Food Science & Technology · Food Science Core · also Food Chemistry, Food Chemistry of Macronutrients, Chemistry of Food Constituents
The four constituents that make up most of any food and how they behave during processing and storage: water and its mobility, sugars and starch, proteins and their functional properties, and lipids and their oxidation.
In the exams: GATE XE-G and XL-U Section 1 (carbohydrates, proteins, lipids); ICAR AIEEA-PG Code 18 Unit I; ICAR AICE 35.b Unit 5 (emulsions, foams, gels, functional properties)
Most of the mass of almost every food is made up of four things: water, carbohydrates, proteins and lipids. Their chemistry decides what a food is like and what happens to it. Water decides whether microbes can grow and how fast reactions run; starch decides whether a sauce thickens and whether bread goes stale; proteins decide whether egg white foams, whether meat is tender and whether a cheese curd forms; lipids decide the texture of chocolate and margarine and, when they oxidise, whether a food goes rancid. Food chemistry is the study of these constituents in foods — their structure, their properties, and the changes they undergo when foods are processed and stored.
The subject is the scientific base of food technology. A process engineer who sets a drying end-point needs sorption isotherms and glass transition; a product developer choosing a thickener needs to know the difference between high- and low-methoxyl pectin, or between waxy and normal starch; a quality manager investigating rancidity needs the mechanism of autoxidation and the meaning of peroxide and acid values; a meat technologist needs post-mortem biochemistry. In each case the practical decision follows directly from molecular structure.
Food chemistry is examined everywhere food science is taught. GATE XE-G and XL-U (Food Technology) give a whole section to food chemistry and nutrition, naming starch gelatinisation and retrogradation, pectic substances and dietary fibre, protein structure, post-mortem changes and tenderisation, lipid rancidity, polymerisation and polymorphism. ICAR AIEEA-PG Code 18 Unit I covers the general chemistry of food constituents, colloids, gels and emulsions and physicochemical changes during processing, and ICAR AICE-JRF/SRF 35.b Unit 5 covers emulsions, foams and gels, functional properties of starch, proteins and lipids, hydrocolloids and the oxidation of fat. This manual covers the major constituents; pigments, flavours, vitamins, minerals, enzymes and browning are covered in the companion subject food-chemistry-minor-constituents-enzymes, and the colloid science of emulsions and foams is taken further in food-structure-encapsulation. Numerical values quoted are typical textbook values and vary with the food, the variety and the method of measurement.
Water is the most abundant constituent of most foods — over 90 per cent of many fruits and vegetables, about 60 to 75 per cent of meat, about 35 to 40 per cent of bread and only a few per cent of dry cereals and milk powder. The oxygen atom carries two lone pairs and two hydrogen atoms at an angle of about 104.5°, so the molecule is strongly polar and can both donate and accept hydrogen bonds; each water molecule can take part in up to four hydrogen bonds in a roughly tetrahedral arrangement. This gives water a high boiling point, high specific heat, high heat of vaporisation and high dielectric constant for so small a molecule.
In foods, water interacts with other constituents in several ways. Ions and ionic groups bind water strongly through ion–dipole interactions; polar groups such as hydroxyl, carbonyl and amide groups hydrogen-bond with it; and non-polar groups cause water to arrange itself around them, the hydrophobic effect, which drives proteins to fold and lipids to cluster. Water is also held physically in capillaries and in the networks of gels and tissues. Older texts call bound water the small fraction held so strongly — for example as a monolayer on polar sites — that it does not freeze at −40 °C and is not available as a solvent; free water behaves much like pure water. The distinction is useful but not sharp: water mobility changes continuously from the most tightly held to the most freely moving.
The total water content of a food does not tell us how available that water is for microbial growth or chemical reactions. Honey, with about 17 per cent water, keeps for years; fresh meat with 70 per cent spoils in days, but a dried sausage with 30 per cent water can be stable. The useful measure is water activity, aw, defined as the ratio of the vapour pressure of water in the food, p, to the vapour pressure of pure water at the same temperature, p0: aw = p/p0. It ranges from 0 to 1. A food in equilibrium with air has aw equal to the equilibrium relative humidity of that air divided by 100.
For ideal dilute solutions, Raoult's law gives aw equal to the mole fraction of water. One mole of sucrose in 1 kg of water (55.5 mol of water) gives aw = 55.5/56.5 ≈ 0.982.
Water activity governs microbial growth. Most spoilage bacteria need aw above about 0.90 to 0.91, many yeasts above about 0.88, most moulds above about 0.80, and Staphylococcus aureus can grow aerobically down to about 0.86. Xerophilic moulds and osmophilic yeasts grow down to about 0.61 to 0.65, and no microbial growth occurs below about 0.6. Water activity also controls chemical reaction rates, as described below, and is the basis of the US definition of low-acid foods (aw above 0.85).
A moisture sorption isotherm plots the equilibrium moisture content of a food against water activity at constant temperature. Most foods give a sigmoid (type II) curve. The low-aw region (roughly 0 to 0.2–0.3) corresponds to water held as a monolayer on the most polar sites; the middle region (about 0.3 to 0.7) to multilayer water; and the high-aw region to water condensed in capillaries and pores and acting as a solvent. Foods high in sugars or salts give curves that stay low and then rise steeply at high aw as the crystals dissolve. The curve obtained when a dry food takes up water (adsorption) usually lies below that obtained when a wet food dries (desorption) — this difference is hysteresis — so a dried food may hold more water at a given aw than the same food wetted from dry.
Isotherms are described by models. The BET equation estimates the monolayer moisture content, often taken as the moisture of best stability for dry foods; the GAB model fits most foods over the range aw 0.05 to about 0.9 and is widely used in drying and packaging calculations.
The food stability map relates aw to reaction rates. Microbial growth starts above about 0.6 and rises steeply. Non-enzymatic browning and many enzymatic reactions show a maximum at intermediate aw (commonly 0.6 to 0.8), falling at higher aw by dilution and at lower aw by lack of mobility. Lipid oxidation is unusual: it is fast at very low aw, has a minimum near the monolayer (about aw 0.2 to 0.3), and rises again at intermediate aw. This is why over-dried foods such as some crisps can go rancid faster than foods dried to the monolayer value.
Many food solids are not crystalline. When a sugar solution is dried or frozen quickly, the molecules do not have time to arrange themselves into a crystal lattice; they are trapped in a disordered, amorphous state. Below a characteristic temperature, the glass transition temperature Tg, this amorphous solid is a glass: hard, brittle and with very low molecular mobility, so that diffusion-limited reactions and physical changes are extremely slow. Above Tg the material becomes rubbery or leathery, and further above, a viscous liquid. Viscosity falls by many orders of magnitude over a few tens of degrees above Tg.
Their crispness, stickiness, caking and collapse all depend on whether they are below or above their Tg. A crisp biscuit that has absorbed moisture goes soft because water has lowered its Tg below room temperature; spray-dried powders cake when they pass through Tg in humid storage; freeze-dried foods collapse if the drying temperature exceeds the glass transition of the freeze-concentrated phase.
Water is a powerful plasticiser of food solids: it has a very low glass transition temperature (about −135 °C), and adding it lowers the Tg of the mixture. For a two-component system the Gordon–Taylor equation estimates the mixture's Tg: Tg = (w1 Tg1 + k w2 Tg2)/(w1 + k w2), where w1 and w2 are the mass fractions of solid and water, Tg1 and Tg2 their glass transition temperatures, and k a constant for the system. Because Tg falls steeply with the first few per cent of water, a dry powder that is glassy at room temperature can become rubbery after picking up only a little moisture.
Anhydrous sugars have very different Tg values: fructose and glucose are low, sucrose and lactose intermediate, and maltodextrins high, rising with molecular weight. This is why maltodextrins are added as drying aids to fruit juices and other sugar-rich products: they raise the Tg of the mixture and reduce stickiness in spray dryers.
Above Tg, the rates of diffusion-controlled changes increase much faster than the Arrhenius equation predicts. The Williams–Landel–Ferry (WLF) equation describes this region: log(η/ηg) = −C1(T − Tg)/(C2 + T − Tg), where η is viscosity (or relaxation time) and ηg its value at Tg; the 'universal' constants C1 = 17.44 and C2 = 51.6 K are often used when specific values are not known. The equation explains why stability depends on the temperature difference T − Tg rather than on temperature alone, and why storage a few degrees below Tg can be far more protective than storage a few degrees above.
Freezing creates a related situation. As ice forms, the remaining solution becomes more concentrated until, at the maximally freeze-concentrated state, it becomes a glass at Tg′, with an unfrozen water content Wg′. Below Tg′ the concentrated phase is glassy and deterioration slows greatly; above it, recrystallisation of ice and other changes proceed. Tg′ values for many foods lie between about −10 and −45 °C depending on composition, which helps explain why frozen foods are commonly stored at −18 °C or below and why ice cream, rich in low-molecular-weight sugars with low Tg′, is prone to ice recrystallisation and coarse texture when storage temperature fluctuates. The glass transition concept complements water activity: aw describes the equilibrium availability of water, while Tg describes molecular mobility, and both are needed to predict stability.
Carbohydrates are polyhydroxy aldehydes or ketones and their derivatives. Monosaccharides are the simple sugars. They are classified by the number of carbon atoms (trioses, tetroses, pentoses, hexoses) and by their carbonyl group: aldoses carry an aldehyde group and ketoses a ketone group. Glucose is an aldohexose, fructose a ketohexose, galactose an aldohexose epimeric with glucose at carbon 4; ribose and xylose are pentoses.
In solution, sugars with five or more carbons exist mostly as rings formed when a hydroxyl group reacts with the carbonyl group: six-membered pyranose rings and five-membered furanose rings. Ring closure creates a new chiral centre, the anomeric carbon, with two forms, α and β. In solution the α and β forms interconvert through the open-chain form until an equilibrium is reached; the resulting change in optical rotation is called mutarotation. For glucose, the equilibrium mixture is about one-third α and two-thirds β.
Sugars whose anomeric carbon is free can open to the aldehyde or ketone form and reduce mild oxidising agents such as Fehling's or Benedict's reagent; they are reducing sugars. Reducing sugars take part in the Maillard reaction with amino groups, which is why glucose, fructose, lactose and maltose brown more readily than sucrose. Sugar alcohols (polyols) such as sorbitol, mannitol, xylitol and maltitol are formed by reducing the carbonyl group; they are non-reducing, do not take part in Maillard browning and are used as bulk sweeteners and humectants.
Disaccharides consist of two monosaccharides joined by a glycosidic bond. Sucrose, the common sugar of cane and beet, is glucose and fructose joined between their anomeric carbons (α-1,2 to β); because neither anomeric carbon is free, sucrose is non-reducing. Lactose, the sugar of milk, is galactose β-1,4 linked to glucose, and maltose, produced from starch by amylases, is glucose α-1,4 linked to glucose; both are reducing.
Hydrolysis of sucrose by acid or by the enzyme invertase gives an equimolar mixture of glucose and fructose called invert sugar. Because the optical rotation changes from positive (sucrose) to negative (the mixture, dominated by strongly laevorotatory fructose), the process is called inversion. Invert sugar is sweeter than sucrose, more soluble and more hygroscopic, resists crystallisation and is used in confectionery, jams and baked goods; inversion occurs naturally when sucrose is boiled with acid fruit in jam making.
Relative sweetness depends on concentration, temperature and the other ingredients, so published figures vary. Taking sucrose as 1.0, fructose is usually quoted as sweeter (roughly 1.2 to 1.8), glucose less sweet (about 0.6 to 0.8), and lactose much less sweet (about 0.2 to 0.4). Lactose is also poorly soluble and crystallises readily, producing a sandy texture in sweetened condensed milk and ice cream if not controlled, and it is digested only in people who keep the enzyme lactase into adulthood.
Oligosaccharides contain about three to ten monosaccharide units. Raffinose (a trisaccharide) and stachyose (a tetrasaccharide), found in beans and other legumes, contain α-galactosidic bonds that human digestive enzymes cannot break; they pass to the colon, where bacteria ferment them, producing gas and the flatulence associated with legumes. Soaking, germination, fermentation and enzyme treatment reduce them. Fructo-oligosaccharides and galacto-oligosaccharides resist digestion in the same way and are used as prebiotics that selectively feed beneficial gut bacteria.
Starch hydrolysis produces a family of syrups characterised by their dextrose equivalent (DE), the reducing power expressed as a percentage of that of pure glucose on a dry basis. Starch itself has a DE close to zero, maltodextrins are commonly defined as having DE below 20, glucose (corn) syrups DE 20 and above, and pure dextrose DE 100. As DE rises, sweetness, hygroscopicity, browning, osmotic pressure and freezing-point depression increase, while viscosity, Tg and the ability to prevent crystallisation and to bind and bulk fall. High-fructose syrups are made by enzymatically isomerising part of the glucose in a glucose syrup to fructose with glucose isomerase.
Starch is the main storage carbohydrate of plants and the largest source of energy in human diets. It is made of two glucose polymers. Amylose is essentially linear, with glucose units linked α-1,4, and has a molecular weight in the hundreds of thousands to about a million; it forms helices that can enclose iodine (giving a deep blue colour) and lipids. Amylopectin is very large and highly branched: α-1,4 chains joined by α-1,6 branch points that make up roughly 4 to 5 per cent of the linkages; it gives a red-purple colour with iodine. Most normal starches contain about 20 to 30 per cent amylose. Waxy starches, from waxy maize, rice or other mutants, are almost entirely amylopectin, while high-amylose starches contain 50 per cent or more amylose.
Starch is laid down in granules whose size and shape are typical of the plant source: large oval potato granules, small polygonal rice granules and the bimodal lenticular and spherical granules of wheat. Granules are semi-crystalline: the outer branches of amylopectin form double helices packed in crystalline layers alternating with amorphous layers containing branch points and amylose. X-ray diffraction distinguishes A-type crystallinity (typical of cereal starches), B-type (tuber starches such as potato and high-amylose starches) and C-type, a mixture found in many legume starches. Under polarised light intact granules show birefringence, a characteristic Maltese cross.
Starch granules are insoluble in cold water, but when they are heated in excess water they absorb water and swell, and above a characteristic temperature range their order is lost: crystallites melt, birefringence and the Maltese cross disappear, amylose leaches out and the granules swell many times their original volume. This irreversible process is gelatinisation. The temperature range depends on the starch source and is commonly between about 55 and 80 °C. Gelatinisation is shifted to higher temperatures by sugars (which compete for water and is why cakes set late in baking), and by limited water, as in biscuits and low-moisture doughs, where it may be incomplete.
Continued heating and stirring of the swollen granules is called pasting. It is followed with viscosity-recording instruments such as the Rapid Visco Analyser or the older amylograph. The pasting curve shows the pasting temperature at which viscosity begins to rise, a peak viscosity when swollen granules fill the volume, a fall in viscosity (breakdown) as granules rupture under heat and shear, and, on cooling, a rise to a final viscosity; the increase on cooling is called setback and reflects re-association of amylose. Potato starch gives a high peak and strong breakdown; cross-linked starches resist breakdown; waxy starches give clear, cohesive pastes with little setback.
When gelatinised starch cools and stands, its chains re-associate into ordered, partly crystalline structures. This is retrogradation. Amylose retrogrades quickly, within hours, forming the gel network of a cooled starch paste and contributing to the firmness of freshly baked bread crumb; amylopectin retrogrades slowly over days and is the main cause of bread staling — the firming of the crumb. Retrogradation is fastest at refrigerator temperatures, around 0 to 4 °C, which is why bread stales faster in a refrigerator than at room temperature, and is slowed by freezing. Retrogradation also causes syneresis, the weeping of liquid from starch gels, and loss of clarity.
Some starch escapes digestion in the small intestine and is called resistant starch. Types include physically inaccessible starch in whole grains (RS1), native granules resistant to amylase such as raw potato and green banana starch (RS2), retrograded starch formed when cooked starchy foods are cooled (RS3), chemically modified starches (RS4) and amylose–lipid complexes (RS5).
Native starches have limitations — breakdown under heat, shear and acid, retrogradation and syneresis — so modified starches are made for industry. Pregelatinised starches thicken without cooking; acid-thinned starches give low-viscosity hot pastes and strong gels for gum confectionery; cross-linked starches resist heat, shear and low pH; substituted starches, such as acetylated and hydroxypropylated starches, resist retrogradation and are freeze–thaw stable; oxidised starches give clear, soft gels.
Cellulose is the main structural polysaccharide of plant cell walls and the most abundant organic compound on earth. It is a linear polymer of glucose with β-1,4 links. The β linkage makes each chain flat and ribbon-like, so chains pack side by side through extensive hydrogen bonding into crystalline microfibrils that are insoluble in water and resist human digestive enzymes. Chemically modified celluloses are widely used as food additives: carboxymethylcellulose is a soluble thickener and stabiliser; methylcellulose and hydroxypropyl methylcellulose are unusual in forming gels on heating and melting on cooling, which makes them useful in fried coatings, gluten-free baking and plant-based meat analogues.
Hemicelluloses are a diverse group of cell-wall polysaccharides with shorter, often branched chains — xylans, arabinoxylans, glucomannans and mixed-linkage β-glucans. Cereal arabinoxylans hold large amounts of water and affect dough rheology and bread quality. Mixed-linkage (1→3),(1→4)-β-glucans of oats and barley form viscous solutions in the gut and are associated with lowering of blood cholesterol, the basis of health claims for oat products in several jurisdictions.
Pectins are cell-wall and middle-lamella polysaccharides abundant in citrus peel and apple pomace, the main commercial sources. The backbone is chiefly galacturonic acid linked α-1,4 (the homogalacturonan or 'smooth' region), interrupted by rhamnose-containing 'hairy' regions carrying neutral sugar side chains. Some of the carboxyl groups of galacturonic acid are esterified with methanol. The degree of esterification (DE, or degree of methylation) divides commercial pectins into two classes. High-methoxyl (HM) pectins, with more than 50 per cent of carboxyl groups esterified, form gels only at high soluble solids (commonly above about 55 to 65 per cent sugar) and low pH (commonly about 2.8 to 3.5): the sugar reduces water availability and the acid suppresses charge on the remaining carboxyl groups, so chains associate through hydrogen bonds and hydrophobic interactions. This is the gel of traditional jams and jellies.
Low-methoxyl (LM) pectins, with less than 50 per cent esterified, gel in the presence of calcium ions, which bridge the free carboxyl groups of neighbouring chains in the 'egg-box' arrangement; they can gel with little or no sugar and are used in reduced-sugar jams and dairy desserts. Amidated LM pectins are made by treating pectin with ammonia and are more tolerant of calcium variation. In plant tissue, pectin changes underlie softening during ripening (pectin solubilisation by enzymes such as polygalacturonase and pectin methylesterase), firming of vegetables by low-temperature blanching with calcium, and the cloud stability of juices, which pectinases are used to clarify.
Hydrocolloids are polysaccharides and some proteins that disperse in water to give viscosity or gels. Seed galactomannans — guar gum and locust bean gum — are excellent thickeners; guar is cold-soluble and gives high viscosity, while locust bean gum needs heating and interacts synergistically with xanthan and κ-carrageenan to form gels. Xanthan gum, produced by fermentation with Xanthomonas campestris, gives highly shear-thinning (pseudoplastic) solutions whose viscosity is stable over wide ranges of pH, temperature and salt, ideal for suspending particles in dressings and sauces. Seaweed polysaccharides include agar, which forms gels that set at about 35 to 40 °C and melt only near 85 °C (large thermal hysteresis); alginates from brown seaweeds, which gel cold with calcium through egg-box binding of their guluronic acid blocks; and carrageenans from red seaweeds, of which kappa forms firm gels with potassium, iota soft elastic gels with calcium, and lambda does not gel. Gum arabic, an exudate of Acacia, gives low viscosity even at high concentration and is a valued emulsifier for beverage flavour emulsions.
Dietary fibre is the part of plant food that resists digestion and absorption in the human small intestine. The Codex definition (2009) covers carbohydrate polymers with ten or more monomeric units that are not hydrolysed by endogenous enzymes, and leaves to national authorities whether to include polymers with three to nine units. Fibre is commonly divided into insoluble fibre (cellulose, many hemicelluloses, lignin), which increases faecal bulk, and soluble, viscous or fermentable fibre (pectins, β-glucans, gums, inulin), which slows glucose absorption, can lower blood cholesterol and is fermented by colonic bacteria to short-chain fatty acids.
Proteins are polymers of amino acids. Each standard amino acid has a central carbon carrying an amino group, a carboxyl group, a hydrogen atom and a side chain (R group); all except glycine are chiral, and proteins contain the L-forms. The twenty common amino acids are grouped by side chain: non-polar (alanine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, proline), polar uncharged (serine, threonine, cysteine, tyrosine, asparagine, glutamine; glycine, whose side chain is a single hydrogen atom, is placed in either group by different texts), acidic (aspartic and glutamic acids) and basic (lysine, arginine, histidine). Cysteine is important in foods because two cysteine residues can be oxidised to form a disulphide bond, cross-linking chains; this governs gluten network formation and the gelation of many proteins.
Nine amino acids are essential for adult humans because the body cannot make them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. In foods the limiting amino acid — the essential amino acid present in least amount relative to need — determines protein quality: cereals are typically limited by lysine and legumes by the sulphur amino acids methionine and cysteine, so cereal–legume combinations complement each other.
In solution amino acids exist mainly as zwitterions, with a positively charged amino group and a negatively charged carboxyl group. The pH at which the net charge is zero is the isoelectric point, pI. Proteins also have isoelectric points, determined by all their ionisable side chains, and at the pI their solubility is usually lowest because the lack of net charge reduces repulsion between molecules. Casein, with a pI near 4.6, precipitates when milk is acidified to that pH — the basis of acid curd and of yoghurt and cottage cheese formation.
Amino acids are joined by peptide bonds between the carboxyl group of one and the amino group of the next, with loss of water. The peptide bond has partial double-bond character, so it is planar and rigid, limiting the conformations the chain can adopt.
Protein structure is described at four levels. Primary structure is the sequence of amino acids. Secondary structure is the regular local folding stabilised by hydrogen bonds between backbone groups: the α-helix and the β-pleated sheet, with turns and random coils between them. Tertiary structure is the overall three-dimensional folding of the chain, stabilised by hydrophobic interactions (non-polar side chains buried away from water), hydrogen bonds, ionic bonds (salt bridges) and covalent disulphide bonds. Quaternary structure is the association of two or more chains, as in haemoglobin or the soybean storage protein glycinin.
Food proteins differ greatly in shape. Globular proteins, such as egg albumin, whey proteins and most enzymes, are compact and soluble; fibrous proteins, such as collagen, keratin and the myosin tail, are elongated and often insoluble. Caseins are unusual: they have little ordered secondary structure, are rich in proline and phosphoserine, and associate with calcium phosphate into large colloidal micelles. Structure determines how each protein behaves when food is processed.
Protein in foods is usually estimated from nitrogen. The Kjeldahl method digests the sample in sulphuric acid with a catalyst, converting organic nitrogen to ammonium sulphate; the ammonia is then distilled into acid and titrated. The Dumas method burns the sample and measures the nitrogen gas released. Nitrogen is converted to protein with a factor: the general factor 6.25 assumes protein contains 16 per cent nitrogen, while specific factors are used for some foods — commonly about 6.38 for milk and 5.7 for wheat flour. A sample containing 2.4 per cent nitrogen therefore contains 15.0 per cent protein by the general factor. Because the methods measure all nitrogen, non-protein nitrogen such as urea, free amino acids and nucleic acids is counted as protein, and deliberate addition of nitrogen-rich adulterants such as melamine has been used to inflate apparent protein — a well-known case of food fraud that led to wider use of confirmatory methods.
The classical Osborne classification groups proteins by their solubility. Albumins dissolve in water; globulins in dilute salt solutions but not in pure water; prolamins in aqueous alcohol (about 70 per cent ethanol); and glutelins only in dilute acid or alkali. The scheme was developed for cereal and seed proteins and is still used: the prolamins of wheat are gliadins, of maize zein, of barley hordein and of rye secalin; the glutelin of wheat is glutenin. Proteins can also be classified by shape (globular and fibrous), by composition (simple proteins, and conjugated proteins carrying a non-protein group, such as glycoproteins, lipoproteins, phosphoproteins like casein, and haem proteins like myoglobin), and by function (enzymes, storage, structural, contractile and transport proteins). Solubility classes overlap and depend on the extraction conditions, so Osborne fractions are operational rather than chemical categories; they remain useful because they map onto how the proteins behave in processing — for example, the alcohol-soluble prolamins and acid-soluble glutelins of wheat together form gluten when flour is mixed with water.
Cow's milk contains about 3 to 3.5 per cent protein, of which roughly 80 per cent is casein and 20 per cent whey (serum) protein. The caseins (αs1, αs2, β and κ) are phosphoproteins that, with calcium phosphate, form colloidal micelles; κ-casein on the micelle surface stabilises it, and its cleavage by chymosin (rennet) causes coagulation in cheese making, while acidification to pH 4.6 causes acid coagulation. The main whey proteins are β-lactoglobulin and α-lactalbumin; being globular, they are denatured by heat above about 70 °C, which is exploited in yoghurt manufacture and whey-protein gels. Egg white is about 10 per cent protein: ovalbumin (the most abundant, a heat-setting phosphoglycoprotein), ovotransferrin (conalbumin, which binds iron and copper), ovomucoid (a heat-stable trypsin inhibitor and important allergen) and lysozyme (an antibacterial enzyme). Egg yolk proteins are mostly lipoproteins, the low-density lipoproteins being key emulsifiers in mayonnaise.
Muscle proteins fall into three groups. Myofibrillar proteins — mainly myosin and actin, with tropomyosin and troponin — are soluble in salt solutions and are responsible for contraction and, in meat products, for water binding, emulsification and gel formation. Sarcoplasmic proteins are water-soluble and include glycolytic enzymes and myoglobin, the pigment that gives meat its colour. Stromal (connective-tissue) proteins, mainly collagen and elastin, are insoluble; collagen is converted to soluble gelatin by prolonged moist heating, which is why slow, moist cooking tenderises tough cuts.
Wheat gluten is the protein complex formed when gliadins and glutenins are hydrated and mixed. Gliadins, mostly single-chain molecules, give extensibility and viscosity; glutenins, large polymers linked by intermolecular disulphide bonds, give elasticity and strength. Together they form the viscoelastic network that holds gas in bread dough, so flour quality for bread-making depends on gluten quantity and quality. Gluten also triggers coeliac disease in susceptible people.
Legume and oilseed proteins are mainly globulins. Soybean protein is dominated by two storage globulins, glycinin (the 11S fraction) and β-conglycinin (7S), whose balance affects tofu texture and gel properties; soy protein is made into flours, concentrates (about 70 per cent protein) and isolates (about 90 per cent protein), and textured by extrusion. Pulses such as pea, chickpea and lentil provide protein isolates increasingly used in plant-based foods. Cereal proteins are relatively poor in lysine, while legume proteins are rich in lysine but limited in methionine and cysteine. Plant proteins often carry anti-nutritional factors, such as trypsin inhibitors and lectins in raw legumes, that are inactivated by adequate heat treatment.
Denaturation is any change in the native secondary, tertiary or quaternary structure of a protein that does not break peptide bonds. The chain unfolds, buried hydrophobic groups and reactive side chains become exposed, and properties change: solubility usually falls, viscosity rises, enzymes lose activity, and digestibility often improves because the unfolded chain is more accessible to proteases. Denaturation may be reversible if the stress is mild and removed, but in foods it is usually followed by aggregation and is effectively irreversible.
Agents of denaturation include heat (the most important in food processing), extremes of pH, organic solvents, detergents, high concentrations of urea or some salts, high hydrostatic pressure, mechanical shear and exposure to interfaces such as air bubbles and oil droplets. Each protein has a characteristic denaturation temperature: many whey proteins unfold around 70 to 80 °C, ovalbumin around 80 to 85 °C, while some proteins, such as ovomucoid and certain enzymes, are notably heat-stable. Stability depends on conditions: denaturation temperature is often raised by sugars and polyols and changed by pH and salts. Salts follow the Hofmeister series: some ions (such as sulphate) stabilise proteins and promote 'salting out' at high concentration, while others (such as thiocyanate) destabilise them. At low ionic strength, small amounts of salt increase protein solubility — 'salting in'.
Functional properties are the physicochemical properties of proteins that decide how they behave in foods. Solubility is often the first requirement, because many other functions depend on it; it is lowest near the isoelectric point and is changed by pH, ionic strength and temperature. Water-holding (or water-binding) capacity — the ability to retain water against gravity, pressing or heating — matters for the juiciness of meat and the yield of meat products, and is improved by salt and phosphates that increase the net charge and swell myofibrils.
Gelation is the formation of a three-dimensional network that holds water. Heat-set gels form when globular proteins such as egg white, whey and soy proteins unfold on heating and aggregate through hydrophobic interactions and disulphide bonds; near the isoelectric point or at high salt, aggregation is rapid and the gels are opaque and coarse (particulate gels), while away from the pI and at low salt, gels are fine-stranded and translucent. Cold-set gels form on cooling, as in gelatin, whose chains form triple-helix junction zones and melt again near body temperature — the source of gelatin's 'melt in the mouth' character. Gels can also be formed by acid (yoghurt), enzymes (rennet curd, transglutaminase cross-linking of fish and meat proteins in surimi and restructured products) or added calcium (tofu with calcium or magnesium salts).
Proteins are amphiphilic: they contain both hydrophobic and hydrophilic regions, so they adsorb at oil–water and air–water interfaces, unfold partly, and form a viscoelastic film around droplets or bubbles. This film lowers interfacial tension and protects droplets and bubbles from coalescence by steric and electrostatic repulsion. Emulsifying properties are measured as emulsifying activity (the interfacial area stabilised per gram of protein) and emulsion stability; flexible proteins such as caseins and egg-yolk lipoproteins are excellent emulsifiers, which is why milk, cream and mayonnaise are stable emulsions. Emulsions are least stable near the protein's pI.
Foaming properties are measured as foam overrun (the volume increase on whipping) and foam stability (how slowly liquid drains and bubbles collapse). Egg white is the classic foaming protein: whipping denatures proteins at the bubble surface, and the film is strengthened by ovalbumin's aggregation on heating, which sets meringues and cakes. Small amounts of fat or egg yolk spoil egg-white foams because lipids compete for the interface and break the protein film; sugar stabilises foams by raising viscosity but slows their formation; acid (cream of tartar) and, traditionally, copper bowls improve stability, the latter because copper forms a stabilising complex with ovotransferrin. Over-whipping causes excessive denaturation and a dry, coarse foam that collapses.
After slaughter, blood circulation stops and the muscle is cut off from oxygen, but its cells go on using energy. ATP is regenerated for a while from creatine phosphate and then by anaerobic glycolysis, which breaks down glycogen to lactic acid. Because lactic acid cannot be removed, the pH of the muscle falls from about 7.0–7.2 in living muscle to an ultimate pH of about 5.4 to 5.8 in normal meat, over several hours in beef and lamb and faster in pork and poultry. When ATP is exhausted, myosin and actin bind permanently into actomyosin, and the muscle becomes stiff and inextensible: this is rigor mortis. Meat cooked in full rigor is tough; rigor is followed by a gradual tenderisation during ageing as the structure is broken down by enzymes.
The rate and extent of pH fall decide meat quality. Pale, soft and exudative (PSE) meat, seen mainly in pork, results when pH falls rapidly while the carcass is still warm, often in stress-susceptible pigs (associated with a mutation in the ryanodine receptor gene, the 'halothane gene') or after acute stress just before slaughter; the combination of low pH and high temperature denatures muscle proteins, which then hold water poorly and scatter light. Dark, firm and dry (DFD) meat, seen in beef (dark cutters) and pork, results when glycogen is depleted by prolonged stress, fasting or exhaustion before slaughter, so little lactic acid forms and the ultimate pH stays high (above about 6.0); such meat holds water well but is dark and spoils quickly because the high pH favours bacteria.
The temperature of muscle during rigor development strongly affects tenderness. If pre-rigor muscle — still containing ATP — is chilled very rapidly to below about 10 to 15 °C, it contracts strongly, because cold causes calcium to be released into the cytoplasm while ATP is still present. This cold shortening shortens the sarcomeres and makes meat markedly tougher; it is a particular problem in lamb and beef carcasses with little fat cover chilled quickly. Thaw rigor is an extreme contraction that occurs when muscle frozen before rigor is thawed. Very high temperatures during rigor can also cause shortening (heat or rigor shortening) and PSE-like conditions.
Electrical stimulation of carcasses shortly after slaughter accelerates glycolysis and pH fall, so the muscle passes into rigor before it is cold enough to cold-shorten; it also tears some fibres and helps tenderisation.
Ageing (conditioning) — holding meat chilled for days to weeks after rigor — makes it more tender and develops flavour. The main agents are endogenous proteases. The calpain system, especially μ-calpain (calpain-1), activated by calcium, degrades key myofibrillar proteins such as desmin, titin and troponin T, weakening the structure at the Z-lines; calpastatin is the natural inhibitor of calpains, and its level helps explain why meat from some breeds, such as Bos indicus cattle, tends to be tougher. Lysosomal cathepsins and the proteasome also contribute.
Tenderness also depends on connective tissue: collagen content and cross-linking increase with animal age and vary between muscles, and collagen toughness is reduced not by ageing but by moist cooking that converts collagen to gelatin. Artificial tenderisation methods include plant proteases — papain from papaya, bromelain from pineapple and ficin from figs — applied as powders, marinades or injections; calcium chloride injection, which activates calpains; mechanical methods such as blade tenderising, pounding and grinding; and marinating in acid, which swells and partly solubilises connective tissue. Plant proteases act strongly on connective tissue and myofibrils and can produce a mushy texture if overused.
Lipids are a chemically diverse group defined by solubility in non-polar solvents. A common classification divides them into simple lipids (esters of fatty acids with alcohols — triacylglycerols, the main component of fats and oils, and waxes), compound lipids (containing additional groups — phospholipids such as lecithin, and glycolipids) and derived lipids (fatty acids, sterols such as cholesterol and phytosterols, fat-soluble vitamins and carotenoids). Triacylglycerols make up about 95 per cent or more of most food fats and oils. Phospholipids are amphiphilic and are important emulsifiers and constituents of membranes.
Fatty acids are named by systematic, common or shorthand names. The shorthand gives the number of carbon atoms and double bonds, with the position of the first double bond counted from the methyl end (n- or omega notation): oleic acid is 18:1 n-9 (cis-9-octadecenoic acid), linoleic 18:2 n-6, α-linolenic 18:3 n-3, eicosapentaenoic acid (EPA) 20:5 n-3 and docosahexaenoic acid (DHA) 22:6 n-3; stearic 18:0 and palmitic 16:0 are saturated, lauric acid is 12:0 and butyric acid 4:0. The delta (Δ) system counts from the carboxyl end. Natural unsaturated fatty acids are mostly cis, with methylene-interrupted double bonds; trans fatty acids arise in ruminant fats and, in much larger amounts, from partial hydrogenation of oils, and are restricted in many countries because of their effects on cardiovascular risk. Linoleic and α-linolenic acids are essential in the human diet.
In a triacylglycerol, the three positions of glycerol are distinguished by stereospecific numbering as sn-1, sn-2 and sn-3. The distribution of fatty acids among these positions affects melting, crystallisation and nutrition; for example, palmitic acid is located mainly at sn-2 in human milk fat but at sn-1 and sn-3 in most vegetable oils.
The melting point of a fatty acid rises with chain length and falls with unsaturation; a cis double bond introduces a bend that prevents close packing and lowers the melting point sharply, whereas a trans double bond keeps the chain nearly straight, so elaidic acid (trans-18:1) melts much higher than oleic acid. Natural fats are mixtures of many triacylglycerols and do not melt at one temperature but over a range. The proportion of solid fat at each temperature, the solid fat content (SFC), is measured by pulsed nuclear magnetic resonance and determines texture: a margarine must be spreadable from the refrigerator yet not oily at room temperature, chocolate must be hard at room temperature and melt quickly in the mouth, and a shortening must have a wide plastic range for creaming and baking. Fats are modified to obtain the desired SFC profile by blending, hydrogenation, interesterification and fractionation, as covered in fats-oils-technology.
Fats crystallise in more than one crystal form, a property called polymorphism. The three main forms of triacylglycerols are α (hexagonal sub-cell packing, the least stable, with the lowest melting point, formed on rapid cooling), β′ (orthorhombic, intermediate stability, small needle-like crystals) and β (triclinic, the most stable, with the highest melting point and the largest crystals). Transformations proceed from less stable to more stable forms (α → β′ → β) and are not reversible except by melting. The form present has large effects on texture: β′ crystals give smooth, creamy margarines, shortenings and spreads and good aeration in creaming, whereas transformation to β gives large, grainy crystals and a sandy texture. Fats of varied fatty-acid composition, such as cottonseed, palm and milk fat, tend to stay β′, while fats with uniform triacylglycerols, such as soybean, sunflower and canola after hydrogenation, tend to transform to β.
Cocoa butter, dominated by a few symmetrical triacylglycerols (POP, POS and SOS, where P is palmitic, O oleic and S stearic), has six polymorphs, numbered I to VI. Form V, melting at about 33 to 34 °C, gives chocolate its gloss, snap and clean melt in the mouth. Tempering — a controlled sequence of cooling, gentle reheating and working — produces stable form V seed crystals so that the whole mass crystallises in that form. Untempered or poorly stored chocolate develops fat bloom, a dull, greyish surface, when fat crystals transform to form VI or migrate and recrystallise at the surface; sugar bloom, by contrast, results from moisture dissolving surface sugar that then recrystallises.
Lipid oxidation is the main chemical cause of quality loss in fat-containing foods: it produces rancid off-flavours, loss of essential fatty acids and fat-soluble vitamins, colour changes and potentially harmful compounds. Autoxidation, the reaction of unsaturated fatty acids with atmospheric (triplet) oxygen, proceeds by a free-radical chain mechanism in three stages. In initiation, a hydrogen atom is removed from a carbon next to a double bond, forming a lipid alkyl radical (R•); this step is promoted by heat, light, transition metals such as iron and copper, and existing hydroperoxides. In propagation, R• reacts very rapidly with oxygen to give a peroxyl radical (ROO•), which abstracts hydrogen from another fatty acid to form a hydroperoxide (ROOH) and a new R•, so the chain continues. In termination, radicals combine to form non-radical products.
The bis-allylic hydrogens between two double bonds (in linoleic and linolenic acids) are much easier to remove than allylic hydrogens in oleic acid, so oxidation rates rise steeply with the number of double bonds: linoleate oxidises roughly ten or more times faster than oleate, and linolenate faster still; exact ratios quoted vary between sources. Autoxidation typically shows an induction period, during which little change is detected while antioxidants are consumed and radicals accumulate, followed by rapid oxidation. Hydroperoxides themselves are tasteless, but they decompose into volatile secondary products — aldehydes such as hexanal and 2,4-decadienal, ketones, alcohols and acids — that cause rancid, painty, cardboard and fishy flavours at very low concentrations.
Photo-oxidation occurs when light energy is absorbed by a photosensitiser — chlorophyll in vegetable oils, riboflavin in milk, myoglobin in meat — which transfers energy to oxygen and forms singlet oxygen, an excited, highly reactive form. Singlet oxygen adds directly to the double bonds of unsaturated fatty acids, forming hydroperoxides that include isomers not produced by autoxidation, at a rate much greater (often quoted as about a thousand times or more) than triplet oxygen. Because it is not a radical chain reaction, it has no induction period and is not prevented by ordinary radical-scavenging antioxidants; it is controlled by excluding light (opaque packaging), removing sensitisers and adding singlet-oxygen quenchers such as carotenoids and tocopherols. Light-induced off-flavour in milk in clear containers is a familiar example.
Enzymatic oxidation is catalysed by lipoxygenases, iron-containing enzymes in legumes, cereals and many vegetables, which oxidise fatty acids with a cis,cis-1,4-pentadiene structure (such as linoleic acid) to specific hydroperoxides. They cause beany and grassy flavours in soy products and off-flavours in unblanched frozen vegetables, but they are also used deliberately: lipoxygenase in soy flour bleaches carotenoid pigments in bread flour and improves dough. Blanching inactivates lipoxygenase before freezing.
Antioxidants slow oxidation by several mechanisms. Primary or chain-breaking antioxidants donate a hydrogen atom to peroxyl radicals, forming a stable antioxidant radical that does not continue the chain; examples are the tocopherols, the synthetic phenolics BHA, BHT, TBHQ and propyl gallate, and natural phenolics in rosemary extract and green tea. Secondary or preventive antioxidants act indirectly: chelating agents such as citric acid, phosphates and EDTA bind pro-oxidant metals; oxygen scavengers such as ascorbic acid and its fat-soluble ester ascorbyl palmitate remove oxygen and regenerate tocopherols; singlet-oxygen quenchers such as carotenoids deactivate singlet oxygen. Synergists improve the action of primary antioxidants. Packaging under vacuum or nitrogen, oxygen scavengers, low temperature, protection from light and avoidance of metal contamination are equally important.
Oxidation is measured by primary and secondary indices. Peroxide value (PV) measures hydroperoxides, usually in milliequivalents of active oxygen per kilogram of fat, by iodometric titration; it rises during early oxidation and may fall later as hydroperoxides break down, so a low PV alone does not prove a fat is fresh. The p-anisidine value (AV) measures aldehydes, particularly unsaturated ones, and the total oxidation (totox) value is calculated as 2PV + AV. The thiobarbituric acid reactive substances (TBARS) test measures malonaldehyde and related compounds and is common for meats. Hexanal measured by gas chromatography is a sensitive marker of linoleate oxidation. Oxidative stability is tested by accelerated methods such as the Rancimat or oxidative stability index, which record the induction period of a fat heated with air flowing through it.
Hydrolytic rancidity is the release of free fatty acids (FFA) from triacylglycerols by water, catalysed by lipases — from the raw material (for example in milk, cereals and oilseeds), from microorganisms, or by heat and moisture during frying. In fats containing short-chain fatty acids, such as milk fat, which contains butyric (4:0), caproic (6:0) and caprylic (8:0) acids, the released acids are volatile and have strong rancid, cheesy odours at low concentration. In lauric fats such as coconut and palm kernel oils, release of lauric acid gives a soapy flavour. In long-chain fats, FFA are less odorous but lower the smoke point of frying oil and are a quality index. Hydrolytic rancidity in rice bran, wheat germ and whole-grain flours is controlled by heat stabilisation to inactivate lipases; in milk, by prompt cooling, gentle handling and pasteurisation.
Several classical constants describe fats. The acid value (AV) is the milligrams of potassium hydroxide needed to neutralise the FFA in one gram of fat; FFA expressed as oleic acid is approximately AV divided by 1.99. The saponification value (SV) is the milligrams of KOH needed to saponify one gram of fat; it is inversely related to the mean molecular weight of the triacylglycerols (mean molecular weight ≈ 3 × 56.1 × 1000/SV), so fats rich in short-chain acids, such as milk fat and coconut oil, have high SVs. The iodine value (IV), grams of iodine absorbed by 100 g of fat, measures unsaturation; it is used to classify oils (drying, semi-drying, non-drying) and to follow hydrogenation.
Deep-fat frying exposes oil to temperatures of about 160 to 190 °C, to air at the surface and to water and food components released from the food, for long periods. Three main groups of reactions occur. Hydrolysis by steam releases FFA, mono- and diacylglycerols. Thermal oxidation proceeds by radical mechanisms like autoxidation but much faster, producing hydroperoxides that decompose immediately into volatile compounds (some of which give fried flavour, others off-flavours) and non-volatile oxidised triacylglycerols. Polymerisation, both oxidative and thermal (including Diels–Alder type reactions of dienes), forms dimers, oligomers and polymers of triacylglycerols, which raise viscosity, cause foaming, darken the oil and form gummy deposits on fryers.
As oil degrades, its smoke point falls, colour darkens, viscosity and foaming rise and fried products absorb more oil and develop off-flavours. Oil quality is monitored by total polar compounds (TPC, measured by column chromatography or rapid sensors) and polymerised triacylglycerols; many countries set discard limits for TPC in the mid-20s per cent (about 24 to 27 per cent, depending on the jurisdiction).
Processing changes all four major constituents, sometimes beneficially and sometimes not. Water is removed by drying, frozen, or redistributed; changes in water activity and glass transition alter the rates of every other reaction. Carbohydrates undergo starch gelatinisation (improving digestibility and texture), retrogradation, partial hydrolysis (dextrinisation in toasting and extrusion), sucrose inversion in acid foods, caramelisation of sugars at high temperatures, and Maillard browning with amino groups. Maillard reactions create the desirable colour and flavour of bread crust, roasted coffee and fried foods, but also reduce the availability of lysine and can form acrylamide when asparagine reacts with reducing sugars at high temperature and low moisture, as in fried potatoes and baked cereal products.
Proteins are denatured by heating, which usually improves digestibility, inactivates enzymes and anti-nutritional proteins and creates gel and texture, but severe treatments cause damage: losses of lysine through Maillard reaction, formation of cross-links such as lysinoalanine under alkaline conditions, racemisation of amino acids, and oxidation of sulphur amino acids. Lipids may oxidise during heating, drying and storage, especially when surface area is high, as in powders and extruded snacks; hydrogenation can create trans fatty acids; and frying both absorbs oil into the food and degrades it. Many processes also cause interactions between constituents — amylose–lipid complexes in extruded and cooked starches, protein–polysaccharide complexes, and the binding of oxidised lipids to proteins — that change texture and nutritive value. Understanding these reactions lets the technologist choose conditions — temperature, time, pH, water activity and oxygen exposure — that deliver the wanted changes while limiting the unwanted ones.
A student uses this subject to explain, not just memorise, what happens in the laboratory and the pilot plant: why a starch paste thickens and then sets, why a jam with too little acid stays runny, why egg white will not whip in a greasy bowl, why chilled bread stales, and why a fat with a high peroxide value can still smell fresh. Exam questions in GATE XE-G and ICAR Code 18 lean on exactly these links between structure and behaviour, together with short calculations of water activity, protein from nitrogen and fat constants.
A product developer or quality technologist five years into the job uses food chemistry daily: choosing a waxy or cross-linked starch for a retorted sauce, an LM pectin for a reduced-sugar jam, xanthan for a pourable dressing or a β′-stable fat blend for a spread; setting moisture and water-activity targets for a snack; running and interpreting peroxide, anisidine, acid and iodine values on incoming oils; tracking total polar compounds in fryers; and investigating complaints of rancidity, staling, caking, syneresis, fat bloom or tough meat back to their chemical cause.
A senior R&D or quality leader uses this knowledge to set ingredient specifications and shelf-life strategies across product ranges, to decide on reformulation — reducing trans and saturated fat, replacing synthetic antioxidants with natural ones, replacing animal proteins with plant proteins — and to judge the technical claims of ingredient suppliers. They weigh the cost and regulatory status of modified starches, hydrocolloids and antioxidants in each market, and they translate chemistry into process limits: drying end-points from isotherms and glass transition, frying oil discard rules, chilling regimes that avoid cold shortening.
A research leader works on the frontiers of the same chemistry: structure–function relationships of plant proteins for meat and dairy analogues, mechanisms of lipid oxidation in emulsions and the role of interfaces, glass-transition and mobility models for stability prediction, starch digestibility and resistant starch for health, protein–polysaccharide interactions in structured foods, and process contaminants such as acrylamide and glycidyl esters. They supervise doctoral students, review evidence for nutrition and safety regulators, and decide which classical teaching (Osborne fractions, bound water) still serves students and which should be replaced by modern concepts.
This manual covers the major constituents only. Pigments, flavours, vitamins, minerals, food enzymes and the detailed chemistry of Maillard browning, caramelisation and acrylamide are in food-chemistry-minor-constituents-enzymes; the colloid science of emulsions, foams and gels is taken further in food-structure-encapsulation; industrial fat modification (hydrogenation, interesterification, fractionation) is in fats-oils-technology; and analytical procedures are in food-analysis. Quantitative tables of glass transition temperatures, sorption isotherm parameters and fatty-acid composition of individual oils are not given, because published values vary with source and method; questions supply the values they need. Legal limits for antioxidants, modified starches and frying-oil polar compounds differ by country and are mentioned only in outline.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions. Every numerical answer was computed by script; constants such as glass transition temperatures are given in the question because published values vary.
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