chemistry and food

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47.2 Food Preservation
47.3 Food Additives
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6 basic ingredients supply from a nutritional diet:
Proteins
Carbohydrates
Fats
Minerals
Vitamins
Water
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Essential food components serve 3 main functions:
1. Provide energy for the essential biological processes to take place
2. Provide materials for building and repairing body tissues
3. Provide materials need for regulation of life processes such as the transport of oxygen and digestion
47.1 Principal Components of Food (SB p.279)
New Way Chemistry for Hong Kong A-Level Book 4
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Fats and carbohydrates supply most of the energy needs
Proteins supply most of the building blocks for tissue
Trace amount of vitamins and minerals acts as regulators of vital body processes
Food provides energy for the athletes to run
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Importance of Proteins in Our Body
Vital activity
Digestive enzymes, e.g. trypsin, amylase, lipase
Catalyzes the hydrolysis of proteins to polypeptides; Catalyzes the hydrolysis of starch to maltose; Catalyzes the hydrolysis of fats into fatty acids and glycerol
Respiration and transport
Antibodies
Essential to the defence of the body, e.g. against bacterial invasion
Growth
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Some functions of proteins in our body (cont’d)
Vital activity
Sensitivity and coordination
Hormones, e.g. insulin
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Food rich in proteins
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Structure of Proteins
Proteins are organic compounds of large molecular masses
In addition to C, H, O, they always contain N, usually S and sometimes P
The basic structural units of proteins is amino acids
Amino acid is a group of chemical and around twenty of them occurred in proteins
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Amino acid contains a basic amino group (–NH2) and an acidic carboxyl group (–COOH)
Variation in the structure of R gives rise to the 20 amino acids which are the building blocks of different proteins
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Common proteins are considered as macromolecules of amino acids
Different proteins are formed when the 20 different amino acids are linked up in specific sequences
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The first step in the synthesis of a protein molecule is the condensation reaction between two amino acids with a water molecule being eliminated
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Either ends of the peptide can undergo condensation reaction with another amino acid to form a tripeptide
Further combinations with amino acids extend the length of the chain to form a polypeptide
The molecule formed from two amino acids is called dipeptide
The group is called amide group or the peptide linkage
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The peptide linkages in the protein molecules are broken when it hydrolyzed by mineral acids or enzymes
Hydrolysis of Proteins
On complete hydrolysis, the protein molecule is broken down into its constituent amino acids
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Separation of Amino Acids by Paper Chromatography
As each amino acid has its own adsorption ability on a piece of paper, they can be separated and identified by means of paper chromatography
1. In paper chromatography, a piece of filter paper is used as the adsorbent
2. A capillary tube is used to spot the paper with a solution of the mixture of amino acids (U) to be analyzed
3. By using different capillary tubes, a number of solutions of known amino acids (A to F) are also spotted for comparison
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4. In order to achieve a good separation, the spots should be placed along the pencil line near the bottom of the paper
5. When the paper become dry, the paper is then rolled into a cylindrical tube with the ends stapled together
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6. The paper is then placed into a beaker containing a small amount of a suitable solvent such as a mixture of NH3 and propan-2-ol
The spot must be above the initial level of the solvent, otherwise they will be removed completely from the paper
8. The beaker is covered tightly with a piece of aluminium foil, so that the air inside is saturated with the solvent vapour
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9. The solvent moved up the filter paper by capillary action. As solvent moved across spots, partition of the spots between the stationary phase of trapped water in the paper and the mobile phase of the solvent occurs
The states of partition of the spots depend on:
1. The tendency of the amino acids to attach to the adsorbed water,
2. The tendency of the amino acids to dissolve in the solvent
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As different amino acids have different states of partition between the mobile and stationary phase, they would be moved up in different extent
Amino acids which are more soluble in the solvent than in water are moved up by the mobile phase at a faster rate
Amino acids which are more soluble in water than in the solvent are moved up by the mobile phase at a slower rate
As fresh solvent is moving up continuously, solvent extraction occurs continuously along the filter paper.
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Sufficient time is allowed for the solvent to rise up the paper so that the amino acids are separated far enough for easy identification
The paper is then removed from the solvent, and a pencil is used to mark the solvent front
The paper is allowed to dry and a developing agent (e.g. ninhydrin) is then sprayed on the paper to reveal the positions of the colourless amino acids
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A pencil is then used to mark the positions of the spots. For each amino acid, a Rf value (retardation factor) is calculated by using the following formula
Rf value must be smaller than 1
At the same temperature and with the same solvent, an amino acids always has the same Rf value
When the Rf value of various amino acids under a particular condition are known, it may be possible to identify an unknown amino acid by paper chromatography
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The Rf value of some amino acids can be very similar in particular solvent
amino acids would not appear as separate spots on the developed chromatogram
2-D chromatography is applied for further separation
a chromatogram is produced in the usual way and dried
It is then turned 90° and placed in the second solvent with polarity different from that of the first one
The second solvent completes the separation
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2-D paper chromatography
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Example 47-1
The figure below shows the chromatogram of an unknown sample U. The distances travelled by the three amino acids A1, A2 and A3 are 5.2 cm, 3.4 cm and 7.5 cm respectively. The distances travelled by the spots above sample U are 5.3 cm and 7.6 cm and the distance travelled by the solvent front is 10 cm. Calculate the Rf values of the three amino acids and determine the amino acids in the unknown sample U.
Answer
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Solution:
i.e. 0.53 and 0.76.
By comparing these values with the Rf values of A1, A2 and A3, we can deduce that the unknown sample contains A1 and A3.
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2-aminopropanoic acid
A peptide linkage is a bond formed by the condensation reaction between two amino acids with a water molecule being eliminated.
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Carbohydrates are the important source of energy in the diet
They comprise a large group of organic compounds which C, H and O. The general formula is CxH2yOy
Light
chlorophyll
6CO2(g) + 6H2O(l) C6H12O6(s) + 6O2(g)
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Carbohydrates provide a significant amount of the energy for our daily activities
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Carbohydrates are divided into 3 groups:
monosaccharides
disaccharides
polysaccharides
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Monosaccharides
A group of sweet, soluble crystalline molecules of relatively low molecular masses which cannot be further hydrolyzed into simpler compounds
General formula: C6H12O6
fructose (found in many fruits, honey;
glucose is found in the blood
of animals)
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Disaccharides
General formula: C11H22O11
May be formed from the condensation reaction of two monosaccharide molecules
Under suitable conditions, they may be hydrolyzed by water to yield its constiuent monosaccharides
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Common dissarcharides:
1. Sucrose (found in sugar cane), consists of a glucose unit and a fructose unit
2. Maltose (found in malt), consists of 2 glucose units
3. Lactose (found in milk), consists of a glucose unit and a galactose unit
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Polysaccharides
Examples of polysaccharides:
1. Starch; it is commonly found in rice, bread and potatoes
2. Cellulose; it is found in fruits, vegetables, cotton and wood
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Glucose can exist in acyclic (also describe as open-chain) and cyclic forms
In acyclic structure, it contains an aldehyde group
known as aldohexose
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In solid state, glucose does not exist as open-chain structure
It exists in one of the two cyclic structures which differ in the carbon C1 only
-glucose and -glucose
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Two cyclic forms of glucose can be converted to the other form when dissolved in water
In equilibrium mixture, 2 forms will exist together with trace amount of acyclic form
Although the acyclic form only exist in trace amount, most of the reaction of glucose in aqueous solution are due to the presence of free aldehyde group in open-chain form
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Fructose can exist as an acyclic form, 6-membered cyclic form and 5-membered cyclic form
It contains a keto group in open-chain form
known as ketohexose
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Similar to glucose, most of the reactions of fructose in aqueous solution are due to the presence of free keto group in the open-chain form
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Glycosidic Linkage in Carbohydrates
The bond formed between two monosaccharides is called glycosidic linkage
formed from the condensation reaction between 2 –OH groups of two monosaccharides with the elimination of H2O molecule
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Sucrose is made up of a glucose unit and fructose unit with the elimination of H2O molecule
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Maltose is made up of 2 glucose units with the elimination of H2O molecule
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Food containing sucrose and maltose
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The condensation process can be repeated to build up a giant molecule of polysaccharides
e.g. Starch and Cellulose
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Cellulose:
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Hydrolysis of Sucrose and Starch
Sucrose is hydrolyzed by dilute mineral acids (or enzymes) to give glucose and fructose
C12H22O11 + H2O C6H12O6 + C6H12O6
sucrose
glucose
fructose
H+
starch
maltose
amylase
The solution of starch is hydrolyzed into maltose in the presence of suitable enzyme
2(C6H10O5)n + nH2O nC12H22O11
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When maltose is further treated with a dilute acid or enzyme maltase, 2 glucose units will be formed
C12H22O11 + H2O 2C6H12O6
maltase
glucose
maltose
starch
H+
When starch is boiled with dilute sulphuric(VI) acid, it is hydrolyzed to give glucose
(C6H10O5)n + nH2O nC6H12O6
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Fehling’s test
A reducing sugar can reduce a basic solution of Cu2+(aq) (Fehling’s solution) or Ag+(aq) (Tollen’s solution)
Reducing sugars reduce Fehling’s solution
The Fehling’s solution changes from blue to green and a yellow-orange ppt. of Cu2O(s) is produced
Glucose, fructose, maltose are reducing sugars
Sucrose, starch and cellulose are non-reducing sugars
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Results of Fehling’s test on glucose, maltose and sucrose (from left to right)
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Fehling’s solution is used to distinguish reducing and non-reducing sugars
Glucose is a reducing sugar
the presence of the aldehyde group in the acyclic form in aqueous solution in equilibrium
Fructose is a reducing sugar
the presence of the keto group in the acyclic form in aqueous solution in equilibrium
Disarccharides also need to have free carbonyl groups for them to have reducing action on Fehling’s solution
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Maltose is formed by the condensation of 2 glucose units which are joined up by a glycosidic linkage
The glycosidic linkage is formed by the condensation of the –OH group of C1 of one glucose ring with the –OH group on C4 of another glucose ring
The ring can open up to form acyclic structure with a free aldehyde group when dissolved in water
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In sucrose, the glycosidic linkage is formed between C1 of glucose and C2 of fructose
Both constituent rings in sucrose cannot equilibrate with acyclic form containing a free carbonyl group which possess the reducing properties
sucrose is a non-reducing sugar
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Check Point 47-2
(a) Name two important hexoses and draw their linear structures. State one difference between them.
Answer
(a) The two important hexoses are glucose and fructose
Glycose is an aldohexose whereas fructose is a ketohexose.
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Check Point 47-2 (cont’d)
(b) Given three unknown solutions labelled A, B and C, suggest chemical tests to determine which solution contains starch, which contains glucose and which contains sucrose.
Answer
(b) Firstly, a few drops of iodine solution are added to the three unknown solutions. The one which gives a blue black coloration contains starch. Then the other two solutions can be distinguished by conducting Fehling’s test. As glucose is a reducing sugar, it can reduce Fehling’s solution. The original blue colour of Fehling’s solution becomes green, and finally a yellow-orange precipitate of copper(I) oxide is produced. However, as sucrose is a non-reducing sugar, it does
not have any effect on the Fehling’s solution
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(c) What is a glycosidic linkage? Draw the structure of sucrose and indicate such a linkage.
Answer
(c) A glycosidic linkage is a bond formed from the condensation reaction between two –OH groups of two monosaccharides.
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Fats and oils provide energy and essential fatty acids (carboxylic acid) to our body
Fats are kept in the animal’s body as energy reserve and also keep the animal warm in cold climates
Solid fats and liquid oils
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Vegetable oils are extracted from various nuts and seeds while marine oils may be extracted from herring or sardine
Fats and oils are chemically similar although fats are solids while oils are liquids at room temperature
Food rich in fats and oils
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Structure of Fats and Oils
Most natural fats and oils are mixed glycerides
Glycerides are esters formed from propane-1,2,3-triol (also known as glycerol) and a mixture of carboxylic acids with different hydrocarbon chains
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where R1, R2, R3 represent long hydrocarbon chains
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The carboxylic acids making up fats and oils are mainly unbranched and divided into 2 groups: saturated and unsaturated
Saturated carboxylic acids are those that have single bonds in their hydrocarbon chain only
Unsaturated carboxylic acids contains at least one C = C double bond in the chain.
Carboxylic acids with more than one C = C double bond in chain are known as polyunsaturated carboxylic acids
A Component of Fats and Oils —Carboxylic Acids
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Saturated carboxylic acids are solids at room temperature
regular nature of aliphatic chains allows the molecules to be closely packed in an orderly fashion
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the hydrocarbon chains are bent
unsaturated carboxylic acid are less dense and attractions between adjacent molecules are weaker
The larger no. of double bond, the lower is the melting point
The longer the hydrocarbon chain, the higher is the melting point
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Formulae, melting points and sources of some common carboxylic acids
Name
14 16 18
CH3(CH2)12COOH CH3(CH2)14COOH CH3(CH2)16COOH
54 63 70
Unsaturated carboxylic acids:
18 18 18
16 –5 –11
Olive oil Sunflower oil, soya bean oil, corn oil Soya bean oil, linseed oil
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When a glycerol (i.e. propane-1,2,3-triol) reacts with a stearic acid (fatty acid), glyceryl monostearate is formed
Formation of Fats and Oils
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The product formed may react with another stearic acid to form glyceryl distearate, which may further react to give glyceryl tristearate
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The glyceryl tristearate formed is a solid, i.e. it is a fat
If glycerol reacts with fatty acids to form a liquid product, then the product is oil
Most natural fats and oils are mixed glycerides (esters formed from propane-1,2,3-triol and a mixture of different long-chain carboxylic acids, such as linoleic acid, stearic acid and oleic acid)
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Hydrolysis of Fats and Oils
Fats and oils ingested in our body are hydrolyzed by enzymes into carboxylic acids and glycerol
The hydrolyzed products are then used as fuels in our body, or used in building cell membranes and fatty tissues
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Hydrolysis of fats and oils are carried out by acids or bases in the laboratory
The hydrolysis carried out in an alkaline medium (e.g. NaOH) is more practical as the reaction is irreversible
this is called saponification
The products are propane-1,2,3-triol and a mixture of long-chain carboxylic acids
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Iodine Value
The difference between fats and oils is the degree of unsaturation
To measure the degree of unsaturation quantitatively, iodine value is employed
Its determination is based on the reaction between iodine and the C = C bonds in fats or oils
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The amount of iodine reacted depends on the degree of unsaturation of the fats and oils
The higher the degree of unsaturation of a fat or an oil, the greater the amount of iodine it react with
The iodine value of a fat or an oil is defined as the number of grams of iodine that reacts with 100 grams of the fat or oil
Vegetable oils have higher iodine values than animal fats.
indicates vegetable oils are more unsaturated
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Fats / oils
Iodine values
Animal fats
80 – 140
Ground-nut oil
85 – 105
Olive oil
80 – 90
Almond oil
90 – 110
Corn oil
115 – 130
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Since most fats and oils are mixed glycerides, the iodine values for a particular fat or oil will usually have a range of values
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Hardening of Vegetable Oils
Unsaturated oils usually have a lower melting point and exist as a liquid at room temperature
The C = C double bonds in these unsaturated oils may be converted to single bonds by the addition of hydrogen in the presence of catalyst
known as hydrogenation
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Hydrogenation of some of the carbon-carbon double bonds in oils converts them into solid fats
The hardening of oils is the basis for industrial production of margarine
Margarine is prepared by the hydrogenation of certain oils with the addition of flavourings and colourings, plus vitamins A and D
The oils will not completely hydrogenated, otherwise the solid fats would be too hard and brittle for cooking
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Hydrolytic and Oxidative Rancidity
Fats and oils would develop an unpleasant odour if they are kept too long
They are liable to spoilage which results in the production of an “off” odour and a flavour described as rancidity
Rancidity is mainly caused by hydrolytic or oxidative reactions which release foul smelling aldehydes and fatty acids
Different fats and oils go rancid at different rates
e.g. Oils from sea animals contain high proportion of highly unsaturated carboxylic chains which deteriorate rapidly
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e.g. frying potato chips in oil at a high temperature
the water released from potato may cause the hydrolysis of oil
The reaction is speeded up in the presence of certain micro-organisms or in the presence of enzymes
The liberated carboxylic acids may be volatile and have extremely unpleasant odours and flavours (e.g. butanoic acid)
Hydrolytic Rancidity
Hydrolytic rancidity is due to the presence of moisture in fats and oils, which causes the hydrolysis of the glyceride molecules to propane-1,2,3-triol and free carboxylic acids
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Fats and oils having a high degree of unsaturation are more susceptible to oxidation
The oxidation is a free radical mechanism and is accelerated by trace metals, light and free radical initiators
Oxidative Rancidity
Oxidative rancidity occurs when fats or oils are exposed to air and undergo oxidation and results in the production of flavours such as “tallowy”
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The products, hydroperoxides, are flavourless and odourless, but they decompose easily to form highly reactive hydroperoxide free radicals
These radicals set up a chain reaction and produce volatile, flavoured compounds of aldehydes, ketones and carboxylic acids, which are responsible for their rancid flavour
known as autoxidation
Cleavage of
double bonds
free radical
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Addition of antioxidants can slow down oxidative spoilage
Natural antioxidants (e.g. vitamin E) are found in vegetable oils
The oil can withstand autoxidation for a longer time
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Check Point 47-3
(a) State one similarity and one difference between fats and oils.
Answer
(a) Both fats and oils are esters formed from propane-1,2,3-triol and a mixture of carboxylic acids with different chains of hydrocarbons. However, fats are solids whereas oils are liquids at room temperature.
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(b) (i) What is the difference in the structure between an unsaturated carboxylic acid and a polyunsaturated carboxylic acid of the same length of hydrocarbon chain?
Answer
(b) (i) An unsaturated carboxylic acid has one carbon-carbon double bond in its hydrocarbon chain whereas a polyunsaturated carboxylic acid has more than one carbon-carbon double bond in its hydrocarbon chain.
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(b) (ii) How can you convert unsaturated carboxylic acids into saturated carboxylic acids?
Answer
(b) (ii) The conversion can be made by hydrogenation of the unsaturated carboxylic acids with a suitable catalyst.
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(b) (iii) What physical change would be observed accompanying the reaction in (b)(ii)?
Answer
(b) (iii) The carboxylic acids change from liquid state to solid state.
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(c) Which one, an animal fat or a vegetable oil, is more susceptible to rancidity? Explain your answer.
Answer
(c) Vegetable oil is more susceptible to rancidity as it contains more C = C double bonds.
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(d) Hydrolysis of fats can be carried out in either an acidic or an alkaline medium. State one similarity and two differences between these two reactions.
Answer
(d) Whether the hydrolysis is carried out in an acidic or an alkaline medium, it is the ester linkage being ruptured in both cases. However, the hydrolysis occurring in an alkaline medium is irreversible, whereas that in an acidic medium is reversible. Besides, one of the products formed in the hydrolysis taking place in an alkaline medium is sodium carboxylate, instead of carboxylic acid formed
in an acidic medium.
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1. Fresh and raw food is limited in supply
2. Food is not always consumed immediately after harvest or slaughter, and it has to be transported from where it is produced to where it is consumed
3. Many agricultural products are seasonal. We may not have delicious fresh summer fruits in winter time
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47.2 Food Preservation (SB p.299)
10% of the world’s total supply of food deteriorated each year
food spoilage and its prevention raise the global concern in terms of health and economy
Food spoilage is mainly due to the activities of micro-organisms (bacteria, moulds and yeasts) and, to a lesser extent, chemical reactions which are often catalyzed by enzymes
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Food Spoilage due to Microbial Activities
Micro-organisms, in the form of fungi, yeasts and bacteria, are responsible for food spoilage
Micro-organisms spoil the attractiveness, palatability, nourishment of food
e.g. moulds on bread, lactic acid bacteria in the souring of milk
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Some micro-organisms are either themselves toxic in nature or produce toxic chemicals when they act on a food substrate and cause food poisoning
e.g. Clostridium botulinum produces botulin which is probably the most poisonous substance known
Clostridium botulinum
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Food Spoilage due to Chemical Changes
The rancidity of fats and oils and the browning of fruits and vegetables are examples of food spoilage due to chemical or biochemical changes
Browning of apple occurs when a freshly cut apple is left in air
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This phenomenon may be accounted for by an initial enzymatic oxidation of phenols present in apples by atmospheric oxygen to diquinones which readily polymerize
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The initially oxidative step is catalyzed by enzymes present in the apple, but subsequent polymerization requires neither oxygen nor enzymes to carry on
browning of apple may be prevented by either removing O2 supply or inhibiting the enzymatic activities
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To slow down the deterioration in quality and to increase the food’s shelf-life
The principle of food preservation techniques is to suppress the microbial or chemical changes resulting in food spoilage
Ways to do: killing the micro-organisms, inhibition of microbial growth, or retardation of chemical changes
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Main Principles of Food Preservation
The presence of water is one of the necessary conditions for microbial growth in food
Removing the moisture from food would slow down the microbial growth rate
The presence of water in food also leads to hydrolytic reaction
∴ removal of moisture helps suppress chemical and biochemical changes
Examples: drying and dehydration
Removal of Moisture
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Warm environment is a favourable condition for microbial growth in food
At low temperatures, the growth of micro-organisms is retarded
At high temperatures, bacteria and spores are destroyed and micro-organisms are killed
e.g.
Food is sterilized at high temperatures in canning industry
microbial activities are reduced to minimum and the rates of chemical and biochemical changes are retarded
Altering Temperature
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Most micro-organisms prefer a neutral or slightly acidic medium for growth
pH ranging from 4.5 to 10 are suitable for their survival and growth
Creating an unfavourable condition by increasing or decreasing the pH values can retard their growth
Examples:
2. Use of lactic acid in yoghurt
Changing pH
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Micro-organisms require a minimum moisture content of 20% to grow
decreasing the moisture content by increasing the solute concentration of food would inhibit microbial growth
Putting food in concentrated salt or sugar solutions
due to the difference in osmotic potential, water inside the cells would be lost to the surrounding solution by osmosis
micro-organisms cannot multiply, therefore they would not cause food spoilage
Use of Osmotic Process
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Nitrates(V) and nitrates(III) are used in meat curing because they can inhibit or slow down microbial activities in food
Antioxidants are added to certain food products such as potato crisps to discourage rancidity of fats and oils
extend the shelf-life of food
Ascorbic acid and BHA are antioxidants and used in pork sausages and cooking oil respectively
Use of Chemical Additives
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1. retain the original characteristics of the food
2. must not have detrimetal effect on the nutritive value of the food
3. must not be potentially harmful to health
Many of food preservation practiced nowadays utilized a combination of the food preservation principles
e.g. freezing helps preserve food by lowering temperature and making water unavailable for bacterial growth
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All forms of cooked food are preserved by this method
Another well-known example is the sterilization of milk by UHT (ultra high temperature)
Heat Treatment
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Food can be preserved by exposing to ionizing radiation,
e.g. -rays from a cobalt-60 source or fast-moving electrons from a high voltage
The radiation destroys enzymes and micro-organisms in food
Examples of food preserved by employing this method:
Peaches and potatoes
Irradiation
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the oldest practice of food preservation
Food is dried under sunlight or in special ovens or freeze-driers
water is unavailable for chemical reactions and micro-organisms growth
Drying and Dehydration
Fruits, coffee powder, milk powder, soups, fish, meat and vegetables
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In chilling, food is kept at 0°C – 4°C
Chilling slows down the microbial and enzymatic activities, as well as chemical and biochemical changes
food spoilage is avoided
Refrigeration (Chilling and Freezing)
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In freezing, food is kept at –18°C
Freezing turns the water present in food to ice which is unavailable for hydrolytic reactions to occur and for microbial to grow
Examples:
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Concentrated sugar solution can preserve food from spoilage
Water in food is lost by osmosis to the sugar solution and is no longer available for enzymatic
activities and microbial growth
Examples: Jam and fruits
Sugaring
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Food can be treated with salt or concentrated salt solutions
Water in food is lost by osmosis to the salt solution and is no longer available for enzymatic
actions and microbial growth
Salting
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Cooking destroys enzymes and micro-organisms in food
Sealed cans ensure that no air and no micro-organisms can get in
Examples:
Canning
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By adding salt (NaCl) to meat or treating with concentrated salt solutions containing nitrates(V) and nitrates(III)
NaCl ties up the water in food so that micro-organisms cannot grow, and chemicals such as nitrates(III) prohibit their growth
Examples:
corned beef, bacon, sausages,
Meat-curing
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Examples:
Pickling in Vinegar
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The growth of micro-organisms and chemical spoilage are controlled by the substances added
The added substances inactivate or kill micro-organisms, retard chemical spoilage, but considered relatively safe to humans
Examples:
Use of Food Additives
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(a) State the principles of preservation of the following food.
(i) butter
(ii) wine
Answer
(a) (i) Antioxidation and lowering temperature
(ii) Antioxidation
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(b) Name the techniques used to preserve the following food.
(i) Chinese sausages
Answer
(a) (i) Meat-curing, drying and dehydration
(ii) Use of food additives
(iii) Pickling
(iv) Freezing
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Food additives have been used for centuries in preserving and improving the appearance and colour of food
e.g. salt, sugar, vinegar are used to preserve food; herbs and spices are added to improve the flavour of food
Food additives are any substances added to food
Food additives help maintaining high qualities and constancy of characteristics of food demanded by consumers
47.3 Food Additives (SB p.305)
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1. To maintain palatability and wholesomeness
Food loses flavour and freshness naturally due to aging and exposure to natural elements such as moulds, air, bacteria, fungi or yeasts
Ascorbic acid, BHA, BHT and nitrates(V) are preservatives added to slow down spoilage and rancidity while maintaining the taste
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2. To enhance flavour or import desired colour
Spices and natural or synthetic flavours are added to enhance the taste of food
Colours are added to enhance the appearance of certain foods to meet consumer’s expectations
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Certain ingredients such as emulsifiers, stabilizers and thickeners help ensure consistent food texture and characteristics
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Leavening agents help cakes and biscuits to rise during baking
Other additives help modify the acidity and alkalinity of food for proper flavours, tastes and colours
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Nutrients in food may be lost during processing
Foods such as cereals, milk, margarine can be enriched or fortified with additives such as vitamins A and D, iron, ascorbic acid, calcium carbonate, zinc oxide and thiamin
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The use of chemical preservatives is the most common method adopted in food industry
The principle of preservatives is to kill micro-organisms in food and prevent microbial growth
Common chemical preservatives: nitrates(V), sulphates, and acids
Function of Food Additives
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slow down microbial growth;
prevent botulism in
Nitrates(III) and Nitrates(V)
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help maintain the colour and the vitamin C content of food
as bleaches and antioxidants to
prevent browning in alcoholic beveages, fruit juices, margarine, salads, jams and pickled products
Sulphur Dioxide and Sulphates(IV)
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widely used in oyster sauce, fish sauce, ketchup, non-alcoholic beverages,
fruit juices, margarine, salads, jams and pickled products
Benzoic Acid and Sodium Benzoates
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Sorbic Acid and Sorbates
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Propanoic Acid and Propanoates
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The natural flavours (taste and aroma) of food are lost during processing
many processed food are dependent on additives for their flavours
Some common preservative also serve to flavour the food
Some flavourings do not have any flavour themselves but
enhance the natural tastes or aroma of the food
As Flavourings
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Foods contain flavourings
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Natural flavours are extracted from crushed plants with appropriate solvents
Natural flavours are complex mixtures of numerous compounds
Now, many of the compounds can be synthesized in the laboratory and mixed to produce synthetic flavourings
There are a slight difference between synthetic and natural flavourings
synthetic products lack many of trace compounds present in natural products
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Some common flavourings used as food additives
Flavour
Food additive
Camphor Cinnamon Ginger Grape Lemon Orange Pear Peppermint Rum Spearmint Spicy Vanilla Wintergreen
Bornyl acetate Cinnamaldehyde Ginger oil Methyl anthranilate Citral Orange oil Amyl butyrate Menthol Ethyl formate Carvone Ethyl cinnamate Ethyl vanillin Methyl salicylate
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Common used in: chicken broth,
oyster sauce, fish sauce, soup
bases for instant noodles and
fastfood and restaurant dishes
Spices
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MSG does not add any taste to food but is a flavour enhancer
does not change the original flavour but make it more noticeable and tasty
Common used in: chicken broth,
oyster sauce, soup bases for
instant noodles and dim sums
Monosodium Glutamate
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responsible for the sweet smell of fruits
The fruit flavours can be extracted from fruits but most of fruit flavourings used in the food industry are synthetic compounds
Esters are used in ice-creams and many fruit juices
Typical example:
Esters
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a kind of sweetener used as a sugar substitute or diet sugar
has no food value but intense sweetness which about 500 times sweeter than table sugar
Used in sugar-free food for
diabetics and diet soft drinks
Saccharin
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its sweetness is about 160 times that of sucrose
Aspartame
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Food colourings are a mixture of dyes added to make foods look more attractive to consumers
They replace the colours of food lost in processing or increase the colour appeal of foods only with no other functions
Mainly use for commercial and cosmetic reasons
As Colourings
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Examples:
Caramel
a brown colouring which is often used in oyster sauce
Carotene
a yellow dye present in carrots which is used to colour butter and margarine
Chlorophyll
a green dye extracted from plants which is used to colour edible oils
Natural Colourings
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also called synthetic dyes, organic dyes or coal tar dyes
derivatives of organic compounds
more stable, more attractive in appearance and cheaper to manufacture
Examples:
Indigo carmine
a purple blue dye used in combination with yellow colourings for confectioneries, candies and liqueurs
Synthetic Colourings
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a yellow dye used in some fruit squash and cordial
Sunset yellow
an orange azo dye used in candies, honey-like products, salmon and crabs
Foods with synthetic colourings
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Emulsifiers are substances that keep very small droplets of non-polar liquid (usually oil) evenly dispersed in a polar liquid (usually water)
Widely used in dairy products, ice-cream, mayonnaise, margarine and peanut butter
Stabilizers and thickeners improve the texture, blend, smoothness and other consistencies of food
As Emulsifiers, Stabilizers and Thickeners
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Foods containing emulsifiers
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Some examples of emulsifiers, stabilizers and thickeners and their functions
Emulsifiers, stabilizer or thickener
Xanthan gum
Used in seafood dressings, frozen pizza and packet dessert topping
Pectin
Dextrins
Used in icings, frozen desserts, confectioneries, whipped cream, cake mixtures, mayonnaise and salad dressing
Sodium alginate
Commonly used to emulsify and stabilize ice-cream, yogurt, sauces and syrups
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Nutrients are added to replace those lost in processing or improve the nutritional value of food
Examples:
Vitamin C is added into fruit juices and chewing gums
Vitamin B is added to enrich flour
Vitamin D is added into milk and margarine
Ammonium ferric citrate is added into infant milk formulations and bread flour
Iodine is added into iodized salt
As Nutrients
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The main cause of rancidity of fats and oils in food is the atmospheric oxidation
Fats and oils deteriorate rapidly due to autoxidation, producing carboxylic acids, aldehydes and ketones which give their characteristic unpleasant rancid taste and odour
Foods kept wrapped, cold and dry are relatively free of air oxidation
Antioxidant added to food can retard oxidation
As Antioxidants
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Ascorbic acid (vitamin C) and ascorbates are used as antioxidants in pork sausages
Ascorbic Acid and Ascorbates
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common antioxidants used in food to retard the development of oxidative rancidity in unsaturated fats and oils
These antioxidants are added to
vegetable oils, shortening and
products that contain unsaturated
cereals, potato crisps, cheez balls,
bread and sausages
BHA and BHT
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BHA and BHT seem to work by donating the H atom of the –OH group to the free hydroperoxide radical (ROO •) involved in the autoxidation of fats and oils
stop the chain reaction in oxidation spoilage
AH + ROO • ROOH + A •
(AH represents the antioxidant and A • is a radical derived from the antioxidant)
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Example:
the use of baking powder when making bread or cake
During baking, carbon dioxide gas is produced and it expands upon heating
raises the bread a nd cakes
As Leavening Agents
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confer a white colour to food such as flour and cheese
SO2 is used to bleach flour
As Bleaching Agents
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Example:
Use of glycerin which holds moisture in icings and tobacco because of the hydrogen bonds present
As Humectants
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keep powdered food fast flowing even in humid weather
Silicates and sodium ferrocyanide are added to salt, garlic powder, onion powder, milk powder and cream powder to prevent caking
As Anti-caking Agents
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The functions of acids, bases and buffers are various:
e.g. giving mild acid taste; masking undesirable aftertastes; control of pH
Examples:
(b) Adding sodium hydrogencarbonate in tinned custard
As Acids, Bases and Buffers
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It is difficult to comment about the safety of food additives
But it cannot be denied that there are hazards associated with their uses
3 possible harmful effects of food additives are:
1. Allergies
2. Hyperactivities
The Possible Menace of Food Additives
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Some people are allergic to certain additives
It is suspected that MSG and tartrazine may cause rashes, stomach upsets and asthma
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In 1970s, some scientist suggested that hyperactivity and attention deficient disease (ADD) in children are related to some food additives
Some hyperactive children showed improvement when fed with additive-free diets
no evidences of these additives causing hyperactivity or learning disabilities have been shown from a well-controlled study
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Some additives, e.g. sodium nitrate(III) are suspected to be carcinogens
Some additives are believed to be the cause of some long illnesses
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The use of BHA and BHT is extremely controversial
small amount of BHT can prevent cancer, but larger amount of it would cause cancer in laboratory animals
vitamins C and E function similarly as BHA & BHT and are safe
scientists suggest not to take the risk of using BHA & BHT
Controversy over BHA and BHT
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it is the monosodium salt of glutamic acid which is an amino acid
found naturally in many fresh meats and vegetables, e.g. mushrooms, peas and tomatoes
has no significant taste, but it enhances the flavour of food
The use of MSG as a flavour enhancer in food has become controversial over last 30 years
some reports of adverse reactions in people who have eaten food that contain MSG
Side Effects of Monosodium Glutamate
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“MSG Syndrome” has been so-called “Chinese Restaurant Syndrome” due to the heavy use of MSG in Chinese restaurants
The MSG contents of some dim sums
Dim sum
Weight of MSG (g / piece)
Beef cheung fun Beef siu mai Chicken roll Fried taro Fun ko Pork ribs Prawn dumpling Siu mai Shark fin roll Spring roll
0.39 0.62 0.40 0.39 0.22 0.24 0.13 0.18 0.23 0.40
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Possible symptoms and side effects of “MSG syndrome include:
burning sensation, thirst, headache, chest pain, vomiting and abdominal discomfort
The side effects are more prominent in some individuals than others, and are dose-related
The use of MSG in baby food is discontinued
scientists found that injection of high doses of MSG into 10-day-old mice caused brain damage
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Nitrates(III) and nitrates(V) are used together as preservatives in many cured and canned meats
Nitrates(V) are reduced to nitrates(III) which are responsible for developing the cured aroma and fresh pink colour in meats, and for inhibiting bacterial growth
Nitrates(III) are effective in preventing botulism
Excessive ingestion of nitrates(III) causes decreased levels of haemoglobin in the blood. Long-term ingestion may lead to malnutrition, growth retardation, impairment of reproductive capacity and reduced lifespan
Toxicity and Potent Carcinogenic Nature of Nitrates(III)
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Nitrates(V) are fairly harmless, but nitrates(V) ingested from vegetables and meats are reduced to harmful nitrates(III) in body
Studies have shown that unusually high dosage of nitrates(III) would increased incidence of cancer in rats
nitrates(III) possibly give rise to carcinogenic nitrosamines
Nitrates(III) are converted to nitric(III) acid in stomach
NaNO2(aq) + HCl(aq) HNO2(aq) + NaCl(aq)
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Under certain conditions, nitric(III) acid reacts with 2° amines released from digestion of proteins to form nitrosamines
HNO2(aq) + R2NH(aq) R2N – N = O(aq) + H2O(l)
2° amine
Studies also shown that the high incidence of nasal and oesophagus cancer in Guangdong Province has been associated with the consumption of salted fish and cured meat sausages
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“sulphur dioxide” is a collective term for sulphur dioxide gas and its salts, namely sulphates(IV), hydrogensulphates(IV) and disulphates(IV)
They are used to preserve dehydrated fruits and vegetables, wines and other non-alcoholic beverages
Sulphur dioxide is poisonous, it will attack the respiratory system when ingested
Starting from 1986, the use of sulphates(VI) on fresh fruits and vegetables (except potatoes) to be sold or served raw to consumers has been banned in the USA
Toxicity of Sulphur Dioxide
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1. about 500 times sweeter than ordinary sugar
2. slight bitter taste
3. used as a substitute for sugar especially for diabetics
4. low calorific value
Some studies have shown that high doses of saccharin in second generation rats are linked to bladder cancer
Potent Carcinogenic Nature of Saccharin
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aspartame is a better alternative to saccharin
show no allergic reactions in adults or children in carefully controlled clinical studies
aspartame is a sweetener which is about 160 times sweeter than sucrose
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Comparative sweetness of some sugars and artificial sweeteners
Sugar or artificial sweetener
Sweetness (relative to sucrose)
Lactose Galactose Maltose Glucose Sucrose (table sugar) Inverted sugar Fructose Aspartame Saccharin
16 32 33 74 100 125 174 16 000 50 000
Disaccharide Monosaccharide Disaccharide Monosaccharide Disaccharide Mixture of glucose and fructose Monosaccharide Artificial sweetener Artificial sweetener
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The number of types and efficiency of additives have increased and their safety has been ensured over the years
Some substances such as salt, sugar, vitamins and some minerals, which have long been used as additives to food, are “Generally Recognized As Safe” and excluded from the food additive regulation process
Apart from GRAS substances, no food additive may be used unless approval is granted
Monitoring the Use of Food Additives
By Research
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Studies using animals to show the effects caused by the intake of certain food additives are carried out at expected levels of human consumption
The data obtained are then compiled and lists of permitted additives are published
In the mean time, scientists are continuing to study the potent effects of the additives on humans, while watching out for any adverse ones
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The use of food additives is governed by legislation in many countries
In the USA, the Food and Drug Administration is responsible for monitoring the safety, purity and wholesomeness of food and food additives
In the UK, the Food Advisory Committee compiles lists of food additives permitted by the government
For countries in European Union, a permitted food additive is assigned an E number
By Legislation
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Type of additive
Colourings Preservatives Flavourings Antioxidants Emulsifiers and stabilizers Acids, bases and buffers Sweeteners
Most begin with 1 Most begin with 2 Not numbered E300 – E321 E322 and some numbers between E400 and E495 Most begin with 5 Most begin with 4 or 6
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In Hong Kong, the Department of Health is responsible for the monitoring and legislation of food additives
The department is empowered by the Public Health and Municipal Services Ordinance to:
1. legislate permissible additives and maximum limits of additives in particular food;
2. require supply of information on the composition of substances used in the preparation of food;
3. check food labelling;
4. inspect food-processing industries;
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5. take samples of foods and food additives for chemical analysis
6. seize and destroy food;
7. prosecute for false labelling or advertisement, using non-permitted additives or using additives beyond the permissible limits
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The approval of food additives may change since absolute safety of any substance can never be proved
Scientific knowledge is constantly evolving
governments often review earlier decisions so that the assessment on the safety of food substances remains up-to-date
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(i) monosodium glutamate (MSG) in oyster sauce
(ii) butylated hydroxyanisole (BHA) in cooking oil
(iii) E130 in fruit juice
(iv) sodium nitrate(III) in Chinese sausages
Answer
(a) (i) It is used to enhance the flavour of oyster sauce
(ii) It is used as an antioxidant to prevent cooking oil from going rancid
(iii) It is used to make the fruit juice look more attractive to consumers
(iv) It is used to slow down microbial growth, maintain pink colour of meat and prevent botulism
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(b) Give one advantage and one disadvantage associated with the use of the following additive:
(i) sulphur dioxide in dried raisins
Answer
(b) (i) Advantage: Raisins can last longer
Disadvantage: Some people are allergic to it
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(ii) potassium nitrate(III) in luncheon meat
Answer
(b) (ii) Advantage: Luncheon meat can last longer with a pink colour
Disadvantage: Nitrates(III) are associated with high occurrence of nasal and oesophagus cancers
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(iii) saccharin in soft drinks
Answer
(b) (iii) Advantage: It has a low calorific value and is suitable for patients suffered from diabetes
Disadvantage: Saccharin is a suspected carcinogen
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