Food Science & Technology · Food Science Core · also Food Chemistry II, Food Enzymology, Flavour Chemistry, Food Biochemistry
The minor constituents that decide colour, flavour and nutritional value, the enzymes that act in raw and processed food, and the browning reactions — enzymatic and non-enzymatic — that every food technologist must control or exploit.
In the exams: GATE XE-G and XL-U Section 1 (pigments, flavours, enzymes, browning); ICAR AIEEA-PG Code 18 Unit I (food flavours); ICAR AICE 35.a Unit 7 (flavour chemistry technology)
Water, carbohydrates, proteins and lipids make up almost all the mass of food, but the minor constituents decide much of what consumers notice and much of what food technologists must control. Pigments give colour and change it during processing; volatile compounds and taste-active molecules give flavour; vitamins and minerals give nutritional value and are lost or made unavailable by processing; enzymes in raw materials keep working after harvest and slaughter, for good and ill; and the browning reactions, enzymatic and non-enzymatic, create both the attractive colours and aromas of baked, roasted and fried foods and some of their undesirable contaminants.
This manual covers that chemistry. It starts with the natural pigments, chlorophylls, carotenoids, anthocyanins, betalains, tannins and myoglobin, and how processing and storage change them. It then treats the chemistry of aroma compounds and of the five basic tastes, and how flavour is generated by heating, fermentation and maturation. It covers the stability of vitamins and the chemistry and bioavailability of minerals. It explains enzymes as catalysts, with kinetics and inhibition, then the endogenous enzymes that matter most in raw materials and the industrial enzymes used in processing. It ends with enzymatic browning and its control, the Maillard reaction and caramelisation, acrylamide and other process contaminants, and phenolic compounds as natural antioxidants.
Postgraduate entrance examinations in food technology ask about pigment structure and stability, flavour compounds and their sources, relative sweetness, vitamin stability, enzyme kinetics, the steps of the Maillard reaction and the control of browning. Doctoral fellowship examinations add flavour chemistry technology and process contaminants. Licensure and food analyst examinations ask applied questions on colour, discolouration, vitamin losses and mitigation of contaminants. The manual covers all of these with worked calculations where examinations use them.
Chlorophylls are the green pigments of leaves, green vegetables and unripe fruit. Each molecule has a tetrapyrrole ring, a chlorin, with a magnesium ion at its centre, and a long hydrophobic phytol chain esterified to one side. Chlorophyll a, with a methyl group, is blue-green; chlorophyll b, with an aldehyde group in the same position, is yellow-green; plants contain roughly three times as much a as b. Chlorophylls are fat soluble because of the phytol tail.
Carotenoids are yellow, orange and red tetraterpenoids, C40 molecules built from eight isoprene units, with a long chain of conjugated double bonds that absorbs blue light. Carotenes are pure hydrocarbons, such as β-carotene in carrots and lycopene in tomatoes and watermelon; xanthophylls carry oxygen, such as lutein in leafy vegetables and egg yolk, zeaxanthin in maize, capsanthin in red peppers and astaxanthin in salmon and crustaceans. Carotenoids are fat soluble and relatively heat stable, but they oxidise when exposed to oxygen, light and metal ions, especially in dried foods, and heat converts some all-trans forms to cis isomers, slightly lightening colour. Carotenoids with at least one unsubstituted β-ionone ring, notably β-carotene, α-carotene and β-cryptoxanthin, are provitamin A; lycopene and lutein are not. Their absorption from food is improved by heating, which breaks cell walls, and by eating them with fat. Annatto (bixin and norbixin) and saffron (crocin, a water-soluble glycoside) are apocarotenoids used as colourants.
Anthocyanins are water-soluble flavonoid glycosides responsible for most red, purple and blue colours of fruits, flowers and vegetables, such as berries, grapes, red cabbage, purple sweet potato and black carrot. The aglycones, anthocyanidins, include cyanidin (the most common), pelargonidin, delphinidin, peonidin, petunidin and malvidin. Their colour depends strongly on pH: at pH below about 3 the red flavylium cation dominates; at pH 4 to 5 much of the pigment becomes the colourless carbinol pseudobase; at higher pH purple and blue quinoidal bases form, which are unstable. Acylation with aromatic acids and copigmentation with other phenolics stabilise colour.
Betalains are nitrogen-containing water-soluble pigments found in beetroot, prickly pear, amaranth and some fungi, in plants of the order Caryophyllales, where anthocyanins do not occur. Betacyanins such as betanin are red-violet; betaxanthins are yellow. They are fairly stable between pH 3 and 7 but sensitive to heat, light and oxygen.
Tannins are polyphenols that bind proteins. Hydrolysable tannins are esters of gallic or ellagic acid with sugars; condensed tannins, or proanthocyanidins, are polymers of flavan-3-ols. They cause astringency by precipitating salivary proteins and give brown colours on oxidation, as in tea, where catechins are oxidised enzymatically to theaflavins and thearubigins.
Myoglobin is the haem protein that gives meat its colour. Its iron can be ferrous (Fe²⁺) or ferric (Fe³⁺) and can bind different ligands. Deoxymyoglobin, ferrous with no ligand, is purplish red, as in vacuum-packed meat; oxymyoglobin, ferrous with oxygen, is bright cherry red, the bloom of fresh-cut meat; metmyoglobin, ferric, is brown.
Heating green vegetables in acid conditions replaces chlorophyll's magnesium with two hydrogen ions, forming olive-brown pheophytin; if the phytol chain is also removed, pheophorbide forms. Cooking in a covered pan, where volatile plant acids are trapped, and long heating make the change worse. The enzyme chlorophyllase removes phytol to give chlorophyllide, which is still green but water soluble. Keeping water slightly alkaline preserves green colour but softens texture and destroys vitamins such as thiamin and ascorbic acid, so it is not good practice. Zinc and copper can replace magnesium to form stable bright-green complexes, the basis of 'regreening' processes and of copper chlorophyllin colourants, whose use is restricted by additive law. High-temperature short-time processing and quick cooling reduce pheophytinisation.
Anthocyanins are lost on heating, especially at higher pH and in the presence of oxygen. Sulphur dioxide bleaches them by forming colourless adducts, which is used in making glacé cherries. Ascorbic acid accelerates their degradation, and metal ions such as tin, iron and aluminium form blue or purple complexes, so anthocyanin-rich fruits are packed in lacquered cans. Browning of anthocyanin products during storage, such as strawberry jams, combines pigment breakdown with non-enzymatic browning.
Meat colour depends on the myoglobin state. Oxygen at the surface keeps oxymyoglobin red, but at low oxygen partial pressure metmyoglobin forms and the meat turns brown. In cured meats nitrite gives nitric oxide, which binds to give nitrosylmyoglobin, red; on cooking, the globin denatures and the stable pink nitrosylhaemochrome forms. Uncured cooked meat is grey-brown because of denatured globin haemichromes. Carbon monoxide forms bright-red carboxymyoglobin, and its use in meat packaging is permitted in some jurisdictions and banned in others because it can mask spoilage.
Colour is measured instrumentally in a way that matches human perception. The CIE L*a*b* system describes colour by three coordinates: L* for lightness from 0 (black) to 100 (white), a* from green (negative) to red (positive), and b* from blue (negative) to yellow (positive). From a* and b*, chroma, C* = √(a*² + b*²), describes saturation, and hue angle, h = arctan(b*/a*), describes the colour family: 0° is red, 90° yellow, 180° green and 270° blue. A sample with a* = 20 and b* = 20 has a hue angle of 45°, an orange-red.
Total colour difference between two samples is ΔE = √(ΔL*² + Δa*² + Δb*²). If a stored sample differs from fresh by ΔL* = 4, Δa* = 3 and Δb* = 0, then ΔE = √(16 + 9) = 5. As a rough guide, differences of about 1 or less are not seen by most observers, while differences of several units are readily seen, though the visible threshold depends on the colour and on viewing conditions.
Spectrophotometric methods measure pigment concentrations directly: chlorophylls and carotenoids are extracted with organic solvents and read at their absorption maxima; anthocyanins are measured by the pH-differential method, which uses the difference in absorbance between pH 1.0 and pH 4.5 to separate monomeric anthocyanin from browned polymeric material. High-performance liquid chromatography separates individual pigments. In the plant and in quality control, colour specifications are written in L*a*b* or in reference to colour standards, and colour change is often the attribute that ends shelf life.
Aroma is produced by volatile compounds that reach the olfactory receptors in the nose, either directly when food is smelled or retronasally from the mouth during eating. A food may contain hundreds of volatiles, but only some are present above their odour thresholds and contribute; the odour activity value (OAV), the concentration divided by the odour threshold in the same medium, ranks them. A compound at 50 µg/kg with a threshold of 0.5 µg/kg has an OAV of 100. Character-impact compounds alone evoke a food, such as 1-octen-3-ol for mushroom or benzaldehyde for bitter almond.
Terpenes are built from isoprene units. Monoterpenes (C10), such as limonene in citrus peel, linalool in flowers and coriander, citral in lemongrass and lemon, and menthol in mint, and sesquiterpenes (C15), such as nootkatone in grapefruit, dominate the aromas of fruits, herbs and spices. Esters give fruity notes: isoamyl acetate is banana-like; ethyl butanoate and ethyl hexanoate give fruity, pineapple and apple notes; they are formed by ripening fruits and by yeasts. Aldehydes are potent: hexanal and (E)-2-hexenal give green, grassy notes; benzaldehyde almond; vanillin vanilla; cinnamaldehyde cinnamon; (E,Z)-2,6-nonadienal cucumber. Ketones include diacetyl (2,3-butanedione), the butter aroma of cultured dairy products, and methyl ketones such as 2-heptanone in blue cheese. Lactones give fruity, creamy and coconut notes; furanones such as furaneol give caramel and strawberry notes; pyrazines give roasted and nutty notes.
Sulphur compounds have very low odour thresholds and characterise many foods. In onions and garlic, the aroma is absent in the intact bulb and forms within seconds of cutting. In garlic, the enzyme alliinase acts on alliin, a sulphur-containing amino acid derivative, to form allicin, which gives fresh garlic its pungent smell and breaks down further to diallyl sulphides. In onion, alliinase acts on a related precursor and the lachrymatory factor synthase forms propanethial S-oxide, the tear-inducing compound. In mustard, horseradish, wasabi and brassica vegetables, the enzyme myrosinase hydrolyses glucosinolates to isothiocyanates, such as allyl isothiocyanate, the pungent principle of mustard.
Other sulphur compounds come from heating: dimethyl sulphide gives the smell of cooked sweet corn and seaweed; methional, from methionine by Strecker degradation, smells of cooked potato; 2-furfurylthiol is a key note of roasted coffee. Light-struck or 'skunky' beer is caused by 3-methyl-2-butene-1-thiol, formed when riboflavin, excited by light, degrades hop iso-α-acids, which is why beer is packed in brown bottles or cans.
The lipoxygenase pathway generates aroma in fresh fruits and vegetables when tissue is disrupted: lipoxygenase oxygenates linoleic and linolenic acids, and hydroperoxide lyase cleaves the hydroperoxides to C6 aldehydes and alcohols, the green notes of cut grass, tomato and cucumber, and to C9 compounds. Aroma compounds are analysed by gas chromatography with mass spectrometry, and gas chromatography-olfactometry, in which a trained assessor sniffs the column effluent, identifies which peaks actually smell.
Taste is detected by receptor cells in taste buds on the tongue and palate. Five basic tastes are generally recognised: sweet, sour, salty, bitter and umami. Sweet taste is detected by a receptor formed from two proteins, T1R2 and T1R3; umami by T1R1 and T1R3; bitter by a family of about 25 T2R receptors that together recognise thousands of structurally different bitter compounds. Sour taste is detected through proton channels, and salty taste largely through sodium channels.
Sweetness is compared against sucrose, given a value of 1. Approximate relative sweetness values on a weight basis, which vary with concentration, temperature and the medium, are about 1.2 to 1.7 for fructose, 0.6 to 0.7 for glucose, 0.2 to 0.4 for lactose, about 1 for xylitol and about 0.5 to 0.6 for sorbitol. High-intensity sweeteners are hundreds to thousands of times sweeter: aspartame and acesulfame potassium about 200, saccharin about 300 to 400, sucralose about 600, steviol glycosides about 200 to 300, thaumatin a few thousand, neotame several thousand. To replace the sweetness of 100 g of sucrose with sucralose at 600 times would need only about 0.17 g, which is why intense sweeteners need bulking agents to replace the body of sugar.
Sour taste comes from hydrogen ions, but at the same pH organic acids such as acetic acid taste more sour than hydrochloric acid because the undissociated acid enters cells and releases more protons. Salty taste is chiefly produced by sodium ions; potassium chloride, used in salt reduction, tastes salty but also bitter and metallic. Umami is the savoury taste of glutamate, as monosodium glutamate, and of the 5′-ribonucleotides inosine monophosphate and guanosine monophosphate; mixtures of glutamate and these nucleotides act synergistically, greatly increasing umami intensity.
Bitterness protects against toxins and is often unwanted in food, but it is valued in coffee, tea, beer, dark chocolate and some vegetables. Important bitter compounds include caffeine and theobromine; quinine, used as a reference standard; naringin, the flavanone glycoside of grapefruit; limonin, which causes 'delayed bitterness' in citrus juices as it forms from a non-bitter precursor after juicing; iso-α-acids from hops in beer; glucosinolates in brassicas; and hydrophobic peptides released by proteolysis, which cause bitterness in some cheeses and protein hydrolysates. Debittering approaches include enzymatic hydrolysis of naringin by naringinase, adsorption on resins, and careful choice of proteases.
Several oral sensations are not tastes but chemesthetic responses mediated by other receptors. Pungency, or 'heat', comes from capsaicin in chilli, acting on the TRPV1 receptor, and from piperine in black pepper; allyl isothiocyanate from mustard and wasabi acts on TRPA1. Cooling comes from menthol acting on TRPM8. Astringency, the drying, puckering sensation of tea, red wine and unripe fruit, results from tannins precipitating salivary proteins. 'Kokumi', described as mouthfulness and continuity, is attributed to γ-glutamyl peptides acting on calcium-sensing receptors and enhancing other tastes.
Tastes interact: sugar suppresses sourness and bitterness, salt suppresses bitterness, and acids enhance saltiness. Temperature changes perception, and taste and aroma combine in the brain into flavour, so a product reformulated for less sugar or salt often needs aroma and texture adjustment as well.
Many of the most appreciated flavours exist only because food is heated: the crust of bread, roasted coffee and cocoa, roasted meat, fried potatoes, toasted nuts and malt. Most of these come from the Maillard reaction and its companion, Strecker degradation. In Strecker degradation, an α-dicarbonyl compound formed in the Maillard reaction reacts with an amino acid, which is decarboxylated and deaminated to an aldehyde with one carbon fewer, the Strecker aldehyde, while the dicarbonyl becomes an α-aminoketone. Strecker aldehydes are potent odorants: 3-methylbutanal from leucine gives malty notes, phenylacetaldehyde from phenylalanine gives honey-like notes, and methional from methionine gives cooked-potato notes. The aminoketones condense to pyrazines, which give roasted, nutty and earthy notes, the backbone of coffee, cocoa and roasted grain aroma.
Thermal degradation of sugars gives caramel-like furanones and maltol; thermal oxidation of lipids gives aldehydes and ketones that contribute to fried and meaty aromas; reaction of sulphur compounds from cysteine and thiamin with Maillard intermediates gives meaty thiols and thiophenes; and degradation of phenolic acids gives smoky and spicy phenols such as guaiacol and 4-vinylguaiacol. The balance of these depends on temperature, time, water activity, pH and the precursors present, which is why roasting profiles are controlled tightly.
Microorganisms generate flavour by fermenting sugars and by breaking down proteins and lipids. In cultured butter and some fresh cheeses, citrate-fermenting lactic acid bacteria, such as Lactococcus lactis biovar diacetylactis and Leuconostoc species, produce diacetyl. In yoghurt, Lactobacillus delbrueckii subsp. bulgaricus produces acetaldehyde, the key note of yoghurt flavour. Yeasts produce ethanol, higher alcohols and esters in bread, beer and wine. Acetic acid bacteria make vinegar. In cocoa, a mixed fermentation of the pulp by yeasts, lactic acid bacteria and acetic acid bacteria kills the bean, allows enzymes to form flavour precursors, and is essential for chocolate flavour on roasting.
Maturation and ripening continue flavour development after the main process. In cheese ripening, proteolysis by rennet, milk proteases and starter and non-starter bacteria breaks casein into peptides and amino acids, which are catabolised to aldehydes, alcohols, acids and sulphur compounds; lipolysis releases free fatty acids, dominant in hard Italian and blue cheeses, where moulds such as Penicillium roqueforti convert fatty acids to methyl ketones. In meat ageing, endogenous proteases tenderise and release flavour precursors. In wine and spirits aged in oak, compounds such as oak lactones, vanillin and other phenolic aldehydes are extracted, slow oxidation occurs and esters form. In curing of vanilla beans, β-glucosidase releases vanillin from glucovanillin. Tea leaf 'fermentation' is an enzymatic oxidation by polyphenol oxidase that forms theaflavins and thearubigins and much of black tea's flavour. Control of time, temperature, oxygen and the microbial population determines the result.
Vitamins are lost during processing and storage by heat, oxygen, light, pH extremes, metal ions, enzymes and leaching into water. Vitamin C (ascorbic acid) is the least stable: it is oxidised to dehydroascorbic acid, which still has vitamin activity but is easily hydrolysed irreversibly to 2,3-diketogulonic acid. Oxidation is accelerated by heat, oxygen, copper and iron ions, alkaline pH and the enzyme ascorbate oxidase in cut plant tissue, and ascorbic acid is very water soluble, so blanching and boiling leach it out. Because it is so labile, vitamin C is often used as an indicator of the severity of a process.
Thiamin (vitamin B1) is destroyed by heat at neutral and alkaline pH but is more stable in acid; it is cleaved by sulphite, which is why sulphites are not used in foods that are important thiamin sources, and it is destroyed by thiaminases present in raw fish and some ferns. Riboflavin (B2) is heat stable in acid but very sensitive to light: it forms lumiflavin in alkaline and lumichrome in neutral or acid solution, and it acts as a photosensitiser that causes light-induced off-flavours in milk, so milk in clear containers loses riboflavin and flavour on retail display. Folate is sensitive to oxidation and heat and is lost by leaching. Vitamin B6 is light sensitive. Niacin is the most stable vitamin. Vitamin B12 is fairly heat stable but damaged by light and by ascorbic acid in some conditions.
Among fat-soluble vitamins, vitamin A and provitamin A carotenoids are destroyed by oxidation and light and isomerised by heat; vitamin E (tocopherols) is consumed as it acts as an antioxidant and lost heavily in deep frying; vitamin D is relatively stable; vitamin K is sensitive to light.
Vitamin degradation in foods usually follows first-order kinetics: the concentration C after time t is C = C0e^(−kt), where k is the rate constant, and the half-life is ln 2/k. If vitamin C in a juice degrades with k = 0.02 per day, its half-life is 0.693/0.02 ≈ 35 days, and after 30 days the retention is e^(−0.6) ≈ 55%. The rate constant rises with temperature, often described by the Arrhenius equation or by Q10 values, and with oxygen and light exposure, so storage temperature and packaging largely decide vitamin shelf life.
Retention is calculated carefully. Apparent retention compares concentrations, but cooking changes mass by water loss or gain, so true retention accounts for both: true retention (%) = (nutrient content per gram of cooked food × grams of cooked food) ÷ (nutrient content per gram of raw food × grams of raw food) × 100. If 100 g of raw vegetable containing 40 mg/100 g of vitamin C becomes 90 g of cooked vegetable containing 35 mg/100 g, true retention = (0.35 × 90)/(0.40 × 100) × 100 = 79%, whereas the apparent retention from concentrations alone would be 87.5%.
Processes are chosen to protect vitamins: steam blanching loses less than water blanching; HTST and UHT treatments lose less than long low-temperature heating; freezing preserves most vitamins if blanching is adequate and storage is cold; deaeration and oxygen-barrier packaging protect vitamin C in juices; opaque packaging protects riboflavin and vitamin A in milk. Fortified foods are formulated with overages, extra vitamin added to ensure that the label claim is met at the end of shelf life.
Minerals are the inorganic elements that remain as ash when food is burned. Macro-minerals needed in amounts of more than about 100 mg a day include calcium, phosphorus, magnesium, sodium, potassium, chloride and sulphur; trace elements include iron, zinc, copper, iodine, selenium, manganese, molybdenum, chromium and fluoride. Ash is determined by dry ashing at about 550 °C or by wet digestion, and individual elements by atomic absorption spectrometry, inductively coupled plasma optical emission or mass spectrometry, and other methods.
The amount of a mineral in food matters less than its bioavailability, the fraction absorbed and used. Iron illustrates this. Haem iron from meat, fish and poultry is absorbed well and little affected by the rest of the diet; non-haem iron from plant foods and fortificants is absorbed much less and is strongly influenced by other components. Ascorbic acid and the 'meat factor' enhance non-haem iron absorption; phytate in whole grains and legumes, polyphenols in tea and coffee, calcium and some proteins inhibit it. Zinc absorption is likewise inhibited by phytate; nutritionists use the phytate-to-zinc molar ratio, and ratios above about 15 indicate poor zinc bioavailability. For a meal with 600 mg phytate (molar mass about 660 g/mol) and 3 mg zinc (65.4 g/mol), the ratio is (600/660)/(3/65.4) ≈ 20. Oxalate in spinach and rhubarb binds calcium. Processing can improve bioavailability: soaking, germination, fermentation and added phytase break down phytate.
Minerals take part in many reactions that matter in processing. Calcium firms plant tissue by cross-linking pectin, especially pectin demethylated by pectin methylesterase, which is why calcium salts are added to canned tomatoes and fruit pieces; calcium is essential to rennet coagulation and to alginate and low-methoxyl pectin gels. Phosphates and citrates bind calcium and are used as emulsifying salts in processed cheese and as stabilisers in evaporated milk.
Transition metals, especially iron and copper, catalyse lipid oxidation by decomposing hydroperoxides into radicals and catalyse the oxidation of ascorbic acid; even traces from equipment or water accelerate rancidity. Chelating agents such as citric acid, phosphates and EDTA bind these metals and act as antioxidant synergists.
Several discolourations are caused by mineral reactions. Iron reacts with tannins and other phenolics to give blue-black or grey-green colours, for example in canned products and in tea made with iron-rich water. Hydrogen sulphide released from proteins during heating reacts with iron to give black iron sulphide, causing the grey-green ring around the yolk of overcooked eggs and black deposits in canned seafood and meats, which is why some cans have special lacquers. Tin can dissolve from unlacquered cans into acidic foods. Glass-like crystals of struvite, magnesium ammonium phosphate, can form in canned seafood and are mistaken by consumers for glass. Sodium chloride and other salts affect protein solubility, water holding and texture, as in processed meats, where salt extracts myofibrillar proteins that bind the product.
Enzymes are protein catalysts that speed up reactions by lowering the activation energy without being consumed. They are highly specific: some act on only one substrate (absolute specificity), others on a group of related substrates or a particular type of bond (group or linkage specificity), and most distinguish between stereoisomers. Enzymes are classified by the reaction they catalyse into seven classes in the Enzyme Commission system: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases and translocases. Most food-processing enzymes are hydrolases, such as amylases, proteases, lipases and pectinases.
Many enzymes need a non-protein partner. A cofactor may be a metal ion, such as the copper of polyphenol oxidase or the zinc of many proteases, or an organic coenzyme, often derived from a vitamin: NAD⁺ and NADP⁺ from niacin, FAD and FMN from riboflavin, thiamin pyrophosphate from thiamin, pyridoxal phosphate from vitamin B6, coenzyme A from pantothenic acid, and biotin. The protein without its cofactor is the apoenzyme; with it, the holoenzyme.
Enzyme activity depends on temperature and pH. Activity rises with temperature, with a Q10 often about 2, until denaturation becomes significant; beyond the optimum, activity falls as the enzyme is inactivated, a process usually following first-order kinetics with D and z values like microbial inactivation. Each enzyme has a pH optimum, reflecting the ionisation of groups in its active site and substrate. Activity is expressed in units: one international unit (U) converts 1 µmol of substrate per minute under defined conditions; the SI unit is the katal, 1 mol per second.
For many enzymes the initial rate v depends on substrate concentration [S] according to the Michaelis-Menten equation, v = Vmax[S]/(Km + [S]). Vmax is the maximum rate when the enzyme is saturated, and Km, the Michaelis constant, is the substrate concentration at which v is half of Vmax; a low Km indicates high affinity. With Vmax = 100 units and Km = 2 mM, the rate is 50 at 2 mM, 75 at 6 mM and 90 at 18 mM; reaching 90% of Vmax needs [S] = 9Km. The Lineweaver-Burk double-reciprocal plot of 1/v against 1/[S] is a straight line with y-intercept 1/Vmax, x-intercept −1/Km and slope Km/Vmax, convenient for estimating the constants and diagnosing inhibition, though it distorts errors at low [S].
Inhibitors reduce enzyme activity. A competitive inhibitor resembles the substrate and competes for the active site; it raises the apparent Km to Km(1 + [I]/Ki) without changing Vmax, and in a Lineweaver-Burk plot the lines meet on the y-axis. High substrate concentration overcomes it. A non-competitive inhibitor binds elsewhere and lowers Vmax without changing Km; lines meet on the x-axis. An uncompetitive inhibitor binds only the enzyme-substrate complex and lowers both Vmax and Km, giving parallel lines. Irreversible inhibitors bind covalently and permanently. In foods, inhibition is used and encountered constantly: sulphite and ascorbic acid inhibit or reverse enzymatic browning, chelators remove essential metal cofactors, protease inhibitors in raw legumes such as soybean trypsin inhibitors reduce protein digestibility until they are inactivated by heat, and amylase inhibitors occur in some beans and cereals.
Raw plant and animal materials contain enzymes that remain active after harvest or slaughter. Lipoxygenase oxygenates polyunsaturated fatty acids with a cis,cis-1,4-pentadiene structure, such as linoleic and linolenic acids, to form hydroperoxides. Its breakdown products cause the beany flavour of soybeans and soy milk, the hay-like off-flavours of unblanched frozen vegetables, and the bleaching of carotenoids; the last effect is used deliberately when small amounts of soy flour are added to wheat flour to whiten bread crumb. Heat treatment, or grinding soybeans in hot water, inactivates it.
Peroxidase is a haem enzyme that oxidises many substrates using hydrogen peroxide. It is among the most heat-stable plant enzymes and can partly regenerate after heating, which is why it is used as the indicator of adequate blanching; its own role in off-flavour is debated, and some processors now use lipoxygenase as a more relevant indicator.
Pectic enzymes act on the pectin of cell walls. Pectin methylesterase removes methyl groups, producing low-methoxyl pectin that can cross-link with calcium. In citrus juices this causes loss of the desirable cloud as calcium pectate precipitates, so juices are pasteurised to inactivate the enzyme, and pectin methylesterase activity is used as an index of juice pasteurisation; in vegetables, controlled activation followed by calcium can firm texture. Polygalacturonase depolymerises demethylated pectin and causes softening during ripening, notably in tomatoes. In tomato processing, 'hot break' heating to about 85 to 90 °C or higher inactivates these enzymes rapidly to give thick, viscous products, while 'cold break' at about 60 to 70 °C lets them act, giving better colour and flavour but lower viscosity for juices.
Polyphenol oxidase is a copper-containing enzyme that, in the presence of oxygen, hydroxylates monophenols to o-diphenols (cresolase or monophenolase activity) and oxidises o-diphenols to o-quinones (catecholase or diphenolase activity). The quinones are highly reactive and polymerise, and react with amino acids and proteins, to form brown, red or black pigments called melanins. Enzymatic browning occurs when tissue is cut, bruised or frozen and thawed, bringing the enzyme, its phenolic substrates and oxygen together: apples, pears, bananas, potatoes, mushrooms, avocados, lettuce, and shrimp, where it causes blackspot. It is desirable in tea, cocoa, dates, raisins and prunes.
Control targets each requirement. Exclude oxygen by immersing in water, brine or syrup, by vacuum or modified-atmosphere packaging or by edible coatings. Lower the pH with citric, malic or ascorbic acid, because polyphenol oxidase has an optimum around pH 5 to 7 and little activity below about pH 3. Inactivate the enzyme by blanching where texture allows. Reduce quinones back to colourless diphenols with ascorbic acid or erythorbic acid, which works until the reducing agent is used up. Chelate the copper with citric acid or EDTA. Use sulphites, which both inhibit the enzyme and combine with quinones; they are very effective but restricted and must be labelled because of sensitive individuals. Use cysteine, which forms colourless adducts with quinones; 4-hexylresorcinol, a specific inhibitor used on shrimp; and cold storage, which slows the reaction. In practice, combinations such as calcium ascorbate with citric acid on fresh-cut apples are used, and breeding has produced low-browning varieties of some crops.
Enzymes are among the most widely used processing aids. Most are produced by fermentation of selected or engineered bacteria and fungi, and are evaluated for safety by bodies such as the Joint FAO/WHO Expert Committee on Food Additives and national authorities; many are classed as processing aids that do not need to appear on the label, although rules differ between jurisdictions.
Starch processing depends on enzymes. Thermostable α-amylase liquefies gelatinised starch by random hydrolysis of α-1,4 bonds; glucoamylase saccharifies the dextrins to glucose by removing glucose units from non-reducing ends, assisted by pullulanase, which debranches α-1,6 bonds; and glucose isomerase, in fact a xylose isomerase used in immobilised form, converts glucose partly to fructose to make high-fructose syrups. In baking, fungal α-amylase supports yeast fermentation and maltogenic amylases slow staling. Invertase hydrolyses sucrose to invert sugar for soft-centred confectionery. Lactase, β-galactosidase, hydrolyses lactose for lactose-free milk and, under transgalactosylating conditions, forms galacto-oligosaccharides.
Proteases include chymosin for cheese, much of it now produced by fermentation in microorganisms carrying the calf gene; the plant proteases papain from papaya and bromelain from pineapple, used for meat tenderising, chill-proofing beer and making protein hydrolysates; and microbial proteases for hydrolysates and flavour. Transglutaminase cross-links glutamine and lysine residues in proteins, binding meat and fish pieces and modifying the texture of dairy and plant-protein products.
Lipases develop flavour in cheese and cheese-flavour concentrates and catalyse interesterification of fats; enzymes specific for positions 1 and 3 of the triglyceride allow structured lipids to be made. Phospholipases are used in degumming vegetable oils and in modifying egg yolk for heat-stable mayonnaise. Pectinases and cellulases increase juice yield from fruit mash and clarify juices by breaking down pectin; xylanases improve bread dough handling. Glucose oxidase, often with catalase, removes oxygen or glucose, for example from egg white before drying to prevent Maillard browning and from packaged products to protect flavour. Catalase removes residual hydrogen peroxide. Naringinase debitters citrus juice. Asparaginase converts asparagine to aspartic acid and so reduces acrylamide formation in baked and fried products.
Using enzymes well requires attention to conditions: temperature and pH must be in the enzyme's working range; dose and time must be controlled; and the enzyme must be inactivated afterwards, usually by heat, unless continued activity is wanted. Immobilised enzymes, bound to or entrapped in a solid support and used in packed columns, can be reused and give continuous processes, as in high-fructose syrup production. Enzyme activity is specified in defined units, and suppliers' units are not always comparable, so processors validate doses in their own conditions. Enzymes are proteins and can be respiratory sensitisers for workers handling powders, so dust control is part of safe use.
The Maillard reaction is a complex network of reactions between reducing sugars and amino compounds, named after Louis-Camille Maillard, who described it in 1912; the scheme still taught was set out by Hodge in 1953. In the early stage, the carbonyl group of a reducing sugar condenses with a free amino group, typically the ε-amino group of lysine or a free amino acid, to give an N-glycosylamine, which undergoes the Amadori rearrangement (for aldoses) to a 1-amino-1-deoxy-2-ketose, the Amadori product, or the Heyns rearrangement for ketoses. These products are colourless. In the intermediate stage, they degrade to reactive α-dicarbonyls (deoxyosones), furfurals such as hydroxymethylfurfural, reductones and fission products, and Strecker degradation produces aldehydes and pyrazines. In the final stage, these intermediates polymerise into brown, nitrogen-containing melanoidins.
The rate rises with temperature and time, is faster at higher pH, peaks at intermediate water activity of about 0.6 to 0.8, and depends on the reactants: pentoses react faster than hexoses, sucrose reacts only after hydrolysis, and lysine is the most reactive amino acid. Nutritionally, the reaction blocks lysine, the limiting amino acid in many foods; the Amadori product of lysine and lactose in heated milk is measured as furosine after acid hydrolysis, as an indicator of heat damage. Sulphite inhibits the reaction by binding carbonyl intermediates.
Caramelisation is the browning of sugars heated without amino compounds, at higher temperatures, catalysed by acids or bases. It produces furans, maltol, diacetyl and brown polymers. Caramel colours are made commercially in four classes: plain caramel (class I), caustic sulphite caramel (II), ammonia caramel (III) and sulphite ammonia caramel (IV); classes III and IV can contain 4-methylimidazole, which is limited in specifications.
Acrylamide was found in heated foods in 2002. It forms mainly from free asparagine and reducing sugars through the Maillard reaction, at temperatures above about 120 °C in low-moisture conditions, so it occurs in fried potato products, crisps, roasted coffee, bread crust, biscuits, crackers and breakfast cereals. The International Agency for Research on Cancer classifies it as probably carcinogenic to humans (Group 2A). Codex adopted a Code of Practice for the Reduction of Acrylamide in Foods, and several jurisdictions set benchmark or indicative levels. Mitigation includes choosing potato varieties low in reducing sugars and storing them above about 6 to 8 °C to avoid cold-induced sweetening, blanching or soaking to leach sugars and asparagine, using asparaginase, lowering frying and baking temperatures and aiming for a lighter colour, lowering pH, replacing ammonium bicarbonate raising agents, and adding calcium salts.
Other process contaminants arise in heating. Furan forms in canned, jarred and roasted foods such as coffee. 3-Monochloropropane-1,2-diol esters and glycidyl esters form during high-temperature deodorisation of refined vegetable oils, especially palm oil, from acylglycerols and chlorine sources. Polycyclic aromatic hydrocarbons such as benzo[a]pyrene form in grilling, smoking and direct-fire drying. Heterocyclic aromatic amines form from creatine, sugars and amino acids in meat cooked at high temperature. Ethyl carbamate forms in stone-fruit spirits and fermented foods from cyanate or urea and ethanol. Nitrosamines can form in cured meats; benzene can form from benzoate and ascorbic acid in soft drinks exposed to heat or light. These contaminants are monitored by sensitive chromatographic methods, mostly liquid or gas chromatography with mass spectrometry, and controlled by managing precursors and process conditions rather than by removal after formation.
Phenolic compounds have one or more hydroxyl groups attached to an aromatic ring. They are secondary metabolites of plants and influence colour, taste, astringency, browning and stability. Phenolic acids include hydroxybenzoic acids such as gallic acid, and hydroxycinnamic acids such as caffeic, ferulic and p-coumaric acids; chlorogenic acid, an ester of caffeic and quinic acids, is abundant in coffee and in apples and potatoes, where it is a substrate for enzymatic browning.
Flavonoids share a C6–C3–C6 skeleton of two aromatic rings joined by a three-carbon unit. The main subclasses are flavonols, such as quercetin and kaempferol in onions, apples and tea; flavones, such as apigenin and luteolin in herbs; flavanones, such as hesperidin and naringin in citrus; flavan-3-ols, the catechins of tea, cocoa, grapes and apples, including epigallocatechin gallate in green tea; anthocyanidins; and isoflavones, such as genistein and daidzein in soybeans. Other phenolics include stilbenes such as resveratrol in grapes, lignans in flaxseed and sesame, hydroxytyrosol and oleuropein in olives, curcumin in turmeric, and the tannins.
Phenolics are measured together as 'total phenolics' by the Folin-Ciocalteu method, expressed as gallic acid equivalents. This reagent reacts with any reducing substance, including ascorbic acid and some sugars and amino acids, so it is not specific; individual compounds are identified by liquid chromatography.
Antioxidants delay oxidation, chiefly of lipids. Primary, or chain-breaking, antioxidants are mostly phenols. They donate a hydrogen atom to lipid peroxyl radicals, stopping the propagation step of autoxidation; the resulting phenoxyl radical is stabilised by resonance and does not continue the chain. Tocopherols, natural in vegetable oils, are the most important; in foods, their relative antioxidant activity tends to be the reverse of their vitamin E activity, with δ- and γ-tocopherol more effective in oils than α-tocopherol. Rosemary extract, rich in carnosic acid and carnosol, green tea catechins, and sesamol from sesame oil are effective natural antioxidants; synthetic phenolic antioxidants and gallates are used where permitted.
Secondary antioxidants act in other ways. Chelating agents such as citric acid, phosphates and EDTA bind iron and copper that would decompose hydroperoxides into radicals. Oxygen scavengers such as ascorbic acid and its fat-soluble ester ascorbyl palmitate remove oxygen and regenerate tocopherol from its radical, a classic synergy. Carotenoids quench singlet oxygen, which is important in light-exposed foods.
Antioxidant capacity is measured in vitro by assays such as DPPH and ABTS radical scavenging, ferric reducing antioxidant power and oxygen radical absorbance capacity, but these test chemistry in solution, not effectiveness in a food or in the body. Effectiveness in a food is tested by accelerated storage or by measuring the induction period in an oxidative stability instrument such as the Rancimat. Antioxidants work best when added early, before oxidation begins, and some phenolics and ascorbic acid can act as pro-oxidants at high concentration in the presence of free metal ions.
A student uses this subject to link chemistry to what food looks, smells and tastes like. When a practical turns cut apples brown, overcooks green beans or bakes biscuits too dark, the student can name the reaction, the molecules involved and the control. In examinations the subject provides many direct questions on pigments, flavour compounds, sweetness, vitamins, enzyme kinetics and the Maillard reaction, plus calculations on colour difference, Michaelis-Menten rates, vitamin retention and molar ratios. In laboratory classes the same knowledge guides pigment extraction, enzyme assays and browning experiments.
A product developer or quality technologist with five years' experience applies this chemistry to formulation and troubleshooting. They choose anti-browning treatments for a fresh-cut line, set blanching to protect colour and vitamins, select natural antioxidants to extend the life of a snack, pick enzymes and doses for juice yield or bread softness, and build an acrylamide mitigation plan for a fried product. They also explain to marketing why a natural colour fades and what packaging or formulation will protect it.
A senior technical manager uses this subject to set policy and manage risk. They decide on process contaminant monitoring programmes and mitigation investments, approve clean-label reformulations that replace synthetic colours and antioxidants with natural ones, negotiate vitamin overages and shelf-life claims, and evaluate enzyme suppliers and regulatory status in each market. They respond to regulators and customers when new limits on acrylamide, 3-MCPD esters or other contaminants are set. They also judge whether a supplier change in raw material variety or storage will shift colour, flavour or contaminant levels, and commission the data needed before approving it.
A senior researcher uses this subject to design studies of reaction mechanisms and kinetics in real foods: tracing Maillard and Strecker pathways with isotope labelling, modelling pigment and vitamin degradation, engineering enzymes and processes for lower contaminant formation, and testing natural antioxidants in realistic matrices rather than only in vitro assays. They advise standard-setting bodies on contaminants and train students to treat each colour, flavour and nutrient change as chemistry that can be predicted and controlled. Their reviews and models are used by industry and regulators to set realistic targets.
This manual gives representative values; relative sweetness, odour thresholds, enzyme constants and vitamin degradation rates vary with concentration, matrix and conditions, and published figures differ between sources. It does not give detailed structures or reaction mechanisms beyond what examinations require, nor analytical method protocols. Permitted colours, enzymes, antioxidants and contaminant limits differ by jurisdiction and change; the manual describes principles and Codex texts, not national lists. The roles of some taste receptors, such as those for kokumi and fat, are still being researched.
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.
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