep-fat frying The
application of edible film to foods before frying decreases the
degradation of frying oil during deep-fat frying. a coating of hydroxy-
propylmethylcellulose films on fried chicken strips decreased free fatty
acid formation in peanut oil during deep-fat frying (Choe and Min,
2007).
2.12e Types of fryer:
The types of fryer affect the frying oil deterioration. Even and fast
heat transfer to the oil can prevent hot spots and the scorch of oil.
Polymerized fat deposited on the fryer causes gum formation, the
formation of foam, color darkening, and further deterioration of frying
oil. A small surface-to-volume ratio of fryer for minimum contact of oil
with air is recommended for deep-fat frying. Negishi et al. (2003)
reported that oil oxidation was slowed down by modifying a fryer to have
a ratio of oil depth (D) to oil area (A) with D/A1/2 = 0.93. Copper or iron
fryer accelerates the oxidation of frying oil.
2.12f Antioxidants:
The naturally present or added antioxidants in oils and foods
influence oil quality during deep-fat frying. Tocopherols, butylated
hydroxyanisole (BHA), butylated hydroxytoluene (BHT), propyl gallate
(PG), and tert-butylhydroquinone (TBHQ) slow down the oxidation of
oil at room temperature. However, they become less effective at frying
32
temperature due to losses through volatilization or decomposition. Tyagi
and Vasishtha (1996) reported the ineffectiveness of 0.01% BHA and
TBHQ during deep-fat frying of potato chips in soybean oil. The
decompositions of tocopherols in soybean oil, beef tallow, and palm oil
after 8-h frying of steamed noodles at 150°C were 12.5, 100, and 100%,
respectively. The retention of tocopherols in soybean oil decreased the
oxidation of soybean oil than beef tallow or palm oil without any
retention of tocopherols. Palm oil contained tocotrienols in addition to
tocopherols at 169 ppm, all of which was decomposed during 8-h frying
of steamed noodles. Soybean oil contained more unsaturated fatty acids
than beef tallow or palm oil.
Carotenes do not protect the oil from thermal oxidation in the
absence of other antioxidants. Carotenes are major compounds that react
with oil radicals in red palm olein. Tocotrienols regenerate carotenes
from carotene radicals. The combination of tocotrienols and carotenes
decreased the oxidation of oil synergistically during frying of potato
slices at 163°C.
Lignan compounds in sesame oil, sesamol, sesamin, and
sesamolin, are stable during heating and contribute to high oxidative
stability of roasted sesame oil during heating at 170°C. The blended
soybean oil with roasted sesame oil lowered the formation of conjugated
dienoic acids than soybean oil during frying at 160°C in spite of the
higher unsaturated fatty acids of the blended oil than soybean oil. As the
sesame oil contents in the blended oil increased, the formation of
conjugated diene decreased, possibly due to the antioxidants in sesame
33
oil. The addition of sesame oil and rice bran oil enhanced the oxidative
and flavor stability of high oleic sunflower oil, possibly due to the
avenasterol, which is stable at high temperature (Choe and Min, 2007).
Ascorbyl palmitate lowered dimers in oil during deep-fat frying.
Sterols and their fatty acid esters improved the oxidative stability of oil
during deep-fat frying. Silicone protected the oil from oxidation during
deep-fat frying (Choe and Min, 2007). By the formation of a protective
layer at the air-oil interface and the low convection currents of frying oil .
The combination of silicone and antioxidants (Choe and Min, 2007).
synergistically decreased the oxidation of oil during deep-fat frying.
Rosemary and sage extracts reduce oil deterioration during a 30-h
intermittent deep-fat frying of potato chips. the synergistic antioxidant
effects of rosemary, sage, and citric acid on palm olein during deep-fat
frying of potato chips. the hexane-extracts of burdock decreased the
formation of conjugated dienoic acids and aldehydes in lard significantly
(P< 0.05) at 160°C. The hexane-extract of burdock is a potential
antioxidant of oil for deep-fat frying (Choe and Min, 2007).
2.12.g Dissolved oxygen contents in oil:
Nitrogen or carbon dioxide flushing decreased the dissolved
oxygen in oil and reduced the oxidation of oil during deep-fat frying
.Carbon dioxide gives better protection from oxidation due to its higher
solubility and density than nitrogen. Przybylski and Eskin (1988)
suggested that a minimum of 15 min of nitrogen or 5 min of carbon
dioxide flushing prior to heating decreases the oxidation of oil during
deep-fat frying.
34
CHAPTER THREE
MATERIALS AND METHODS
3.1 Materials
3.1.1 Raw materials
Both refined, bleached and deodorized sunflower and corn oils
were purchased from local market. Potatoes used in the Frying
experiments were also purchased from local market.
3.1.2 Chemicals, reagents and instruments:
They are brought from University of Khartoum and Arabian
Company for Plant Oils Productions.
3.2 Preparation of raw materials:
Potato tubers were peeled, sliced to a thickness of 2 – 4 mm using
a manual slicer, dipped in water to reduce browning, then used in frying.
3.3 Frying processes:
Frying processes on the two oils were done simultaneously: 1.5 kg
oils were heated up to 180oC, two hundred grammes potato slices were
fried. The frying was carried out with sunflower and corn oils for five
consecutive days. At the end of each frying experiment, the oil was
allowed to cool; 100 gm oils were sampled from, in to a clean glass dark
container and stored in the deepfreeze until assessment.
35
3.4 Analytical procedures:
3.4.1 Oil characteristics:
3.4.1.1 Refractive index:
The refractive index (RI) was determined by Abbe 60 refracto-
meter as described by the A.O.A.C. method (2000). A double prism was
opened by means of screw head and few drops of oil were placed on the
prism. The prism was closed firmly by tightening the screw head. The
instrument was then left to stand for few minutes before reading in order
to equilibrate the sample temperature with that of the instrument (32 ±
2oC). The refractometer was cleaned between readings by wipping of the
oil with soft cloth, then with cotton moistened with petroleum ether and
left to dry.
3.4.1.2 Colour:
The colour intensity of oils was recorded using a Lovibond
Tintometer as units of red, yellow and blue according to the A.O.A.C.
method (2000).
Samples of oils were filtered through filter paper immediately
before testing. An appropriate cell (2" cell) was filled with oil and placed
in the tinomeer nearby the window for light. The instrument was
switched on and looked through the eye piece. The yellow colour was
adjusted to 25, then slides were adjusted until a colour match was
obtained from a combination of red and blue. The values obtained by
matching were recorded as red, yellow and blue.
36
3.4.1.3 Viscosity:
The viscosity of the oil samples was recorded using an Ostwald-U-
tube viscometer according to Cocks and Van Rede (1966). The
viscometer was suspended in the constant temperature bath (32 ± 2oC) so
that the capillary was vertical. The instrument was filled to the mark at
the top of the lower reservoir with the oil by means of pipette inserted
into the side arm, so that the tube wall above the mark is not wetted. The
instrument was then left to stand for few minutes before reading in order
to equilibrate the sample temperature with that of the instrument (32 ±
2oC). By means of the pressure on the respective arm of the tube, the oil
moved into the other arm so that the meniscus is 1 cm above the mark at
the top of the upper reservoir. The liquid was then allowed to flow freely
through the tube and the time required for the meniscus to pass from the
mark above the upper reservoir to that at the bottom of the upper
reservoir was recorded.
Calculation:
T – T0 V= T0 Where:
T : Flow-time of the oil T0: Flow-time of distilled water
3.4.1.4 Peroxide value (PV):
The PV of the oil samples was determined according to the
A.O.A.C. method (2000). One gram of the oil was accurately weighed
into 250 ml conical flask. Thirty ml of a mixture of glacial acetic acid
and chloroform (3:2) were added and the solution was swirled gently to
37
dissolve the oil. A 0.5 ml of 0.1N KI was added to the flask, and then the
contents of the flask were left to stand for one minute then adding 30 ml
of distilled water. The contents were titrated with 0.01N sodium
thiosulphae until the yellow colour almost disappeared. A 0.5 ml of 1%
starch solution was added, and the titration continued with vigorous
shaking until the blue colour completely disappeared. Volume of 0.01N
sodium thiosulphae required (a) were recorded. The same process was
repeated for blanks. Volume of 0.01N sodium thiosulphate required by
the blank (B) was recorded.
Calculation:
(b-a) x N x 1000 P.V. = S Where:
b : Volume of sodium thiosulphate required for blank (ml) a : Volume of sodium thiosulphate required for oil sample (ml) S : Weight of oil sample used (gm)
3.1.4.5 Free Fatty Acids (FFA):
FFA determination was carried out according the A.O.A.C.
method (2000). Five grams of the oil was weighed accurately into 250 ml
a conical flask. Fifty ml mixture of 95% alcohol and ether solvent (1:1)
were added. . The contents of the flask were then heated with caution
until the oil was completely dissolved the solution was neutralized after
addition of one ml of phenophtholin indicator. The contents of the flask
were then titrated with 01N KOH with continuous shaking until a pink
colour persisted for 15 seconds. The number of ml of 0.1N KOH required
(a) was recorded.
38
Calculation:
V (ml) x N x M.W. X 28.2 FFA= S Where:
V : Volume of KOH used (ml) N : Normality of KOH M.W. Molecular weight of oleic acid =28.2 S : Weight of oil sample used
3.5 Statistical analysis:
Triplicate of each sample was analyzed using statistical analysis
system. The analysis of variance was performed to examine the
significant effect in all parameters measured. Least significant difference
(LSD) was used to separate the means.
39
CHAPTER FOUR
RESULTS AND DISCUSSIONS
4.1 Physicochemical properties of sunflower and corn oils:
Table (1) shows the initial values of the physicochemical
properties of sunflower and corn oils used in experiments. The general
feature of the oils reflects good colors, acceptable levels of acidity and
peroxides.
4.2 Changes in physical parameters of oils during frying:
4.2.1 The effectiveness on the oil refractive index during frying:
Table (2) shows changes in refractive index (RI) of both oils
during frying of potato chips( corn oil, from 1.4750 in control sample to
1.4820 after the fifth frying process, sunflower oil, from 1.4750 in
control sample to 1.4810 after the fifth frying process). The RI exhibited
significant increase (P< 0.05) in both oils by frequent frying. Increase in
RI of oils by frying was reported recently by Tyagi and Vasishtha (1996)
in soybean oil.
4.2.2 The effectiveness on the oil viscosity during frying:
Table (3) shows changes in viscosity of corn and sunflower oils as
a result of frequent use in frying of potato chips(corn oil, from 28.7 in
control sample to 31.1 after the fifth frying process, sunflower oil, from
28.3 in control sample to 31.0 after the fifth frying process). The
viscosity increased significantly (P< 0.05) directly after second frying
time.
40
Table (1): Physicochemical properties of corn and sunflower oils
Parameter Corn oil Sunflower oil
Refractive index 1.4750 1.4750
Viscosity at 30 c 28.7 28.3
Yellow 25 25 colour
Red 0.9 0.4 Peroxide value (meq/kg) 1.0 1.0
Free fatty acids % 0.125 % 0.110 %
41
Table (2): The effectiveness on the oil refractive index* during frying
Frying times Corn oil Sunflower oil
0.0 1.4750e 1.4750d
1st 1.4760d 1.4760c
2nd 1.4763cd 1.4762c
3rd 1.4767bc 1.47675c
4th 1.4800b 1.4803b
5th 1.4820a 1.4810a
* values having different superscript letter(s) differ significantly (P< 0.05)
42
1.47
1.472
1.474
1.476
1.478
1.48
1.482
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (1): The effectiveness on the oil refractive index during frying
43
Table (3): The effectiveness on the oil viscosity* during frying
Frying times Corn oil Sunflower oil
0.0 28.7e 28.3e
1st 28.8e 28.6e
2nd 29.3d 29.0d
3rd 29.9c 29.5c
4th 30.4b 30.1b
5th 31.1a 31.0a
* values having different superscript letter(s) differ significantly (P< 0.05)
44
26.527
27.528
28.529
29.530
30.531
31.5
0 1 2 3 4 5
Frying time
Corn Sunflow er
Fig. (2): The effectiveness on the oil viscosity* during frying
45
The increase in viscosity of both oils was more or less similar.
Increase in viscosity of oils by frying was found earlier by Thompson
et al. (1967) in cottonseed oil. Also increase in viscosity of oils by frying
was reported by Tyagi and Vasishtha (1996) in soybean oil.
4.2.3 The effectiveness on the oil colour during frying:
Table (4) shows changes in colour of corn and sunflower oils as a
result of frequent use in frying of potato chips (corn oil, from 0.9 in
control sample to 6.4 after the fifth frying process, sunflower oil, from
0.4 in control sample to 5.1 after the fifth frying process). The colour
increased significantly (P< 0.05) redness. The increase in corn oil was
more than sunflower oil. Increase in colour of oil by frying was reported
by Sonia and Badereldeen (1983), and was found earlier by Augustin
et al. (1987).
4.3 Changes in chemical parameters of oils during frying:
4.3.1The effectiveness on the oil peroxide value during frying:
Table (6) shows changes in peroxide value (PV) of corn and
sunflower oils as a result of frequent use in frying of potato chips(corn
oil, from 1.0 m.Eq/kg in control sample to11.9 m.Eq/kg after the third
frying process to 9.8 m.Eq/kg after the fourth frying process to 13.9
m.Eq/kg after the fifth frying process. Sunflower oil, from 1.0 m.Eq/kg in
control sample to 8.8 m.Eq/kg after the fourth frying process to 4.7
m.Eq/kg after the fifth frying process). The PV increased significantly
(P< 0.05) directly after second frying time. Similar increase in PV of
sesame oil were observed earlier by deep frying (Khattab et al., 1974).
46
Table (4): The effectiveness on the oil colour* during frying
Corn oil colour Sunflower oil colour Frying times
Yellow Red Yellow Red
0.0 25 0.9f 25 0.4f
1st 25 1.8e 25 1.2e
2nd 25 2.7d 25 1.9d
3rd 25 4.1c 25 2.8c
4th 25 4.9b 25 3.5b
5th 25 6.4a 25 5.1a
*Mean values having different superscript letter(s)
differ significantly (P< 0.05)
47
0
1
2
3
4
5
6
7
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (3): The effectiveness on the oil colour* during frying
48
Table (5): The effectiveness on the oil peroxide value (m.Eq/kg)* during frying.
Frying times Corn oil Sunflower oil
0.0 1.0f 1.0f
1st 2.9e 2.2e
2nd 6.7d 4.0d
3rd 11.9b 6.9b
4th 9.6c 8.8a
5th 13.9a 4.7c
*Mean values having different superscript letter(s) differ significantly (P< 0.05)
49
0
2
4
6
8
10
12
14
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (4): The effectiveness on the oil peroxide value (m.Eq/kg) during frying.
50
Table (6): The effectiveness on the oil free fatty acids value* during frying
Frying times Corn oil Sunflower oil
0.0 0.125f 0.110f
1st 0.130e 0.115e
2nd 0.135d 0.120d
3rd 0.180c 0.140c
4th 0.185b 0.145b
5th 0.200a 0.150a
*Mean values having different superscript letter(s)
differ significantly (P< 0.05)
51
00.020.040.060.080.1
0.120.140.160.180.2
0 1 2 3 4 5
Frying time
Corn Sunflower
Fig. (5): The effectiveness on the oil free fatty acids value* during frying
52
4.3.2 The effectiveness on the oil free fatty acids value during frying (FFA):
Table (7) shows changes in free fatty acids (FFA) of corn and
sunflower oils as a result of frequent use in frying of potato chips(corn
oil, from 0.125 in control sample to 0.200 after the fifth frying process,
sunflower oil, from 0.110 in control sample to 0.150 after the fifth frying
process). The FFA significantly decreased (P< 0.05) in both oils by
frequent frying. The increase of FFA in both oils was more or less
similar.
53
CHAPTER FIVE
CONCLUSIONS AND RECOMMENDATIONS
5.1 Conclusions:
Fat frying causes the hydrolysis, oxidation, and polymerization of
the oil. Hydrolysis increases the amount of free fatty acids, mono- and
diacylglycerols, and glycerols in oils. Oxidation occurs at a greater rate
than hydrolysis during deep-fat frying. Oxidation produces
hydroperoxides and then low molecular volatile compounds such as
aldehydes, ketones, carboxylic acids, and short-chain alkanes and
alkenes. Dimers and polymers are also formed in oil by radical and Diels-
Alder reactions during deep-fat. Replenishment of fresh oil, frying
conditions, quality of frying oil frying oil, food materials, fryer,
antioxidants, and oxygen concentration affect the quality and flavor of oil
during deep-fat frying. Intermittent frying with a lower turnover rate and
higher temperature accelerates the oxidation and polymerization of oil
during deep-fat frying. The frying quality of oil with high unsaturated
fatty acids and free fatty acids is not as good as oil with low unsaturated
fatty acids and free fatty acids. Addition of spinach and ginseng extract to
the dough increases the oxidative stability of oil. Frying in a fryer with a
small surface-to-volume ratio is desirable to slow down the oxidation of
frying oil. Tocopherols, BHA, BHT, PG, and TBHQ decrease oil
oxidation, but they become less effective at frying temperature. Lignan
compounds in sesame oil are more stable than tocopherols, BHA, BHT,
PG, and TBHQ at frying temperature and are effective antioxidants
during deep-fat frying. Desirable fried flavor compounds such as
54
hydroxynonenoic acid, 2, 4-decadienal, and nonenlactone are produced
during deep-fat frying at an optimal concentration of oxygen.
5.2 Recommendations:
- This study is conducted using one item of food which is potatoes,
while so many other items exist and they may give different results
if tested, especially in connection with physical characteristics.
Therefore, similar studies using other food items are
recommended.
- More research is needed in this field to cover more times of points
and food products.
- Fatty acids composition and iodine value are important to test in the
next researches.
55
REFERENCES
AOAC (2000). Official Methods of Analysis of the Association of Official Agricultural chemists, 14th ed. Washington, D.C.
AOCS (2000). American Oil Chemists Society. Official and Tentative Methods of American Oil Chemists Society, 2nd. Aocs-Champaign Illinois.
Augustin, M.A.; Hena, L.K. and Teah, Y.K. (1987). Comparison of frying performance of market samples of palmolein, corn oil and soya oil in Malaysia. J. Pertanika, 19: 295.
Augustin, M.A.; Heng, L.K. and N.A. (1989). Experiments with hydrogenated vegetable shortenings and palmolein, J. Pertanika, 12: 167.
Augustin, M.A.; Telingal, A. and Teah, Y.K. (1987). Heating experiments with Boskou, G.; Salta, F.N.; Chiou, A.; Troullidou, E.and Andrikopoulos, N.K. (2006). Content of trans, trans-2,4-decadienal in deep-fried and pan-fried potatoes. Eur J Lipid Sci Technol 108:109–15.palmolein and groundnut oil. J. Pertanika, 10: 142.
Bheemreddy, R.M.; Chinnan, M.S.; Pannu, K.S.and Reynolds, A.E. (2002). Active treatment of frying oil for enhanced fry-life. J Food Sci 67:1478–84.
Blumenthal, M.M.( 1991). A new look at the chemistry and physics of deep-fat frying. Food Technol 45(2):68–71, 94.
Buziassy, C. and Nawar, W.W. (1968). Specificity in thermal hydrolysis of triglycerides. J. Food Science. 33: 305.
Celegg, A.J. (1973). Composition and related nutritional and organoleptic aspects of palm oil. J. Am. Oil Chem. Soc. 50: 312.
56
Choe, E.; Kang, W.S.and Chang, Y.S. (1993). Kinds and changes in the amount of flavor compounds formed during storage of the ramyon. Korean J Food Sci Technol 25:52–6.
Choe, E.and Min, D.B.( 2005). Chemistry and reactions of reactive oxygen species in foods. J Food Sci 70:R142–59.
CHOE, E. and MIN, D.B. (2007) Chemistry of Deep-Fat Frying Oils Journal of Food Science 72 (5) , R77–R86 doi:10.1111/j.1750-3841.2007.00352.x
Chung, J.; Lee, J.and Choe, E.( 2004). Oxidative stability of soybean and sesame oil mixture during frying of flour dough. J. Food Sci 69:574–8.
Clark, W.L.and Serbia, G.W.( 1991). Safety aspects of frying fats and oils. Food Technol 45:84–9.
Cocks, L.U. and Van Rede (1966).Laboratory Handbook for oil and fat analysis. Academic Press. Inc. Itd. London; New York.
Cramptom, E.W.; Common, R.H.; Florence, A.; Farmer, A.F.; Wells and Donna Crawford (1953). Studies to determine the nature of the damage to nutritive value of some vegetable oils from heat treatment. J. Nutrition. 49: 333.
Cuesta, C.; Sanchez-Muniz, F.J.; Garrido-Polonio, C.; Lopez-Varela, S.and Arroyo, R.( 1993). Thermooxidative and hydrolytic changes in sunflower oil used in frying with a fast turnover of fresh oil. J. Am Oil Chem Soc 70:1069–73.
Dana, D.; Blumenthal, M.M.and Saguy, I.S. (2003). The protective role of water injection on oil quality in deep fat frying conditions. Eur Food Res Technol 217:104–9.
Elgaili, A.T. (2000). Effect of Frequent frying and storage on quality of Sudanese Edible oils .U of K Library.
Frega, N.; Mozzon, M.and Lecker, G. (1999). Effects of free fatty acids on oxidative stability of vegetable oil. J. Am. Oil Chem. Soc. 76:325–9.
57
Fritisch, C.W. (1981). Measurements of frying fat deterioration. A review. J. Am. Oil Chem. Soc. 58: 272.
Fritisch, C.W.; Egberg, D.C. and Magnuson, J.S. (1979). Changes in dielectric constant as a measure of frying oil deterioration. J. of Am. Oil Chem. Soc. 56: 746.
Fritisch, C.W.; Weiss, V.E. and Anderson, R.H. (1971). Stability of coconut oil in food products. J. of Am. Oil Chem. Soc. 48: 392.
Gray, J.I. (1978). Measurement of lipid oxidations. J. of Am. Oil Chem. Soc. 55: 539.
Hartley, C.W.S. (1967). The oil palm, oil palm products, their formation and characteristics 2nd ed. West Africa Institute for Oil Palm Research, London, New York, 693-703.
Johnson, O.C. and Kummerous, F.A. (1957). Chemical changes which take place in an edible oil during thermal oxidation. J. of Am. Oil Chem. Soc. 34: 407.
Joslyn, M.A. (1970). Methods and food analysis: Viscosity consistency and texture, conductivity measurements and gas analysis. 2nd ed. Academic Press, New York and London. 375-395.
Kaufmann, H.P. (1954). Vegetable fats and oils. Edited by E.W. and Lawrence, P. Miller. Reinhold Publishing Crop. 83-87.
Khalil, S.A. (1983). Chemical and physical changes in heated cottonseed oil during simulated frying conditions and their correlation with foaming properties. Research Reports, No. 313. Nov. Leather Head, Food R.A.
Khattab, A.H.; El Tinay, A.H.; Khalifa, H.A. and Mirghani, S. (1974). Stability of peroxidised oils and fat to high temperature heating. J. Sci. Food Agric. 25: 689.
Lea, C.H.; Parr, L.J. and Canpenter, K.J. (1960). Chemical and nutritional changes in stored herring meal. Brit. J. Nutr. 14: 91.
58
Lee ,J.; Lee, S.; Lee, H.; Park, K.and Choe, E. (2002). Spinach (Spinacia oleracea) powder as a natural food-grade antioxidant in deep-fat-fried products. J. Agric. Food Chem. 50:5664–9.
Lillard, D.A. (1983). Effect of processing on chemical and nutritional changes in the food lipids. J. of Food Protection. 46: 61.
Lopez-Varela, S.; Sanchez-Muniz, F.J. and Cuesta, C. (1995). Decreased food efficiency ratio, growth retardation and change in liver fatty acid composition in rats consuming thermally oxidized and polymerized sunflower oil used for frying. Food and Chemical Toxicology. 33: 181.
Maskan, M.and Bagci, H.I. (2003). The recovery of used sunflower seed oil utilized in repeated deep-fat frying process. Eur Food Res Technol 218:26–31.
Mauric, H. and Sambuc, E. (1968). Analysis and characteristics of oils and fats and fat products. Edited by H.A. Boeckenoo. Gen. Intenscience Publisher A Division of John Wiely and sons Ltd. London, New York, Sydney, 46-47.
Mehlenbacher, V.C. (1960). The analysis of fats and oils. The Carrd Press Publishers, Champaign.
Melnick, D. (1957). Nutitional quality of frying fats in commercial use. J. of Am. Oil Chem. Soc. 34: 578.
Min, D.B.and Boff, J.M.( 2002). Lipid oxidation of edible oil. In: Akoh CC, Min DB, editors. Food lipids. 2nd ed. New York: Marcel Dekker Inc. p 344.
Moreira, R.G.; Castell-Perez, M.E.and Barrufet ,M.A.( 1999a). Fried product processing and characteristics. In: Deep-fat frying: fundamentals and applications. Gaithersburg, Md.: Chapman & Hall Food Science Book.p 11-31.
Moreira, R.G.; Sun, X.and Chen, Y. (1997). Factors affecting oil uptake in tortilla chips in deep-fat frying. J. Food Eng. 31: 485–98.
59
Nawar, W.W. (1984). Chemical changes in lipids produced by thermal processing. J Chem Edu 61:299–302.
Naz, S.; Siddiqi, R.; Sheikh, H.andSayeed, S.A.(2005). Deterioration of olive, corn, and soybean oils due to air, light, heat, and deep- frying. Food Res. Intl. 38: 127–34.
Negishi, H.; Nishida, M.; Endo, Y.and Fujimoto, K. (2003). Effect of a modified deep-fat fryer on chemical and physical characteristics of frying oil J. Am. Oil Chem. Soc. 80:163–6.
Noble, A.C.; Buziassy, C. and Nawar, W.W. (1967). Thermal hydrolysis of some natural fats. J. Lipids. 2: 435.
Peled, M.; Tamar, C. and Arich, I. (1975). Effect of water and BHT on stability of cottonseed oil during frying. J. Sci. Fd. Agric. 26: 165.
Przybylski, R.and Eskin, N.A.M. (1988). A comparative study on the effectiveness of nitrogen or carbon dioxide flushing in preventing oxidation during the heating of oil. J. Am. Oil Chem. Soc. 65: 629–33.
Robern, H.I. and Gray, L. (1981). Oxidative products formed in frying fats and oils heated. Can. Int. Fd. Sci. Tech. J. 14: 150.
Robertson, J.A. (1972). Sunflower: Americans neglected crop. J. Am. Oil Chem. Soc. 49: 239-244.
Robertson, J.A. and Morrison, W.H. (1977). Effect of heat and frying on sunflower oil stability. J. Am. Oil. Chem. Soc. 54: 77A.
Sonia, Z.E. and Badereldeen, A.E. (1983). Changes of cottonseed oil during deep fat frying of food. Larivista Italiana Delle Sostanze Grasse. Vol. IX: 73.
Stevenson, S.G.; Vaisey-Genser, M.and Eskin, N.A.M. (1984). Quality control in the use of deep frying oils. J. Am. Oil Chem Soc 61: 1102–8.
60
Taha, F.S.; Helmy, H.E. and El-Nockrashy, A.S. (1988). Changes in cottonseed oil when used for fyring vegetable products containing chlorophyll. J. of Am. Oil Chem. Soc. 65: 267.
Takeoka, G.R.; Full ,G.H.and Dao, L.T. (1997). Effect of heating on the characteristics and chemical composition of selected frying oil and fat. J. Agric. Food Chem. 45: 3244–9.
Tompkins, C .and Perkins, E.G.( 2000). Frying performance of low-linolenic acid soybean oil. J Am Oil Chem. Soc., 77: 223–9.
Toschi, T.G.; Costa ,A.and Lercker, G. (1997). Gas chromatographic study on high-temperature thermal degradation products of methyl linoleate hydroperoxides. J. Am. Oil Chem. Soc. 74: 387–91.
Tseng ,Y.C. ;Moreira, R.G.and Sun, X. (1996). Total frying-use time effects on soybean oil deterioration and on tortilla chip quality. Intl J Food Sci Technol 31:287–94.
Tyagi, V.K. and Vasishtha, A.K. (1996). Changes in the characteristics and composition of the oil during deep-fat frying. J. of Am. Oil Chem. Soc. 73499.
Warner, K.and Nelsen ,T. (1996). AOCS collaborative study on sensory and volatile compound analysis of vegetable oils. J. Am. Oil Chem. Soc. 73: 157–66.
Warner ,K.; Orr, P.and Glynn, M. (1997). Effect of fatty acid composition of oils on flavor and stability of fried foods. J. Am. Oil Chem. Soc. 74: 347–56.
Warner, K.; Orr, P.; Parrott, L.and Glynn, M. (1994). Effects of frying oil composition on potato chip stability. J. Am. Oil Chem. Soc. 71: 1117–21.
Xu ,X.Q.; Tran, V.H.; Palmer, M.; White, K.and Salisbury, P. (1999). Chemical and physical analyses and sensory evaluation of six deep- frying oils. J. Am. Oil Chem. Soc. 76: 1091–9.
Yoon, S.H.; Jung, M.Y.and Min, D.B. (1988). Effects of thermally oxidized triglycerides on the oxidative stability of soybean oil. J Am. Oil Chem. Soc. 65(10): 1652–6.
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Appendix:
Figure 1: Physical and chemical changes of oil during deep-fat frying (Choe and Min, 2007).
62
Figure 2: Free fatty acid formation in soybean and sesame oil mixture during consecutive frying of flour dough at 160 °C (Chung et al. 2004)
63
Figure 3: The initiation, propagation, and termination of thermal oxidation of oil(Choe and Min,2007).
64
Figure 4: Acyclic polymer formation from oleic acid during deep-fat frying(Choe and Min,2007).
65
Figure 5: Polymer contents of cottonseed and corn oils heated at 190°C and 204°C (Takeoka et al. 1997)
66
Figure 6: Formation of cyclic dimers and polymers from linoleic acids during deep-fat frying(Choe and Min, 2007).
67
Figure 7: Cyclic compound formation from linoleic acid by Diels-Alder reaction during deep-fat frying(Choe and Min,2007).