1 · Web viewThose found in fruit and vegetables contribute 30–100% of the vitamin A requirement in humans. Red astaxanthin when complexed with protein gives the blue or green hue

Worksheet 17.2Chapter 17: Food chemistry – fast factsF.1 Food groups A food is any substance, whether processed, semi-processed or raw that is intended for human

consumption, and includes beverages, chewing gum and any substance that has been used in the manufacture, preparation or treatment of ‘food’. It does not include cosmetics, tobacco or substances used only as drugs (Codex definition 2005).

Nutrients are any substance obtained from food and used by the body to provide energy and to regulate growth, maintenance and repair of the body’s tissues. Proteins, lipids, carbohydrates, vitamins, minerals and water are considered nutrients.

Fats and oils are triesters (triglycerides) formed from three long-chain fatty acid (carboxylic acid) molecules and one glycerol molecule.

The nature of the R group determines the physical and chemical properties of the lipid. Fats are solid at room temperature; oils are liquids.

The simplest carbohydrates are monosaccharides. They contain one carbonyl group (C = O) and at least two hydroxyl (–OH) groups, and have the empirical formula CH2O. Monosaccharides are the building blocks of disaccharides and polysaccharides.

Disaccharides are formed from the condensation reaction of two monosaccharides.

Polysaccharides are condensation polymers formed from monosaccharides with the elimination of water molecules. Glucose is the most important monomer of the naturally occurring polysaccharides. Starch and cellulose are examples.

Proteins are condensation polymers of 20 2-amino acids. All proteins contain C, H, O and N and some also have P and S in their chemical composition.

F.2 Fats and oils Most naturally occurring fats contain a mixture of saturated, mono-unsaturated and poly-

unsaturated fatty acids and are classified according to the predominant type of unsaturation present.

Saturated fatty acids, which are often animal in origin, are carboxylic acids with the general formula CnH2n + 1COOH.

The carbon chain is made from only single carbon–carbon bonds with carbon atoms bonded in a tetrahedral arrangement which allows the chains to pack closely together.

The van der Waals’ forces are sufficiently strong between the chains to make the compounds solid at room temperature.

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Unsaturated fats contain the carbon–carbon double bond. This produces a ‘kink’ in the chain, which prevents the molecules from packing closely together and reduces the intermolecular forces.

Unsaturated oils, which are generally vegetable in origin, are liquids.

The greater the number of C = C double bonds, the greater the separation between the chains and the lower the melting point.

Solid fats are more likely to be crystalline, more saturated and/or have longer fatty acid hydrocarbon chains. Mono-unsaturated (olive, canola and peanut) and poly-unsaturated fats (safflower, sunflower, corn, fish, linoleic, linolenic) are liquids, and saturated fats (palm, coconut, lard, butter, shortening) are solids at room temperature.

The melting point of fatty acids increases with increasing relative molecular mass and increasing degree of saturation. Fats and oils are chosen for cooking on the basis of their melting temperature. For example, cocoa butter melts at close to body temperature, and fats chosen for cake-making melt over a wide range of temperatures.

Cis fatty acids have hydrogen atoms on the same side of the carbon–carbon double bond.

The cis isomer is the most common form of unsaturated fat; the trans form only occurs in animal fats and in processed unsaturated fats, such as margarine.

The molecules of the cis isomer cannot easily arrange themselves side by side to solidify, so they tend to have lower melting points than the corresponding trans isomer.

Trans fatty acids have hydrogen atoms on opposite sides of the carbon–carbon double bond.

The cis and trans isomers are examples of geometric isomers.

Saturated fats are more stable than unsaturated fats. The carbon–carbon double bonds in unsaturated fats react with oxygen (auto-oxidation), hydrogen (hydrogenation), light (photo-oxidation) and enzymes/heat/water (hydrolysis).

The addition of hydrogen to the carbon–carbon double bond of a fatty acid in the presence of heat (140–225ºC), pressure and a finely divided metal catalyst (Zn, Cu, Ni) increases the degree of saturation, which can be partial or full.


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The advantages of hydrogenating fats and oils:

Changes a liquid oil to a semi-solid or solid, which makes the melting point of an unsaturated fat more like that of a saturated fat.

Decreases the rate of oxidation (stability increases with increasing saturation).

Increases hardness.

Controls the feel and plasticity (stiffness).

The disadvantages of hydrogenating fats and oils:

Mono- and poly-unsaturated fats are healthier for the heart than saturated fats.

In partial hydrogenation, trans fatty acids can form which are hard to metabolize, accumulate in fatty tissue, and are difficult to excrete from the body. They increase levels of LDL (bad) cholesterol and are a low-quality energy source.

F.3 Shelf life A food reaches its shelf life when it no longer maintains the expected quality desired by the

consumer because of changes in flavour, smell, texture and appearance (colour, mass) or because of microbial spoilage.

Chemical factors that cause a decrease in the shelf life include:

Water content: exposure to air can make a food dry and change its texture. This increases the rate of oxidation and leads to rancidity, discolouration of the surface and a general decrease in nutrient value. Alternatively if dry foods become moist they are susceptible to microbial spoilage.

Chemical changes occurring within the food can result in a pH change (e.g. becoming sour), produce undesirable flavours, change colours and lead to a loss of nutrients.

Light provides energy for the photochemical reactions which lead to rancidity, the fading of the colour and the oxidation of nutrients, particularly vitamins.

An increase in temperature increases the rate of reactions which degrades the food. A decrease of temperature slows the rate of these reactions which lead to spoilage.

Rancidity is the production of flavours in lipids that our senses perceive as off, because they have a disagreeable smell, taste, texture or appearance.

Hydrolytic rancidity is due to the breaking down of a lipid into its component fatty acids and glycerol.

It takes place more rapidly in the presence of enzymes (lipase), heat and moisture.


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In deep frying, for example, the water present in food and high temperatures increase the rate of hydrolysis.

Examples of off-flavoured fatty acids include:

butanoic, hexanoic and octanoic acid in rancid milk.

palmitic, stearic and oleic acids give chocolate an oily or fatty flavour.

lauric acid gives palm and coconut oil, in cocoa butter substitutes, a soapy flavour.

butanoic acid in butter.

Oxidative rancidity is due to the oxidation of fatty acid chains, typically by the addition of oxygen across the carbon–carbon double bond of the unsaturated fatty acid. Volatile, unpleasant-tasting aldehydes and carboxylic acids are formed in these free-radical reactions.

The oxidation of unsaturated fats by molecular oxygen, which occurs in air in the absence of enzymes, is called auto-oxidation.

Oily fish, such as mackerel and herring, contain a high proportion of unsaturated fatty acids and are prone to oxidative rancidity. Extensive oxidation can lead to some polymerization with consequent increases in viscosity and browning.

The rate of rancidity can be reduced and the shelf life extended by:


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Processing Packaging reduces contact with oxygen in the air by:

Adding additives:

• refrigeration of dairy products

• reducing light levels during storage

• reducing moisture levels by adding salt or sugar, or smoking

• using an inert gas• hermetic sealing• removing air from

packaging

• Na2SO3, NaHSO3 and citric acid reduce non-enzymic browning

• curing meat with NaNO2 and NaNO3 fixes colour and inhibits micro-organisms

• C6H5COOH and C6H5COONa are antimicrobial agents in fruit juices, carbonated beverages, pickles and sauerkraut

• sorbic acid, propanoic acid, calcium propanoate and sodium propanoate delay mould and bacterial growth in breads and cheeses.

• CH3COOH and C6H5COOH add flavour and delay mould and bacterial growth in pickled meats and fish.

Traditional methods of extending the shelf life include fermentation, preserving, pickling, salting, drying and smoking.

An antioxidant is a substance that delays the onset or slows the rate of oxidation. It is used to extend the shelf life of food.

Naturally occurring antioxidants include:

Vitamin E: a fat-soluble vitamin found in foods such as wheat germ, nuts, seeds, whole grains, green leafy vegetables and vegetable oils like canola and soya bean.

Vitamin C (ascorbic acid): a water-soluble vitamin found in citrus fruits, green peppers, broccoli, green leafy vegetables, strawberries, red currants and potatoes.

β-carotene is found in carrots, squash, broccoli, sweet potatoes, tomatoes, kale, cantaloupe, melon, peaches and apricots.

Selenium is found in fish, shellfish, red meat, eggs, grains, chicken and garlic.

Synthetic antioxidants include BHA, BHT, PG, THBP and TBHQ.

Almost all have phenolic-type structures: that is a hydroxyl group attached to a benzene ring.

The tertiary butyl group, which has three methyl groups bonded to one carbon atom, is found in BHA, BHT and TBHQ.


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The advantages of antioxidants in food: The disadvantages of antioxidants in food:

Some consumers perceive natural antioxidants to be safer because they occur naturally in food.

Naturally occurring vitamins C, E and carotenoids reduce the risk of cancer and heart disease by inhibiting the formation of free radicals.

Vitamin C is vital for the production of hormones and collagen.

β-carotene can be used as an additive in margarine to give colour (yellow) and act as a precursor for vitamin A.

They enhance the health benefits of existing foods and boost overall health and resilience.

Some consumers perceive synthetic antioxidants to be less safe because they are not naturally occurring in food.

Natural antioxidants are more expensive than synthetic antioxidants, can add colour and an aftertaste to food, and can be less effective at slowing down the rate of rancidity.

Synthetic antioxidants need to be regulated by policies and legislation to ensure their safe use in food.

Policies regarding the labelling and safe use of food additives can be difficult to implement and monitor internationally.

Many substances traditionally used in different cultures to promote good health are rich in natural antioxidants.

Examples include: green tea, turmeric, oregano, blueberries, cranberries and dark chocolate, which some claim reduce levels of LDL cholesterol, lower blood sugar levels and blood pressure, and prevent cancer.

F.4 Colour A dye is a food-grade, synthetic, water-soluble colorant.

A pigment is a naturally occurring colorant found in the cells of plants and animals.

Foods have colour because of their ability to absorb light in the visible region of the electromagnetic spectrum. Other colours are reflected or emitted and we see the complementary colour.

Naturally occurring pigments include anthocyanins, carotenoids, chlorophyll and heme.

Anthocyanins The anthocyanins all have very similar three-ring C6C3C6 structures, with conjugated double

bonds, but vary in the number and position of the hydroxyl groups and alkoxy side chains.

They are a sub-class of flavonoids responsible for a range of colours in fruits and vegetables including yellow, red and blue.


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They are the most widely distributed pigment in plants and are present, for example, in strawberries, plums, cranberries, blueberries and raspberries.

Many anthocyanins are red in acidic conditions and turn blue at higher pH.

Carotenoids The carotenoids are derived from a 40-carbon polyene chain, with, in some cases, ring structures at

the ends of the chain.

The colours range from yellow to orange to red. They are present in bananas, carrots, tomatoes, watermelon, red/yellow peppers and saffron.

They are the most widespread pigments in nature with the large majority produced by algae.

They act as a precursor for vitamin A.

Those found in fruit and vegetables contribute 30–100% of the vitamin A requirement in humans.

Red astaxanthin when complexed with protein gives the blue or green hue found in live lobsters and crabs and the pink colour of salmon.

Heme and chlorophyllHeme and chlorophyll both contain a planar heterocyclic porphyrin ring which consists of four pyrrole rings linked by a single bridging carbon atom.

The porphyrin ring acts as a polydentate ligand and forms a dative covalent bond with a central metal ion.

The central ion is Mg2+ in chlorophyll and Fe2+ in heme.

The compounds absorb light in the visible region of the spectrum, as they have alternate single and double (conjugated) bonds.

Chlorophyll is the green pigment responsible for the colour of vegetables.

There are two closely related forms of chlorophyll that have different R groups. Chlorophyll a has a methyl group (CH3) and chlorophyll b has an aldehyde CHO group.

Chlorophyll absorbs the red and blue light needed for photosynthesis.

Heme is the red pigment found in the red blood cells of higher animals.

Myoglobin, an extremely compact heme protein found in muscle tissue, is responsible for the purplish-red colour of meat.

The colour stability of naturally occurring pigments is affected by degree of oxidation, temperature, pH and the presence of metal ions.


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Stability of anthocyanins In aqueous solution anthocyanins exist in four possible structural forms, depending on the pH and

temperature.

They are most stable and highly coloured at low pH and temperature.

They form deeply coloured coordination complexes with Fe3+ and Al3+ ions, when the fruit is exposed to metal cans.

They become less stable when exposed to heat, causing a loss of colour and browning.

Stability of carotenoids The multiple conjugated carbon–carbon double bonds are susceptible to oxidation catalyzed by

light, metals and hydroperoxides, allowing them to behave as antioxidants.

The bread-making properties of flour improve with prolonged storage as carotenoids are bleached to give the bread a more ‘attractive’ whiter crumb.

Oxidation leads to a loss of vitamin A activity and produces ‘off’ odours.

The carotenoids are stable up to 50°C and at pHs in the range 2–7. They are not generally degraded by processing.

The naturally occurring trans isomer rearranges to the cis isomer when heated.

Stability of chlorophyll The green colour of vegetables can fade to yellow and brown as they are cooked, owing to the

thermal instability of chlorophyll, which depends on pH.

When heated in acidic solution, the cell membrane of the plant deteriorates, and Mg2+ ions are replaced by two H+ ions. The COOC20H39 ester group is hydrolyzed to leave a brown colour.

Chlorophyll is more stable in alkaline solution.

This cell degradation caused by heat also makes chlorophyll more susceptible to photo-degradation.


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Stability of heme In muscles heme is associated with the purple–red protein myoglobin molecule, which binds to

oxygen molecules to form the red oxymyoglobin molecule:

Fe2+ is more stable than Fe3+ ion in the non-polar environment provided by the side chains in the complex. The red oxymyoglobin does, however, undergo a slow auto-oxidation reaction to form a Fe3+ complex with an undesirable brown colour known as metmyglobin:

The stability of colour and the rate of brown metmyglobin formation is decreased if the meat is stored in oxygen-free conditions.

Air is removed and meat is stored in CO2.

There are safety concerns over the use of synthetic colorants. Different colorants are permitted in different countries which makes international legislation difficult.

Cooking often causes food to turn brown. The two processes involved are the Maillard reactions and caramelization.

Caramelization occurs with foods of high carbohydrate content and low nitrogen content.

The process of caramelization starts with the melting of the sugar at temperatures above 120°C.

The compounds are dehydrated and double bonds are introduced. The small sugar molecules react together by condensation reactions to produce polymers with conjugated double bonds, which absorb visible light to give brown colours.

Smaller volatile molecules are also formed by fragmentation reactions, which give the food unique flavours and fragrances.

Caramelization produces desirable colours and flavours in bakery goods, coffee, soft drinks, beer and peanuts.

Undesirable effects occur if all the water is removed and carbon is produced: CnH2mOm → nC + mH2O.

The highest rate of browning occurs in fructose, as caramelization starts at a lower temperature.

The rate and products of caramelization can be controlled by pH.

Examples include the browning on the top of baked egg dishes.


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Maillard browning involves a complex series of reactions between amino acids and reducing sugars, usually at increased temperatures.

The first step is the condensation reaction of a reducing sugar, such as glucose, with an amino acid, which leads to the replacement of a C = O in the aldehyde group of the sugar by a C = N–R bond.

The larger the sugar, the slower the reaction with amino acids. Lysine is the amino acid which reacts the fastest and causes darkest colour as it has two amino

groups. Cysteine, with only one amine group and a sulfur group, produces the least colour of the amino

acids. A series of dehydration, fragmentation and condensation reactions result in a complex mixture of

products. The reaction is affected by pH, type of amino acid and sugar, temperature, time, presence of

oxygen, water activity and other food components. Examples include heating sugar and cream to make toffees and fudges.

F.5 Genetically modified foods A genetically modified food is one derived or produced from a genetically modified organism.

The food is either substantially different or essentially the same in composition, nutrition, taste, smell, texture and functional characteristics, to the conventional food.

Benefits of GM foods Concerns about GM foods

• Improved flavour, texture and nutritional value.• Longer shelf life.• GM organisms can be more resistant to disease and pests.• Increased crop yields and feed efficiency in animals.• Increased resistance to herbicides and fungicides.• Environmentally ‘friendly’ bio-herbicides and bio-insecticides

can be produced.• Conservation of soil, water and energy.• Improved waste management.• Increased production of substances which can improve human

health. such as vitamins A and C; anti-cancer substances and vaccines.

• Decreases amounts of unhealthy fats.• GM plants grow in a wider range of climatic conditions.

• Uncertainties about the outcomes of genetic modifications given its relatively recent development.

• Increased allergies (for people involved in the processing of GM foods).

• Risks of changing the composition of a balanced diet by altering the natural nutritional quality of foods.

• Contamination of ‘normal’ crops or the wild population by pollen from GM crops.


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F.6 Texture A dispersed system is a kinetically

stable mixture of one phase in another largely immiscible continuous phase.

Emulsifiers help with the formation of emulsions and foams.

An emulsifier generally has a polar head which is hydrophilic and is attracted to the water, and a non-polar tail which is hydrophobic.

They are soluble in fat and water and act as the interface (surface) between the liquid, solid and gas phases in the dispersed system.

Mechanical energy (beating, mixing) is needed to make an emulsion of oil and water.

Lecithin present in egg yolk is widely used as an emulsifier. It is added to oil and water mixtures to make mayonnaise and other salad dressings.

Stabilizers such as trisodium phosphate, Na3PO4, are added to prevent the emulsions from separating out into the separate phases.


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Documents

1 · Web viewThose found in fruit and vegetables contribute 30–100% of the vitamin A requirement in humans. Red astaxanthin when complexed with protein gives the blue or green hue