deep fat frying of potato

DEEP FAT FRYING OF POTATO

STRIPS—QUALITY ISSUES

M. K. Krokida,* V. Oreopoulou, Z. B. Maroulis, and

D. Marinos-Kouris

Department of Chemical Engineering, National TechnicalUniversity of Athens, Zografou Campus,

15780 Athens, Greece

ABSTRACT

Moisture loss and oil adsorption kinetics, structuralproperties (apparent density, true density, specific volumeand internal porosity), color changes and viscoelastic beha-vior (compression tests, crispness) were investigated duringdeep fat frying of french fries. The effect of frying conditions(oil temperature, sample thickness and oil type), dryingpretreatment and osmotic dehydration pretreatment on theabove properties was also examined. The results showedthat oil temperature and thickness of potato strips have asignificant effect on oil uptake, moisture loss and color para-meters of french fries, while the use of hydrogenated oil in thefrying medium does not affect these properties. The porosityof french fries increases with oil temperature increases andsample thickness and it is higher for products fried withhydrogenated oil. Maximum stress and maximum strainincrease during frying, while crispness of potato strips is

879

Copyright & 2001 by Marcel Dekker, Inc. www.dekker.com

*Corresponding author. E-mail: [email protected]

DRYING TECHNOLOGY, 19(5), 879–935 (2001)

higher for hydrogenated oil, and lower for refined oil. Airdrying and osmotic pretreatment increase porosity of friedpotatoes but decrease their oil and moisture content. A nega-tive effect on color development with drying time was alsoobserved. Pre-fry drying as well as osmotic pre-treatmentincreases the maximum stress and maximum strain of frenchfries during frying. Air drying pre-treatment increases thecrispness of potato strips while osmotic pre-treatment doesnot affect it, with the exception of sugar solutions.

Key Words: Apparent density; Color; Compression tests;Oil uptake; Porosity; Water loss.

1. INTRODUCTION

Deep fat frying is a process of simultaneous heat and mass transfer.Heat is transferred from the oil to the food, water is evaporated from thefood material and oil is absorbed in it. Factors that affect heat and masstransfer are the thermal and physicochemical properties of the food and theoil, the geometry of the food, the temperature of the oil, as well as the typeof pre-treatment before the frying process.

Mass transfer kinetics (moisture and oil transfer) are essential for thedesign of the deep fat frying processes and for the efficient operation andcontrol of processing plants.

The comprehensive term ‘‘quality’’ comprises a number of parametersof the frying material, either in a mid-state (at intermediate stages of thefrying process) or after the completion of the frying. Although these proper-ties were not necessary for the prediction of frying time and for processmodeling until last decade, they became very important for the character-ization and prediction of the quality of the fried product during last years.They are also very important for the development of new industrial productswith desirable properties or for quality improvement of already existingones, e.g. fried products that have reduced fat level. The quality relatedproperties could be grouped into the following:

. Structural properties (density, porosity, pore size, specific volume).

. Optical properties (color, appearance).

. Textural properties (compression strength, stress relaxationbehavior, tensile strength).

880 KROKIDA ET AL.

. Mechanical properties (state of product: glassy, crystalline,

rubbery).

. Sensory properties (aroma, taste, flavor).

. Nutritional properties (vitamin content, proteins, etc.).

The industrial product quality aspects usually include control of oil

and moisture content of the fried product, minimization of chemical degra-

dation reactions, control of structural changes and achievement of the

desired taste, texture and color of the product.

Frying conditions and physicochemical changes that occur during

frying seem to affect the quality properties of the fried product. More spe-

cifically, oil temperature and processing time are critical factors. The frying

medium, i.e. the oil type, is another factor that affects mainly degradation of

the product after frying but also the quality properties of the fried products.

To retard degradation partial replacement of conventional oils by hydro-

genated ones has been suggested (Hawrysh et al., 1995; Melton et al., 1993).

So far as the raw material is concerned, in addition to the inherent

properties that depend on the variety and cultivation conditions, sample

geometry and especially sample thickness affect significantly the oil and

moisture content as well as the color, texture, density and porosity of the

fried product.

The increasing need for producing low fat snacks has increased sub-

stantially. The new fat-free tortilla chips are baked rather than fried, how-

ever they have different flavor and textural properties compared to the fried

chips (Rickard et al., 1993). There are alternative methods to the manufac-

turing of fried products with reduced fat, which are based on partial moist-

ure removal before or after frying. The most commonly used methods are

the following:

. Conventional frying with premature removal from the fryer at a

high (�10%) moisture content and finish processing using super-

heated steam (Li et al., 1999).

. Hot air and microwave finishing (Blau, 1965; Blau et al., 1965;

Smith and Davis, 1965).

. Pre-fry drying pretreatment (Gamble and Rice, 1987; Gupta et al.,

2000; Krokida et al., 2000c).

. Osmotic pretreatment of potatoes by immersion or spraying with

sugar solutions (Krokida et al., 2000d).

Another approach, suggested mainly for batter coatings, is the addi-

tion of powdered cellulose that proved efficient in fat reduction (Ang, 1989,

1990, 1993; Ang et al., 1990).

DEEP FAT FRYING OF POTATO STRIPS 881

Although the quality of fried products is of major importance in recentyears, limited information is available in the literature on the quality proper-ties of fried foods.

This review attempts to describe the effect of frying conditions andtype of pretreatment on some quality related properties of french fries.Frying kinetics, structural, optical and textural properties of french friesare examined. The following topics are discussed for each property:

– Definition.– Literature data.– Experimental procedure.– Effect of various factors.– Mathematical modeling.

2. FRYING KINETICS (MOISTURE AND OIL CONTENT)

2.1. Definitions

Two quantities may represent adequately the deep fat frying process:the moisture content, indicating the water loss from the potato strips duringfrying, and the oil content, indicating the amount of oil that the sampleuptakes during frying.

The moisture content (X) and the oil content (Y) of potato strips aftertime (t) of frying are defined as:

X ¼ mw=ms ð1Þ

Y ¼ mL=ms ð2Þ

where: mw is the mass of water remaining in the sample after time t offrying,ms is the dry mass of the sample after time t of frying,mL is the mass of oil contained in the sample after time t of frying.

A first order kinetic model was chosen to describe the mass trans-fer phenomena within the frying process. It is based on the followingassumptions:

1. The oil temperature is constant during frying.2. The initial water content of potato strips is constant.3. The two mass streams (water from the potato strips into the oil

and oil into the potato strips) were considered to be independentof each other.

882 KROKIDA ET AL.

Moisture transfer kinetics

dðXÞ=dt ¼ �KXðX�XeÞ ð3Þ

Oil transfer kinetics

dðYÞ=dt ¼ �KYðY�YeÞ ð4Þ

where: Xe is the moisture content at infinite process time (kg/kg db)KX is the rate constant of moisture loss (min �1)Ye is the oil content at infinite process time (kg/kg db)KY is the rate constant of oil uptake (min �1)

2.2. Literature Data

The effect of frying conditions on moisture loss and oil adsorptionkinetics has been examined by many researchers. Literature data that areavailable have been chosen to be presented for the needs of the presentchapter.

Pravisani & Calvelo (1986) studying the heat and mass transfermechanisms in potato strips proposed the existence of a moving boundarylayer that separates the core and crust which is maintained at 103�C.Gamble & Rice (1987, 1988) and Rice & Gamble (1989) noted that thefree water at the surface of potato chips evaporated rapidly, the surfacebecame dry and the inner moisture was converted to vapour, creating avapour gradient.

Several models have been developed to describe the moisturedesorption characteristics of biological products (Moreira and Bakker-Arkema, 1989). Askenazi et al. (1984) determined that the water diffusionduring frying of french fries was proportional to the square root of thefrying time. Gamble et al. (1987) used the same model to describe thedrying rate of potato chips in deep fat frying. Kozempel et al. (1991) usedFick’s law of diffusion to model moisture loss and zero order kinetics topredict oil adsorption during deep fat frying. Moreira et al. (1991) also usedthe diffusion model to predict moisture loss of tortilla chips during frying.

Although several researchers have described the moisture loss as adiffusion mechanism, it is still not clear how and when the oil is adsorbedby the product (Moreira & Chen, 1997). The distribution and the amount ofoil absorbed were affected by the pre-drying treatment, frying time, surfacetreatments, initial interfacial tension and crust size (Blumenthal, 1991;Farkas et al., 1991; Pinthus & Saguy, 1993; Pravisani & Galvero, 1986).Guillaumin (1988) reported that there was a linear relationship between the


thickness of potato chips and the amount of oil absorbed. Moreira et al.(1991) showed that most of the oil in tortilla chips was not uniformly dis-tributed and it concentrated around the edges and in chips puffed areas.Gamble et al. (1987) indicated that as the food material fries, the innermoisture is converted to steam causing a pressure gradient and as the surfacedries out, the oil adheres to the product surface and enters the surface atdamaged areas. They suggested that most of the oil enters the chips from theadhering oil being pulled into the chips when they are removed from thefryer due to concentration of steam. Matz (1993) commented that if potatochips are removed from the fryer while their temperature is still rising, only15% of the oil will be absorbed, the remainder will be held on the surface.He added that large part of this oil is drawn into the pores as the chipcools, and the rest runs off. McDonough et al. (1993) concluded that theoil diffused into tortilla chips through small channels formed as waterevaporated from the product. Pinthus & Saguy (1993) demonstrated thatinterfacial tension significantly affected oil uptake in deep fat frying ofpotato products, suggesting that the mechanism for oil adsorption is dueto capillary forces.

In recent years, much research has been concentrated on the develop-ment of food products that have reduced fat and cholesterol levels. The mostclearly defined factor influencing oil uptake during chips production is theinitial solids content of the tubers (Lulai & Orr, 1979). A tuber with highsolids content will yield a chip of low final oil content (Gamble & Rice,1987). The initial solids content can be artificially increased by pre-dryingthe potato slices prior to frying and Smith (1951) showed that hot airand infra red drying gave a lower oil content product. Gupta et al. (2000)studied the effect of pre-drying duration on the kinetics of moisture removaland oil uptake.

2.3. Experimental Procedure

The determination of frying kinetics is based on the moisture and fatcontent determination.

Moisture content is measured by drying the samples to constant weightin 30mbar vacuum oven at 70�C (Van Arsdel, 1964).

Fat content is determined by Soxhlet extraction (AOCS, 1993). Thedried samples are ground in a Waring blender and extracted with petroleumether (b.p. 40–60�C) for 4 hours. Petroleum ether is removed under vacuumat 90�C by a rotary evaporator. The recovered oil is left for 24 h in a vacuumoven at 70�C and weighted.

884 KROKIDA ET AL.

2.4. Factors Affecting Frying Kinetics

During frying, heat is transferred from the oil to the food, water isevaporated from the food and oil is absorbed in it. The main aspects, as faras oil and moisture contents are concerned, is to control the product oil andmoisture content in order to produce products of low fat level.

Factors that affect heat and mass transfer are the following:

. Frying conditions

– oil temperature– oil type (hydrogenated or non-hydrogenated oil)– sample thickness

. Type of pre-treatment

– air drying pretreatment (drying duration)– osmotic dehydration pretreatment (salt, sugar, maltodextrine

solutions)

2.4.1. Effect of Frying Conditions on Frying Kinetics

Both mass transfer phenomena (water loss and oil uptake) that takeplace during frying are affected by process parameters (oil temperature andsample size), while the oil type does not affect significantly mass transferduring frying, water loss and oil uptake phenomena are getting more intenseat higher temperatures and thinner sample. Figure 1 shows the variation ofoil and moisture content of french fries during deep fat frying and the effectof process variables on moisture loss and oil uptake.

Moisture Content Kinetics

The moisture content of potato strips decreases significantly duringfrying. The oil temperature has a negative effect on the moisture content offried potatoes. As the temperature of frying increases, the moisture contentfor the same frying time decreases. This difference gets bigger as the fryingproceeds, giving equilibrium moisture content values ranging from 0.3 to0.8 kg/kg db.

The size of potato strips also affects significantly the moisture contentof samples during frying. For the same frying time, the moisture content ofpotato strips is higher for thicker strips. This difference is larger for short


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Figure 1. Effect of process variables on moisture and oil content during frying.

frying times (1min), while it gets smaller as frying proceeds. The effect of oil

type on moisture content is negligible.

The effect of oil temperature and sample size on the equilibrium moist-

ure content and moisture loss rate is shown in Figure 2. Equilibrium

moisture content decreases as oil temperature increases. The effect of oil

temperature on equilibrium moisture content of potato strips is more pro-

nounced for thicker samples. The equilibrium moisture content values are

higher for thicker sample.

As far as rate of moisture loss is concerned, it increases with tempera-

ture increment. This is more intense for thinner samples. The rate constant is

also affected by the sample size. More specifically we notice a dramatic

decrement of rate constant as the sample size increases.


Figure 2. Effect of process variables on equilibrium moisture and oil content and

rate of moisture loss and oil uptake during frying.

Oil Content Kinetics

The oil content of potato strips increases significantly during frying.Oil temperature has a positive effect on the oil content of fried potatoes. Asthe temperature of frying increases the oil content for the same frying timeincreases. This difference gets bigger as frying proceeds, giving equilibriumoil content values, which range from 17 to 35%.

The size of potato strips also affects significantly the oil uptake of thesamples during frying. For the same frying time, the oil content of potatostrips is higher for smaller strips. The effect of oil type on oil content isnegligible.

The effect of oil temperature and sample size on equilibrium oil con-tent and rate of oil uptake is shown in Figure 2. Equilibrium oil contentincreases as oil temperature increases. The effect of oil temperature on equi-librium oil content of potato strips is more intense for lower values ofsample thickness. The equilibrium oil content values are higher for thinnersample. As far as rate of oil uptake is concerned, it decreases with tempera-ture increment and sample size decrement.

2.4.2. Effect of Pre-fry Drying on Frying Kinetics

Pre-fry drying changes the moisture content of the material to be friedand therefore affects mass transfer phenomena during frying. The rates ofboth mass transfer phenomena (water loss and oil uptake) that take placeduring frying of potato strips decrease due to the drying pretreatment beforefrying.

The drying curve is shown in Figure 3, in which the two processes arepresented as serial phenomena. The moisture content of the material followsthe drying curve until the start point of the frying curve. The initial moisturecontent of the frying process depends on the time of pre-fry drying. InFigure 3, it is obvious that the frying kinetic curves depend on the initialmaterial moisture content.

Water Loss and Oil Uptake

As shown in Figure 4, the pre-fry drying reduces the initial moisturecontent of french fries. Therefore, increased duration of pre-drying impliesthat less amount of free moisture is available for removal during frying andless amount of oil is absorbed. Thus the pre-fry drying decreases the oilcontent of potato strips during frying. As the pre-fry drying time increases,

888 KROKIDA ET AL.

the oil content for the same frying time decreases, giving equilibrium oilcontent values ranging from 0.1 to 0.2 kg/kg db.

The moisture loss rate, being dependent to free moisture content,decreases as pre-fry drying duration increases (Figure 5). The rate of oiluptake decreases, too. The equilibrium moisture content and oil contentdecrease as drying time increases.

2.4.3. Effect of Osmotic Pretreatment on Frying Kinetics

Both mass transfer phenomena (water loss and oil uptake) that takeplace during the frying of potato strips get less intense due to the osmoticpre-treatment before frying.

Water Loss and Oil Uptake

The osmotic pretreatment decreases the initial moisture content ofpotato strips, which is further decreased during frying, as shown in Figure 6.The moisture content for the same frying time is lower for osmotically


Figure 3. Moisture content kinetics during drying and frying procedures.

treated samples than for untreated ones, and the equilibrium moisture con-tent values range from 0.3 to 0.1 kg/kg db depending on the type of solution.The lowest moisture content values are given by the pre-treatment withsugar solution, followed by NaCl solution and maltodextrine-21 solution,while the moisture content of samples treated with maltodextrine-12 solu-tion are closest to those of untreated samples.

Oil content experimental data are also shown in Figure 6. Osmoticpretreatment decreases the oil content of potato strips during frying invarying proportions, depending on the type of solution used. The equili-brium oil content of fried potatoes is reduced, too. The lowest values areobserved for samples pretreated with sugar solution (60% reduction of oilcontent), while NaCl solution, maltodextrine 21 and 12 solutions give higherequilibrium oil content values (35%, 20% and 15% reduction, respectively).

2.5. Mathematical Modeling

A first order kinetic model was chosen to describe the mass transferphenomena within the frying process (Krokida et al. 2000a). The proposedmathematical model is summarized in Table 1. Thismodel gives an acceptable

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Figure 4. Effect of pre-fry drying on moisture loss and oil uptake of french fries.

accuracy (9–13%) between calculated and experimental values for all theexamined cases.

3. STRUCTURAL PROPERTIES

(DENSITY AND POROSITY)

3.1. Definitions

Structural properties are important for the characterization of thequality of a fried product. Food structure has a pronounced effect on thetransport properties of foods (e.g. diffusivity, permeability and thermal con-ductivity), it is therefore important to know the physical structure of a foodmaterial. Food structure is of fundamental importance in the developingfield of Food Materials Science. The structure of a food material may becharacterized by its apparent density, solids density, bulk porosity, pore sizedistribution, specific volume, etc.

– Apparent density (rb) concerns powdered and porous materials andit is defined as themass of the sample divided by its apparent volume.


Figure 5. Equilibrium content and rate constant values versus pre-fry drying times.

The terms bulk density and bulk volume are also used for granularmaterials.

– True density (rp) is the density excluding all pores and it is definedas the mass of sample divided by its true volume. The terms particle

density and particle volume are used for granular materials.– Porosity (e) characterizes the overall open structure of a material.

It is the fraction of the empty volume (void fraction) and it isusually estimated from the apparent density and the true densityof the material according to the following equation:

" ¼ 1� rb=rp ð5Þ

– Specific volume (m) is defined by the mass of the dry solids and itsapparent volume.

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Figure 6. Effect of osmotic pretreatment on oil and moisture contents of frenchfries.


Table 1. Mathematical Model for Frying Kinetics

1. Factors

X – moisture content (kg/kg db); Y – oil content (kg/kg db)

2. Frying kinetics

ðX�XeÞ ¼ ðXo �XeÞ expð�KXtÞ; Y ¼ Ye½1� expð�KYtÞ�

3. Parameters

Xe – equilibrium moisture content (kg/kg db)KX – rate constant of moisture loss (min�1)Ye – equilibrium oil content at infinite process time (kg/kg db)

KY – rate constant of oil uptake (min�1)

4. Factors affecting the parameters

. Frying conditions

– oil temperature (T, �C)– oil type (C, % proportion of hydrogenated in refined oil)

– sample size (d, mm)

KX ¼ 0:78T

170

� �1:61 d

10

� ��2:27

Xe ¼ 0:54T

170

� ��3:63 d

10

� �0:89

KY ¼ 0:45T

170

� ��1:7d

10

� �1:73

Ye ¼ 0:26T

170

� �2:35d

10

� ��2:25

. Drying pretreatment

– drying duration (tD min)

Xe ¼ 0:21tD40

� ��0:7

KX ¼ 0:3tD40

� ��0:2

Ye ¼ 0:16tD40

� ��0:3

KY ¼ 0:33tD40

� ��0:05

. Osmotic pretreatment

Type of solution X0 Xe KX Ye KY

No pretreatment 3.9 0.68 0.42 0.25 0.43

Sucrose 1.2 0.05 0.27 0.09 0.83NaCl 1.9 0.19 0.34 0.17 0.55Maltodextrine-12 3.3 0.62 0.52 0.16 0.52Maltodextrine-21 2.8 0.45 0.51 0.20 0.51


During the frying process, the physical, chemical and sensorial char-acteristics of a food are modified. The quality of fried potatoes dependsmainly on their structural, textural and optical properties (Moreira et al.,1995; Pinthus et al., 1995; Farkas et al., 1991; Farkas et al., 1992). Mostimportant structural properties such as apparent and true density, porosityand specific volume of fried potatoes change during frying.

However few data are available on the effect of process variables onphysical properties of the fried potatoes. Porosity is the most commonlyreported in literature structural property. Bulk porosity of french fries hasbeen usually estimated during and after frying. Porosity is strongly affected bymoisture and oil content of the fried product, frying conditions andmethod ofpretreatment. Oil and water content changes with frying time were relatedto pore size and distribution in the sample (Du Pont et al., 1992). Porosityof restructured potato products was evaluated to elucidate its effect on oilabsorption during deep fat frying. Porosity increases significantly duringthe frying process (Saguy and Pinthus, 1994; Pinthus et al., 1995).

Crust is formed during most deep fat frying processes and is one of themost pleasant characteristics of the fried foods (Keller et al., 1986). Crustthickness of the par-fried frozen french fries increased with frying time up to4min (Du Pont et al., 1992). Crust development influences heat and masstransfer phenomena, oil uptake and physical properties of fried products.Several studies have shown that oil uptake during deep fat frying of food islocalized at the crust (Farkas et al., 1992; Gamble and Rice, 1987; Varela,1977). Oil introduced during frying was studied with an oil soluble dyeand showed that the oil layer was approximately 1mm in depth (Farkaset al., 1991).


The determination of structural properties of a material is based onthe mass, apparent volume and true volume determination. Mass is deter-mined by weighing while there are several methods for apparent volume andtrue volume measurement.

– Apparent volume Several methods have been used in order to deter-mine the apparent volume of a solid material. The most commonlyused can be summarized as follows:

– volumetric displacement methods: the apparent volume isdetermined by placing the sample in a container of known

894 KROKIDA ET AL.

liquid volume and measuring the volume displacement(Krokida and Maroulis, 1997).

– dimension methods: the apparent volume is determined byaveraging a number of dimension measurements with micro-meters, assuming spherical or slab shapes (Lozano et al.,1983; Ratti, 1994; Karathanos and Saravacos, 1993).

– stereopycnometer methods: the sample is covered with siliconegrease in order to make it impervious to gases and its appar-ent volume is measured by a stereopycnometer (Bonazzi et al.,1992).

– True volume is usually measured by means of a gas (helium) stereo-pycnometer, which measures the true volume, excluding the inter-particle volume (Mohsenin, 1986; Donsi et al., 1996).

3.4. Factors Affecting Structural Properties

During frying, significant changes in structural properties can beobserved as water is removed from the moist material and oil incomes.The main aspect, as far as structural properties are concerned, is to controlthe product apparent density and porosity and to yield products of differentphysical structures for various uses, choosing appropriate frying conditions.

Changes of structural properties that take place during frying dependon various factors, which specify the structural properties of the fried mate-rial. The factors analytically examined in the following paragraphs are:

. frying conditions

– oil temperature– oil type– sample thickness

. type of pretreatment

– air drying pretreatment (drying duration)– osmotic dehydration pretreatment (salt, sugar, maltodextrine

solutions)

3.4.1. Effect of Frying Conditions on Structural Properties

All the examined structural properties are greatly affected by allprocess variables (oil temperature, sample thickness and oil type).Diffusion of water molecules and oil uptake during frying form cracks to


the solid structure, causing structural damage and significant changes to allstructural properties. Figures 7, 8 and 9 present the variations of true den-sity, apparent density, porosity and specific volume of french fries asaffected by oil temperature, sample size and oil type, respectively.

True density is strongly affected by all process variables. More speci-fically true density of french fries increases during the frying procedure.Mass transfer phenomena that take place during frying affect significantlytrue density. Water loss tends to increase true density while oil gain tends todecrease it. Oil temperature affects significantly true density; as oil tempera-ture increases true density of fried potato decreases. This can be explainedby the higher oil content and lower water content of potato strips as

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Figure 7. Effect of oil temperature on structural properties of french fries.

temperature increases. The opposite result can be noticed for sample thick-ness variable. Increasing sample thickness results in decreasing oil contentand increasing water content for the same frying time. Thus truedensity increases as sample thickness increases. Oil type also affects truedensity values, even though it does not affect mass transfer phenomena.That is because frying takes place at high temperature values, where theproperties, like viscosity, that affect mass transfer phenomena do not differappreciably among the different types of oil, while true density measure-ments take place at room temperature where different types of oil have


Figure 8. Effect of sample size on structural properties of french fries.

different density values. More specifically hydrogenated oil has higher den-sity than refined oil at room temperature, which results in higher true den-sity values as concentration of hydrogenated oil increases.

Apparent density is also greatly affected by process variables duringfrying. Apparent density decreases during frying, which is due to water

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Figure 9. Effect of oil type on structural properties of french fries.

vaporization, air pores development and oil uptake. This means a minimizedshrinkage phenomenon during the frying process. Oil temperature has anegative effect on apparent density, which means that as oil temperatureincreases, mass transfer phenomena gets more intense and apparent densitydecreases. Sample thickness has an opposite effect on apparent density, thusas sample thickness increases, apparent density increases. Oil type alsoaffects apparent density, which is higher for 100% hydrogenated oil.

Porosity and specific volume. It is clear that as temperature increasestotal porosity increases while specific volume decreases, which means thatshrinkage phenomenon gets weaker. As sample thickness decreases porosityincreases and specific volume decreases. Oil type has a smaller effect onporosity and specific volume. Even though it is noticeable that the increaseof hydrogenated oil concentration results in lower porosity values andhigher specific volume values.

3.4.2. Effect of Pre-fry Drying on Structural Properties

The structural properties are greatly affected by the drying pretreat-ment. Figure 10 presents the effect of drying pretreatment duration on thestructural properties of french fries.

True density. It is evident that true density is strongly affected by thepre-fry drying time. In particular, the initial true density of potato stripsincreases due to the drying process. It increases further during frying, due tomass transfer phenomena that take place; water loss tends to increase thetrue density while oil gain tends to decrease it, as already mentioned. Truedensity final values after frying range between 1.2 and 1.5 kg/L.

Apparent density. The apparent density is also strongly affected by thepre-fry drying time. The initial apparent density of potato strips increasesdue to the drying process. It decreases during frying due to the mass transferphenomena that take place; water loss tends to increase the apparent densitywhile the oil gain tends to decrease it. Drying time before frying affectssignificantly the apparent density; as the drying time increases apparentdensity of fried potatoes decreases.

Porosity and specific volume. It is clear that as the drying timeincreases, the total porosity increases while the specific volume decreases,which means that the shrinkage phenomenon is more pronounced (lowerinitial specific volume). The shrinkage phenomenon, which takes placeduring the drying pretreatment, decreases the proportion of open pores,which prevents oil income and consequently the oil content of fried potatoesgets lower. The only structural parameter that is affected by pre-fry drying


pretreatment is the shrinkage coefficient (b’), which is related to the dryingtime through an exponential relation (see Table 2).

3.4.3. Effect of Osmotic Pretreatment on Structural Properties

The structural properties of french fries are also affected by osmoticpretreatment. The effect of the type of solution used for osmotic dehydra-tion on structural properties of French fries is presented in Figure 11.

True density. True density is strongly affected by osmotic pretreat-ment. In particular, the initial true density of potato strips increases dueto osmotic dehydration. It also increases during frying due to the masstransfer phenomena that take place. Samples pretreated with sugar solutionhave the highest true density. True density final values after frying rangebetween 1.2 and 1.5 kg/L.

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Figure 10. Effect of pre-fry drying duration on structural properties of french fries.


Table 2. Mathematical Model for Structural Properties

1. Structural properties

rp True density rp ¼1þX

1

rsþ

X

rw

ðkg=LÞ

rb Apparent density rb ¼1þXþY

1

rboþ b0

X

rwþ

Y

rL

� � ðkg=LÞ

e Porosity e ¼ 1�rbrp

ð�Þ

n Specific volume1

rboþ b0

X

rwþ

Y

rL

� �L=kg dbð Þ

2. Factors

X – moisture content (kg/kg db); Y – oil content (kg/kg db)

3. Parameters

rw – enclosed oil density (kg/L)rL – enclosed oil density (kg/L)rL1 – enclosed refined oil density (kg/L)rL2 – enclosed hydrogenated oil density (kg/L)rs – dry solid true density (kg/L)rb0 – dry solid apparent density (kg/L)b0 – shrinkage coefficient (�)


. Frying conditions

– oil temperature (T, �C)– oil type (C, % proportion of hydrogenated in refined oil)– sample thickness (d, mm)

. Pretreatment

– effect of air drying duration (tD, min)

b0 ¼ b00tD40

� �nb

b00 ¼ 0:6 nb ¼ 0:3

– effect of osmotic pretreatment

Type of solution rbo

No pretreatment 0.5Sucrose 1.0

NaCl 0.5Maltodextrine-12 0.3Maltodextrine-21 0.4

rw 1.09

rL1 0.88rL2 0.96rs 1.8

rb0 0.5b0 0.7

Apparent density. It is evident that apparent density is strongly affectedby the osmotic pretreatment, too. The initial apparent density of potatostrips increases due to the osmotic process. Continuously, it decreases duringfrying due to the mass transfer phenomena that take place. Samples pre-treated with sugar solution have higher final apparent density than theuntreated samples, while the apparent density of samples pretreated withmaltodextrine or NaCl solutions is lower.

Porosity and specific volume. The osmotic pretreatment increases thetotal porosity, for all types of solution with the exception of the sugarsolution, which decreases the total porosity due to the high proportionof solids gain. The specific volume of osmotically pretreated sampleswith sugar or NaCl solution increases in comparison to that of untreated

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Figure 11. Effect of osmotic pretreatment on structural properties of french fries.

samples, while the specific volume of samples pretreated with maltodextrinesolutions decreases.


Many attempts have been made in the literature for the developmentof models for the structural properties. The main purpose of these models isto describe the relation of porosity development with moisture and oil con-tent. Most of these approaches require knowledge of transport propertiesand are rather complicated.

A mathematical model, which predicts the porosity developmentduring frying and correlates it with the material moisture and oil content,would be useful. It should be simple, generalized and contain parameterswith physical meaning.

A simple mathematical model is presented here to correlate true den-sity, apparent density, porosity and specific volume with the material moist-ure and oil content (Krokida et al., 2000b). The proposed model issummarized in Table 2.

This model involves five parameters with physical meaning: the densityof enclosed water (rw), the density of enclosed oil (rL) the dry solids density(rs) the dry solids bulk density (rb0) and the shrinkage coefficient (b0). Truedensity (rp) is presented as a function of moisture content (X), oil content(Y) considering an additive mixing model of a three phase system: the drysolid with density (rs) the water with density (rw) and the oil with density(rL).

Similarly, for the expression of apparent density, a mixing model of athree phase system similar to the previous one can be considered: a dry solidphase having apparent density (rb0) the water phase having density (rw) andthe oil phase having density (rL). This is the additive model when b0¼1, inTable 2.

The total porosity is a function of apparent density and true density.The definition of specific volume involves four parameters: the apparentdensity of dry solids (rb0), the enclosed water density (rw), the oil density(rL) and the shrinkage coefficient (b0) (Equation 4).

The effect of factors such as frying conditions and type of pretreatmenton the examined structural properties is taken into account through theireffect on the five parameters incorporated in the structural model proposed(rs, rw, rL, b

0, rb0) and on moisture and oil content, as expressed throughthe equations in Table 1. The accuracy of the proposed model to the experi-mental data is about 10%.


4. OPTICAL PROPERTIES (COLOR)

4.1. Definition

Color can be defined as the sensation experienced by an individual

when energy in the form of radiation within the visible spectrum fallsupon the retina of the eye. That color is a sensation dependent upon whata person sees can not be overemphasized. Also there are several factors,which influence the radiation and, subsequently, the exact color or sensa-tion, which the individual perceives. These factors include:

– The spectral energy distribution of the light.– The conditions under which the color is being viewed.– The spectral characteristics of the object, with respect to absorp-

tion, reflection, and transmission.– The sensitivity of the eye.

Thus, before measuring the color of any object, it is necessary to

understand the physical, physiological and psychological aspects of the sti-mulation, which ultimately result in the visual perception of the color of anobject.

For the determination of color of solids the CIE system (InternationalCommission on Illumination) has been developed. The CIE has adoptedmethods for the measurement and specification of color which include:

– The use of standard light sources as prescribed the CIE definition.– Exact conditions for the observation or measurement of sample

color.– The use of appropriate mathematical units in which color of an

object can be expressed.– Definition of a ‘‘standard observer’’ curves or tables relating objec-

tive measurement to visual response, and thus, measuring what theeye sees.

The color of the material changes during frying not only due to eva-poration of the surface water and oil uptake but also due to certain

reactions, such as enzymatic browning, non-enzymatic browning and car-amelization reactions (Kudra and Strumillo, 1998). These reactions may beundesirable for many products, thus the regulation of color during frying isthe subject of various procedures, such as temperature deviation of specifiedranges, intermittent drying, use of color protective agents, (e.g., sulfur

dioxide) etc.

904 KROKIDA ET AL.


The color of the fried products, including fried potatoes and potatochips, is one of the most significant quality factors of acceptance. In parti-cular, there are specific values of lightness for chips, which have been estab-lished in the food industry. Heat and mass transfer phenomena that takeplace during frying cause physicochemical changes, which affect the color ofthe products. Process variables such as oil temperature; oil type and sampledimensions are expected to affect the color of the fried products.

It is almost impossible for the food industry to make light coloredchips acceptable to the trade without some treatment of the sliced potatoesin the chip plant. A number of procedures have been found to have somemerit in producing lighter color chips. Many chemical methods such as theuse of sulfites have been used as anti-browning agents in such products.However the safety of sulfiting agents in foods has recently been questionedbecause of their role in the initiation of asthmatic reaction in sensitiveindividuals (Sullivan and Smith, 1985; Taylor et al., 1986). This creates apractical necessity for new approaches meant to prevent undesirable brown-ing in foods.

The color of french fries and potato chips has been related to thereducing sugar content of the potatoes. It has been recognized that thereducing sugar content increases with storage time (Watada and Kunkel,1954; Hyde and Morrison, 1964). Accumulation of reducing sugars intuber results in excessive browning of french fries and potato chips. Tolower the reducing sugars before frying, tubers may be held or reconditionedat about 21�C for 1–3 weeks (Heinze et al., 1955; Kilpatrick et al., 1956)or blanched to leach out soluble sugars (Brown and Morales, 1970).Immersion of potato strips in liquid nitrogen or dichlorodifluoromethanebefore blanching has been shown to be effective in reducing browning ofthe fried products (Schwimmer et al., 1954; Miller et al., 1975). The reasonsfor such variations have been examined by Hautala et al. (1972). The pro-cessing steps taken by manufacturers vary from one plant to another(Salunkhe et al., 1991).

Although, the investigation on color properties of fried potatoes hasstarted many years ago, it continuous with increasing interests in recentyears. Toma et al., (1986) studied the effect of surface freezing pre-treatmenton color changes during deep fat frying. They reported that surface freezingtreatment is an effective means of decreasing oil adsorption and improvingthe color of french fries. Smith (1975) treated potato slices in a solution ofglucose-oxidase, an enzyme that transforms glucose, one of the reducingsugars responsible for color development in chips, to gluconic acid, whichdoes not enter into the browning reactions. The results indicated that it


might be possible to use this treatment in order to reduce the production of

dark color chips. Jiang and Ooraikul (1989) also examined the reduction of

nonenzymatic browning in potato chips and french fries with glucose oxi-

dase. They reported that enzyme treatment results in a lighter and more

uniform color of french fries. Economically, however, this method would

not be justified.

Paul and Mittal (1996) examined how the degradation of oil during

frying of canola affected the color of the fried product. They noted a high

correlation of the color parameters with oil degradation during frying.

Kozempel et al. (1991) developed a simulator for food processes such as

blanching, drying and frying of potatoes. The model was also used to con-

trol the color of fried potatoes. Khalil (1999), examined the quality of french

fried potatoes as influenced by the coating with hydrocolloids. He noted that

coated french fries exhibited higher red and yellow colors.


There are certain methodologies for analyzing the color. The types of

colorimeters that have been employed widely in food applications in

America, to date, are the Hunterlab instruments, the Gardner series, the

Color-Eye, the Colormaster and the Tintometer.

The most common color measurement units are the RGB (Red,

Green, Blue), Lab (L: Lightness, a: Redness-greenness, b: Yellowness-blue-

ness) and XYZ scales that analyze the color into these parameters, so that

each composite color can be easily quantified by a set of three numbers.

Conversion of data from one type of instrument to another is usually

via the CIE XYZ system, and equations are provided with instructions from

each manufacturer. Clydesdale and Podlesney (1968) published a computer

program for these interconversions. The Agtron has also been used widely in

food applications, but there are no satisfactory methods to convert Agtron

data to XYZ.

The measurement of color is done through colorimetric techniques by

analysis of the spectrum of light produced by an instrument and reflected on

the product’s surface. The color may be measured on line and lead to

mechanical sorting of the product, based on color evaluation. Other tech-

niques involved in the browning of foods make use of spectrophotometers to

analyze the extend of browning. The browning may be a very important

quality parameter, since apart from the desirable or undesirable appearance

of food; it may also lead to significant nutrient losses.

906 KROKIDA ET AL.

4.4. Factors Affecting Optical Properties

Color changes, measured by tristimulus reflectance colorimetry, arerelated to browning reactions that take place during frying of french fries.The kinetics of browning reactions defines color changes during frying. Thefactors that affect color kinetics include frying conditions, i.e. oil tempera-ture, type of oil used as frying medium, sample thickness, as well as typeof pretreatment. These factors are analytically examined in the followingparagraphs.

4.4.1. Effect of Frying Conditions on Optical Properties

The frying conditions affect significantly the color of fried products.The experimental and calculated values of lightness (L), redness (a) andyellowness (b) of fried potatoes as affected by frying conditions are shownin Figures 12 through 14.

The lightness of potato strips increases during the early stages offrying, while it remains almost constant afterwards. Oil temperature has anegative effect on the lightness of fried potatoes. As the temperature offrying increases, lightness – for the same frying time – decreases, givingequilibrium lightness values ranging from 74 to 78. The size of potatostrips also affects significantly the lightness of the samples during frying.The lightness of potato strips is lower for smaller thickness values and forthe same frying time. The effect of oil composition on lightness is negligible.

Considering that the production of lighter colored fried potatoes is thepurpose, it is obvious that lower temperatures and higher sample thicknessare indicated.

The parameter ‘‘a’’ is also affected by the process variables. In generalthe ‘‘a’’ parameter increment is not desired because it means more redcoloring, which is not acceptable for fried potatoes. As the temperature offrying increases, the ‘‘a’’ parameter increases for the same frying time, whichis negative for color of the product. The size of potato strips also affects the‘‘a’’ parameter of samples during frying. The ‘‘a’’ parameter increases withsample thickness decrement, for the same frying time. The effect of oil typeon the ‘‘a’’ parameter is negligible.

The positive values of parameter ‘‘b’’ express the intense of yellowness,which is desirable for french fries. All the process variables affect the ‘‘b’’parameter with the same mode that affects the ‘‘a’’ parameter. The onlydifferences that could be noticed are that the only remarkable changeof the ‘‘a’’ parameter is observed with temperature higher than 170�C,


908 KROKIDA ET AL.

Figure 12. Effect of frying conditions on lightness of french fries.


Figure 13. Effect of frying conditions on ‘‘a’’ parameter of french fries.

910 KROKIDA ET AL.

Figure 14. Effect of frying conditions on ‘‘b’’ parameter of french fries.

while the ‘‘b’’ parameter increases constantly with temperature incrementand sample thickness decrement.

In concluding, it could be mentioned that lower oil temperatures, up to170�C, give lighter (less red) and more yellow colored products, which aremore acceptable, while the effect of replacement of refined cottonseed oil byhydrogenated oil is negligible. Thickness of french fries should also be con-sidered as lower thickness resulted in lower lightness and higher yellow colorof the product, while red color development was intense only in case ofhigher temperature.

4.4.2. Effect of Pre-fry Drying on Optical Properties

As seen from Figure 15, lightness of potato strips decreases signifi-cantly due to the pre drying process. It increases during the frying process,reaching values, which range between 40 and 50. The darkening that takesplace during drying decreases the initial lightness values of potato strips. Asthe drying time gets higher, darkening is more pronounced, which is unde-sirable for the color of fried potatoes.

Parameter ‘‘a’’ of potato strips increases significantly due to browningreactions that take place during the drying process. Also, it increases duringfrying process reaching values, which range between 0 and 5. The ‘‘a’’parameter increases with drying time. It can be concluded that the dryingpre-treatment has a negative effect on color of fried potatoes, increasingredness.

Parameter ‘‘b’’ decreases progressively during the drying process. Itcan be concluded that the drying pre-treatment has a negative effect on colorof fried potatoes, decreasing yellowness.

4.4.3. Effect of Osmotic Pretreatment on Optical Properties

Lightness of potato strips decreases significantly due to the osmoticpretreatment process (Figure 16). Although the lightness increases at theearly stages of the frying procedure, this does not happen during frying ofthe osmotic pretreated potatoes. In particular, the lightness of samples pre-treated with NaCl solution is closest to that of fresh samples and remainsconstant during frying. The lightness of samples prepared with maltodex-trine-12, maltodextrine-21 and sugar solutions decreases during frying. Thesamples prepared with sugar solutions showed the lowest lightness values.Osmotic pre-treatment causes darkening, which is more intense for sugarsolutions, probably due to non-enzymatic browning.


912 KROKIDA ET AL.

Figure 15. Effect of pre-fry drying on color parameters of french fries.

Parameter ‘‘a’’ of potato strips increases significantly due to browning

reactions that take place during the osmotic pretreatment process. It

increases during the frying process, reaching values, which range between

4 and 9. The maltodextrine solution gives the highest ‘‘a’’ values, followed

by the sugar solution. As expected, the salt solution causes the smallest ‘‘a’’

increment since salt does not participate in browning reactions.

Parameter ‘‘b’’ – from Figure 16 it is clear that the salt solution gives

the highest ‘‘b’’ values, followed by maltodextrine 12 and 21 solutions, while

the sugar solution gives the lowest ‘‘b’’ values. An overall consideration

of the effect of osmotic solutions on colour parameter shows that salt

dehydration results in the most acceptable, light coloured, yellow products.


Figure 16. Effect of osmotic pretreatment on color parameters of french fries.


Color of fried products is correlated with the frying time, consideringthat color change is caused by various reactions that take place duringfrying and is not related to the material moisture or oil content. In orderto determine the rate of color changes during frying, kinetics of the Hunterparameters, redness (a), yellowness (b) and lightness (L) were investigated,assuming that each parameter followed first order kinetics (Krokida et al.,2000e). The effect of drying pretreatment was also investigated. The math-ematical model is presented in Table 3. This model gives an acceptableaccuracy (12%) between calculated and experimental values for all theexamined cases.

5. TEXTURAL PROPERTIES

(COMPRESSION ANALYSIS)

5.1. Definitions

Texture is one of the most important parameters connected toproduct quality. Textural or rheological properties may be defined asthose having to do with the behavior of the material under appliedforces. Following this broad definition, such properties as stress – strainbehavior of a material under static and dynamic loading as well as flowcharacteristics of a material can be classified as textural or rheologicalproperties.

The viscoelasticity is strongly related to complex quality characteris-tics perceived by people as mouth feeling. Measurement techniques includethe small amplitude oscillatory compression tests, stress relaxation tests,creep tests and other dynamic mechanical analysis tests.


Textural quality is an important attribute for the acceptability offrench fries. It depends on both raw material and process conditions. Theinteraction of raw material properties and the frying process is poorlyunderstood and has to be assessed routinely in industrial practice.

Textural behaviour is related to the structure of foods (Ramana andTaylor, 1994). Textural properties depend on chemical and physical charac-teristics of the products (Mohsenin, 1986; Bourne, 1992; Thiagu et al., 1993).

914 KROKIDA ET AL.


Table 3. Mathematical Model for Color

1. Color parameters

Lightness (L); Red-Green (a); Yellow-blue (b)

2. Color KineticsC� Ce

Co � Ce

¼ exp �Kctð Þ

3. Parameters

Co – initial value; Ce – equilibrium value; KC – rate constant (min�1)


. Frying conditions

– oil temperature (T, �C)– oil type (C, % proportion of hydrogenated in refined oil)– sample thickness (d, mm)

KL ¼ 0:66T

170

� �1:96 d

10

� ��0:1

Le ¼ 75:1T

170

� ��0:21 d

10

� �0:05

Ka ¼ 0:02T

170

� ��7:4 d

10

� �0:21ae ¼ 11:6

T

170

� �2:0 d

10

� ��0:7

Kb ¼ 0:12T

170

� �2:49 d

10

� ��0:44

be ¼ 36:2T

170

� �1:012 d

10

� ��0:2

. Method of pre-treatment

– air drying duration (tD, min)

Lightness: KL ¼ 1:83tD40

� �0:27

Le ¼ 5:6tD40

� ��0:2

Lo ¼ 43tD40

� ��0:1

Parameter ‘‘a’’: Ka ¼ 0:12tD40

� �0:49

ae ¼ 9:2tD40

� �0:02

ao ¼ 2:7tD40

� ��0:4

Parameter ‘‘b’’: Kb ¼ 0:95tD40

� ��0:4

be ¼ 9:3tD40

� ��0:7

bo ¼ 6:7tD40

� �0:4

– osmotic pretreatment

Type of solution Lo Le KL ao ae Ka bo be Kb

No preteatment 62.3 71.2 1.56 �6.6 7.2 1.56 22.6 7.27 1.56

Sucrose 32.4 336 0.00 �1.3 6.7 0.29 2.45 5.60 0.64NaCl 53.1 61.8 0.00 �1.7 5.7 0.21 1.63 19.9 0.39Maltodextrine-12 48.8 67.9 �0.04 �1.0 9.6 0.25 2.46 17.6 0.49

Maltodextrine-21 44.9 287 �0.01 �0.1 10.4 0.26 2.87 13.5 0.5

The rheological behavior of fried products has been studied through both

compression and relaxation stresses (Bagley, 1987; Mohan Rao, 1984; Katz

and Labuza, 1981). Both tests examine the viscoelastic nature of materials,

involving parameters of elasticity such as the elastic modulus (E).

Compression parameters, such as maximum stress and corresponding

strain, are usually investigated by various researchers.

The most important textural attribute of chips and french fries

is crispness. It denotes freshness and high quality. A crisp food should be

firm and snap easily when deformed, emitting a crunchy sound (Vickers and

Christensen, 1980). Early investigators considered crispness to be an audi-

tory sensation that was related to the sound emitted during mastication

(Vickers and Bourne, 1976; Vickers and Christensen, 1980; Vickers and

Wasserman, 1980; Mohamed et al., 1982). Mechanical tests such as com-

pression tests have been used to correlate crispness to a physical parameter

in a force deformation curve (Seymour and Hamman 1987; Bourne et al.,

1987). Usually it is related to the ratio of maximum stress to maximum

strain (Jackson et al., 1996). Difficulties in establishing a relationship

between instrumental measurements and sensorial quality may be caused

by the nature of the mechanical tests. Mechanical properties depend on both

strain and rate, but frequently a single strain measurement is made (Bourne,

1967; De Man, 1969).

Jaswal (1989) noted that high specific gravity potatoes produce french

fries of desirable textural qualities which are crisp, mealy, and firm. In

contrast, low specific gravity fries are not crispy and have undesirable

appearance. The relation of tuber composition to french fry texture has

received a great deal of attention. In general, it has been found that meali-

ness and firmness are positively correlated with high dry matter, starch

content, size of starch granules, specific gravity and alcohol-insoluble

solids (Johnston et al., 1970; Sayre et al., 1975; Smith, 1951). Cell size,

cell wall polysaccharides (Nonaka, 1980; Nonaka and Timm, 1983), non

starch polysaccharides (Jaswal, 1989) and pectic substances have also been

reported to be texture governing factors. Varietal characteristics, growing

location, cultural conditions, storage temperature and methods of cooking

all affect the aforementioned factors and therefore have a bearing on the

texture properties (Bushway et al., 1984; Nelson and Sowokinos, 1983;

Sowokinos et al., 1987; True et al., 1983). The interrelationship of chemical

composition of potatoes to their textural quality has been examined by

Warren and Woodman (1974).

Bushway et al. (1984) compared the texture of microwave pretreated

fries with conventional french fries. They noted an improvement of their

texture caused by microwave pretreatment.

916 KROKIDA ET AL.


The compression test is one of the most common techniques for theestimation of the texture. The simplest approach is to measure the maximumapplied force or stress at fracture of the material and the correspondingstrain, which is called maximum strain. Maximum force or stress is corre-lated with hardness or firmness of the material, while the ratio of maximumstress to maximum strain is correlated to crispness. The determination ofother textural properties such as cohesiveness and chewiness, has been madeby the Texture Profile Analysis methodology.

The compression tests are performed by applying the constant defor-mation rate, and recording force and deformation. Stress-strain compres-sion curves are then constructed. The compression test is usually continueduntil the fracture of specimens.

5.4. Mathematical Model for a Compression Test

Solid and semi-solid foods and agricultural products behave usually asviscoelastic materials, with elastic and viscous components (Mohsenin,1986; Peleg, 1979). The viscoelastic behavior can be determined by compres-sion or tension tests. Mohsenin (1986) suggested that the compressioncharacteristics of a material could be determined from constant deformationrate tests. When agricultural materials and food products are subjected tocompression (or tension) tests, a fixed deformation rate (compression ortension) is applied on a sample until there is a fracture of the sample. Theforce (or stress) is measured as a function of deformation (strain), obtainingthe stress-strain curve, which provides useful information on the viscoelasticproperties of the material.

A mathematical model to describe the non-linear elastic behaviour ofvarious materials has been proposed by Foutz et al. (1993):

� ¼ E"þ d"p

where �¼ stress (kPa)"¼ strain (�l/lo)E¼ elastic parameter (kPa)d¼ viscoelastic parameter (kPa)p¼ viscoelastic exponent

For small deformations (strains) many foods may be assumed tobehave as linear elastic materials. Thus, the first linear part of thestress-strain curve is described by the elastic parameter (E). After that,


the materials seem to follow viscoelastic behavior, which is described by thesecond viscoelastic term of the above equation. The proposed equation doesnot specify the break point of the stress-strain curve, which corresponds tothe maximum stress and maximum strain observed. Thus, it would be usefulif the above equation included parameters such as the maximum stress andmaximum strain, which define strength of the materials against fracture andcompressibility. The mathematical model, which is presented in Table 4, todescribe compression behavior, involves four parameters: the maximumstress (�max), the corresponding strain ("max), the elastic parameter (E)and the viscoelastic exponent (p). The maximum stress and strain representthe break point of the compression test, so they have major importance forthe description of the rheological behavior of the materials. The elasticparameter (E) represents the linear part of the stress-strain curve andshows the elastic nature of the material. The viscoelastic exponent (p) repre-sents the exponential part of the curve. The greater the viscoelastic exponentis, the more the behavior of the product deviates from linearity and becomesmore viscous.

All the above parameters are correlated with the oil content of frenchfries (Krokida et al., 2000f). The maximum stress and strain were experi-mentally measured for various oil contents. They were found to depend onfrying conditions as well as on type of pre-treatment as presented in Table 4.This model gives an acceptable accuracy (11%) between calculated andexperimental values for all the examined cases.

5.5. Effect of Frying Conditions on Compression Behavior

Some typical stress-strain curves obtained from compression tests onmaterial fried at 170�C are presented in Figure 17. Each curve is represen-tative at a specific frying time. Similar curves are extracted for other processconditions. The stress-strain curves indicate that as frying proceeds the samedeformation is accomplished by a considerably lower stress value, showingthat the food becomes less firm but on the other hand a steeper increase instress is noticed towards the end of the compression cycle, indicative of thehigher crispness of the potato. The material shows elastic behavior only forvery small deformations. For larger strains, the stress increased not linearlyuntil the end point, indicating the viscous nature of the material. The cal-culated stress-strain curve was resulted by the stress-strain equation. Thestress-strain equation contains the maximum stress and the maximum strainas parameters, the values of which were calculated for various oil contentsduring frying. The maximum stress and maximum strain were denoted asthe end points of the stress-strain curve (fracture point). Maximum stress is

918 KROKIDA ET AL.

related to hardness or firmness of the product, while the maximum stress tomaximum strain ratio is related to the product crispness (Jackson et al.1996).

The mathematical model for maximum stress and maximum strainwas fitted to experimental data, which were derived from the stress-strain


Table 4. Compression Test Mathematical Model

Compression Test

1. Viscoelastic behavior (stress-strain equation)s ¼ Eeþ smax�E emaxð Þ e=emax

� �p2. Parameters

smax – maximum stress (kPa)emax – maximum strain (�)E – elastic parameter (kPa)p – viscoelastic parameter (�)


. Frying conditions

– oil temperature (T, �C)– oil type (C, % hydrogenated in refined oil)– sample thickness (d, mm)

smax=589+164Y1.7 exp(2286 (1/T� 1/Tr))

emax=0.94+838Y7.2 exp(2999 (1/T� 1/Tr))E=25� 100Y1.5 exp(� 400 (1/T� 1/Tr))p=5+40Y1.2

. Method of pretreatment

– air drying duration (tD, min)

smax=1131 (tD/40)0.35+1446 (tD/40)

0.3 Y1.4

emax=0.63 (tD/40)0.12+300(tD/40)

1.4 Y4.9

E=40 (tD/40)0.2

� 100 Y1.5

p=2 (tD/40)� 0.8+30 (tD/40)

� 0.3 Y1.5

– osmotic pretreatmentsmax¼s0þs1Y

n

emax¼ e0þ e1Ym

Type of pretreatment s0 s1 n e0 e 1 m

Sugar solution 690 500 1.24 0.65 838 4.1Maltodextrine-21 630 2049 1.94 0.63 838 5.2Maltodextrine-12 597 435 2.00 0.60 838 6.0

Salt solution 606 369 1.24 0.60 838 5.6

Where: s – stress (kPa); e – strain (�); Y – oil content (kg/kg db); T – oil temperature(�C); tD – drying time (min); Tr=170�C

curves. The comparison between experimental and calculated values is

shown in Figure 18 as a function of oil content.

The maximum stress increases as the moisture content decreases and

oil content increases during frying. Oil temperature affects the maximum

stress significantly; as the oil temperature increases the maximum stress of

fried potatoes decreases, which denotes the higher firmness of potatoes at

lower temperatures. Oil type also affects the maximum stress significantly

during frying. In particular, the use of hydrogenated oil increases the max-

imum stress values, while the use of refined oil decreases it.

As it may be seen from Figure 18, the maximum strain also increases

significantly during frying. As the oil temperature increases, the maximum

strain decreases for the same oil content. The effect of oil type on the max-

imum strain is negligible.

The crispness (maximum stress to maximum strain ratio) of potatoes

during frying is presented in Figure 19. As it may be seen crispness of potato

strips changes significantly at the beginning of frying procedure, while

becomes constant as frying proceeds. Oil type affects significantly potato

920 KROKIDA ET AL.

Figure 17. Stress-strain curves during deep fat frying.

crispness, more specifically the use of hydrogenated oil instead of non

hydrogenated cottonseed oil increases potato crispness. On the otherhand, oil temperature increment decreases slightly potato crispness.

The proposed model for the maximum stress and strain was fitted to

experimental data and the results of parameter estimation were used conse-quently to extract the stress-strain curve. The other two parameters incor-

porated in the model are the elasticity parameter E, which gives the slope ofthe elastic part, and the viscoelastic exponent p. Both of these parametersdepend on the oil content of the fried potatoes.

The elasticity parameter was found to decrease significantly as themoisture content of fried potatoes decreased and oil content increased

as the frying proceeded. Oil temperature affects significantly the elasticity


Figure 18. Maximum strain and maximum stress versus oil content.

parameter; as the oil temperature increases, the elasticity of fried potatodecreases, which denotes the more elastic nature of potatoes at lower tem-peratures. It can be concluded that higher oil content and lower watercontent of potato strips at elevated temperature result in reduced elasticity.The use of hydrogenated oil as proportion of cottonseed oil increase theelasticity parameter.

The parameter ‘‘p’’ of the model, which expresses the deviation fromlinearity and thus the viscous nature of the material, was found to increaseas the moisture content was decreased and oil content was increased duringfrying.

Concluding, it can be noted that the oil temperature increment makesfried potatoes less firm, while the use of hydrogenated oil increases potatocrispness.

5.6. Effect of Type of Pretreatment on Compression

Behavior of French Fries

Some typical stress-strain curves obtained from compression tests onmaterial fried at 170�C for 15min are presented in Figure 20. Each curve isrepresentative for a fried product pretreated with a specific osmotic solutionor by air drying of different duration, as indicated in Figure 20. The stress-strain curves indicate that for osmotically dehydrated products the samedeformation before fracture is accomplished by a considerably lowerstress value, showing that the food becomes more compressible. For

922 KROKIDA ET AL.

Figure 19. Crispness of fried potatoes versus frying time.

air-drying pretreated products the stress-strain curves indicate that the same

deformation is accomplished by a considerably higher stress value, showing

that the food becomes firmer.

The mathematical models for maximum stress and maximum strain

(presented in Table 4) were fitted to the experimental data, which were

derived from the stress-strain curves.

The maximum stress increases as the moisture content decreases and

oil content increases during frying for all types of osmotic solutions and for

air-dried samples (Figure 21). Osmotic pretreatment affects significantly the


Figure 20. Stress-strain curves for osmotic and air drying pretreated samples friedfor 15min.

maximum stress of french fries; osmotic pretreatment increases the maxi-

mum stress values for the same oil content of fried potatoes, which denotes

the higher strength against fracture of osmotic pretreated potatoes.

The higher stress values were denoted for samples pretreated with sugar

solutions followed by those pretreated with maltodextrine-21, salt and

924 KROKIDA ET AL.

Figure 21. Maximum stress and maximum strain for osmotic pretreated friedpotatoes.

maltodextrine-12 solutions. This can be related with the different percent of

solids gain obtained from each type of solution. Also, air drying pretreat-

ment increases the maximum stress of the fried product and this gets more

intense as the drying duration increases (Figure 22). This can be explained


Figure 22. Maximum stress and maximum strain for air drying pretreated fried

potatoes.

by the increment of maximum stress during air drying (Krokida et al.,

1998b) caused by shrinkage phenomenon that takes place during drying.

As it may be seen from Figure 21, the maximum strain also increases

during frying. Osmotic pretreatment affects significantly the maximum

strain, which is higher for sugar solutions and has lower values for

926 KROKIDA ET AL.

Figure 23. Crispness of fried potatoes versus frying time.

dextrine-21, salt and dextrine-12 solutions. Air drying pretreatmentincreases the maximum strain of fried product and this gets more intenseas the drying duration increases (Figure 22).

The crispness (maximum stress to maximum strain ratio) of osmoti-cally pretreated potatoes during frying is presented in Figure 23. As it maybe seen, the crispness of potato strips changes at the beginning of the fryingprocedure, while it becomes constant as the frying proceeds. Osmotic pre-treatment does not affect significantly the potato crispness, with the excep-tion of sugar-pretreated samples, which show a slight increment of theircrispness. On the contrary, air drying pretreatment increases significantlythe crispness of fried products, which increases as the drying durationincreases.

In conclusion, it can be noted that osmotic pretreatment does notaffect significantly the crispness of fried potatoes, while it increases theircompressibility. Air drying pretreatment increases the product crispnessand makes it firmer.

6. CONCLUSIONS

Mass transfer phenomena – water loss and oil uptake – that take placeduring frying of potato strips can be described by an empirical first orderkinetic model. Water loss and oil uptake phenomena are getting moreintense at higher temperatures and thinner sample. The rates of both masstransfer phenomena (water loss and oil uptake) that take place during thefrying of potato strips decrease due to the drying or osmotic pre-treatmentbefore frying.

All the examined structural properties are greatly affected by all pro-cess variables (oil temperature, sample thickness and oil type). Apparentdensity and specific volume decrease during frying, while true density andporosity increase. A simple mathematical model, which expresses structuralproperties as functions of oil and moisture content, was developed andvalidated. Five parameters were incorporated in the model: the water den-sity, oil density, dry solid true density, dry solid apparent density andvolume shrinkage coefficient. The model can be used in combination withthe mass transfer kinetic model in order to predict structural properties offrench fries during frying. The structural properties are also affected by thedrying or osmotic pretreatment, which increases the densities and the spe-cific volume, while the porosity becomes higher.

Color (L, a, b) changes of potato strips that take place during fryingcan be described by an empirical first order kinetic model. Oil temperatureand sample thickness are the process parameters, which affect significantly


the color parameters during frying, while replacement of the frying oil byhydrogenated oil does not. The color change phenomenon is more pro-nounced at higher temperatures and thinner sample. Color deteriorationthat takes place during drying affects the color of fried products, whichgets browner. Color darkening takes place during osmotic dehydrationand browning reactions during frying are promoted resulting in moredark and red colored fried products. Salt dehydrated products have themost acceptable color.

The effect of frying conditions and type of pretreatment on the com-pression behavior of french fries was investigated, through their effect onfour parameters: maximum stress (�max), maximum strain ("max), elasticityparameter (E) and viscoelastic exponent (p). All the above parameters werefound to be affected significantly by frying conditions. Maximum stress andthe corresponding strain both increases during frying and as the fryingtemperature decreases. Fried potatoes loose their elasticity during frying,an effect, which gets more intense for higher oil temperatures.

Maximum stress and the corresponding strain both increases duringfrying and for both osmotic and air dried pretreated samples. Osmotic pre-treatment decreases the elasticity of fried products but does not affect sig-nificantly their crispness, while air drying decreases the elastic parameterand increases the crispness of french fries.

In conclusion, the quality properties of fried potatoes can becontrolled by choosing the appropriate frying conditions and type ofpretreatment.

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