Dairy Science & Technology · Dairy Chemistry · also Dairy Products Chemistry, Chemistry of Milk Products
Physical and chemical changes in milk as it becomes cream, butter, ghee, concentrated and dried milks, cheese, fermented milks and frozen desserts.
In the exams: ICAR AICE subject 34 Units 5–8; ICAR AIEEA-PG group 16 (chemistry of cream, butter, ghee, ice cream, cheese, condensed and dried milks, infant food)
Chemistry of Dairy Products follows milk as it is turned into cream, butter, ghee, concentrated and dried milks, cheese, fermented milks, ice cream and the heat- and acid-coagulated products of South Asia. In each case it asks what happens to the proteins, fat, lactose and salts during manufacture and storage, why the product has its structure and flavour, and what chemical change causes its typical defects. It is the bridge between the milk-chemistry subjects and the product-technology subjects.
The four milk-chemistry subjects are assumed. Milk Composition and Physical Properties covers composition, acidity and buffering; Milk Proteins and Enzymes covers the casein micelle, its stability, the heat coagulation time-pH profile, whey protein denaturation, plasmin and the indigenous enzymes; Milk Lipids covers fat crystallisation, oxidation, lipolysis and antioxidants; and Lactose, Milk Salts and Vitamins covers lactose crystallisation, the glass transition, Maillard browning and salt balance. This manual refers to those subjects rather than repeating them and concentrates on how those principles operate in each product. Manufacturing procedures are treated in the product-technology subjects (market milk, fat-rich products, concentrated and dried milks, cheese, fermented milks, frozen desserts), so here they are described only as far as needed to explain the chemistry. Values quoted are typical; product standards differ between countries and are given only where noted as examples.
The subject is examined in Units 5 to 8 of ICAR AICE-JRF/SRF subject 34 (Dairy Chemistry) and in group 16 of ICAR AIEEA-PG, whose syllabus names the chemistry of cream, butter, ghee, ice cream, cheese, condensed and dried milks and infant food, the spoilage of ghee, rennet action, fermentation and indigenous products. ASRB NET, food analyst and food safety officer papers and GATE food technology test parts of it. Expect questions on mechanisms (churning, rennet coagulation, ripening, flavour formation, stabiliser and emulsifier action), on defects and their causes, and calculations such as neutraliser requirement, butter overrun, cheese yield, concentration factor and ice cream overrun.
Heat stability is the ability of milk to withstand high-temperature processing without coagulating. It matters for in-container sterilisation, UHT treatment, the manufacture of evaporated milk and recombined products, and for infant formula and high-protein beverages. The standard measure for unconcentrated milk is the heat coagulation time (HCT): the time a sample in a sealed glass tube, rocked in an oil bath at 140 degrees Celsius, takes to show visible coagulation. Concentrated milks are tested at lower temperatures, commonly 120 degrees Celsius, because they coagulate much faster. HCT is usually measured over a range of pH values to give an HCT-pH profile, whose shapes (type A, with a maximum near pH 6.7 and a minimum near pH 6.9, and type B, rising steadily with pH) and molecular basis are described in Milk Proteins and Enzymes. Other heat-stability tests include the alcohol test, which correlates with heat stability because both depend on ionic calcium and casein stability, and pilot sterilisation of small cans of concentrate, used daily in evaporated milk plants to decide stabiliser additions, as described in Lactose, Milk Salts and Vitamins. In practice, a milk is regarded as heat-stable for a process if it gives no coagulation, sediment or excessive viscosity after that process with a safety margin.
Heat coagulation is not simple denaturation of casein, which is very heat-stable. It results from a series of heat-induced changes that destabilise the micelles. Heating at high temperature degrades lactose to organic acids, chiefly formic acid, and precipitates soluble calcium phosphate onto the micelles; both lower the pH, which raises ionic calcium and reduces micelle charge, so heat-induced acidification is a major cause of coagulation. Casein is dephosphorylated and partially hydrolysed, and kappa-casein, alone or complexed with denatured beta-lactoglobulin, dissociates from the micelles, particularly at pH above about 6.7, leaving micelles depleted of their stabilising layer. Maillard reactions between lactose and casein lysine residues alter charge and may cross-link proteins. Coagulation occurs when these changes have reduced repulsion between micelles enough for them to aggregate. Several factors increase heat stability: a slightly higher pH within the stable range, removal of calcium or addition of phosphates and citrates, a higher natural urea content (urea added to milk increases HCT, which partly explains seasonal variation), and appropriate preheating of milk before concentration. Factors that reduce it include high ionic calcium, low pH, concentration, colostrum and late-lactation milk, mastitic milk, and bacterial acid development. Because milk varies, plants that sterilise milk or make evaporated milk monitor heat stability of incoming milk and adjust pH, salts and preheating accordingly.
Age gelation is the progressive thickening of UHT milk during storage, ending in an irreversible gel, often after several weeks to months at ambient temperature. It limits the practical shelf life of UHT milk and especially of UHT concentrated milks, cream and high-protein drinks, and it is often preceded or accompanied by bitterness and sediment. Two broad mechanisms are recognised and often act together. The proteolytic mechanism is caused by heat-stable proteases that survive UHT treatment. One is the indigenous milk protease plasmin, whose heat stability, activation and specificity are described in Milk Proteins and Enzymes; plasmin activity is higher in late-lactation and mastitic milk. The other group is the extracellular proteases of psychrotrophic bacteria, mainly Pseudomonas species, which grow in raw milk during cold storage and secrete enzymes so heat-stable that a large part survives UHT. Proteolysis releases the complex of kappa-casein and denatured beta-lactoglobulin formed during heating from the micelle surface, and these released complexes, together with partly hydrolysed micelles, aggregate into a three-dimensional network. Bacterial proteases tend to attack kappa-casein first and give gels with less bitterness, while plasmin attacks beta- and alpha-s-caseins and gives bitterness. The non-enzymatic (physicochemical) mechanism involves slow changes in the proteins during storage, including Maillard reactions and other cross-linking and changes in calcium phosphate, and is more important in concentrated products.
Age gelation depends on raw milk quality, processing and storage. Raw milk with high psychrotroph counts, from long cold storage before processing, carries more heat-stable bacterial protease; milk with high somatic cell counts or from late lactation carries more plasmin and plasminogen. The type of UHT heating matters: direct heating systems, which give a lower total heat load, leave more plasmin activity than indirect systems, so direct-UHT milk is generally more prone to proteolytic age gelation, while indirect heating causes more whey protein denaturation and cooked flavour. A preheating step before UHT, or a holding step at moderate temperature, can inactivate plasmin and its activators more effectively, because denatured beta-lactoglobulin also inhibits plasmin through thiol-disulphide reactions. Storage temperature affects the rate in a complex way: gelation and proteolysis generally proceed faster at warm ambient temperatures than under refrigeration, and very warm storage also increases browning. Additives such as polyphosphates, where permitted, delay gelation in concentrated UHT products by modifying calcium binding. Control in practice therefore rests on short and cold raw-milk storage with low psychrotroph counts, low somatic cell counts, choice of an adequate heat load, and monitoring of proteolysis during shelf-life trials, for example by the increase in non-protein nitrogen or by electrophoresis of the caseins. Rapid tests for heat-stable bacterial proteases in raw milk are used by some processors to select milk for long-life products.
Cream for butter and ghee in many smallholder systems, and some creamery cream, arrives sour because lactic acid bacteria have fermented lactose during collection. High acidity causes problems: casein coagulates when acid cream is pasteurised, trapping fat in curd particles and giving a cooked or burnt flavour; fat losses in buttermilk rise; and salted butter made from very acid cream develops off-flavours such as fishy and tallowy notes during storage, because low pH and salt promote oxidation. Sour cream is therefore partly neutralised before pasteurisation. Acidity of cream is expressed either on the cream or on its serum (the non-fat part); because lactic acid is in the serum, serum acidity equals cream acidity multiplied by 100 divided by (100 minus fat per cent), so cream with 40 per cent fat and 0.30 per cent acidity has a serum acidity of 0.50 per cent. Neutralisers are lime (calcium hydroxide), magnesium oxide or hydroxide, sodium bicarbonate and sodium carbonate, or mixtures. The quantity needed follows from stoichiometry: one mole of calcium hydroxide (74 grams) neutralises two moles of lactic acid (180 grams), so 1 kilogram of lactic acid needs about 0.41 kilogram of lime; sodium bicarbonate needs about 0.93 kilogram, sodium carbonate about 0.59 kilogram and magnesium oxide about 0.22 kilogram. Neutralisation is never carried to the original acidity of milk: over-neutralisation gives a soapy, alkaline flavour, and sodium neutralisers release carbon dioxide that can cause foaming.
Cream ripening has two meanings. Physical ripening is the controlled cooling and holding of pasteurised cream so that the fat crystallises to the right extent and form before churning; its principles and the temperature programmes used to adjust butter hardness are described in Milk Lipids. Biological ripening is the fermentation of cream with aroma-producing lactic cultures to make cultured (lactic, or ripened-cream) butter, the traditional type in much of continental Europe. The cultures are mesophilic mixed strains containing acid producers (Lactococcus lactis subspecies lactis and cremoris) and aroma producers that metabolise citrate (Lactococcus lactis subspecies lactis biovar diacetylactis and Leuconostoc species). They convert lactose to lactic acid, lowering the pH of the cream serum to about 4.6 to 5.0, and citrate to diacetyl, the key buttery flavour compound, along with acetaldehyde, acetoin and carbon dioxide. The chemistry of diacetyl formation and its reduction to flavourless acetoin is covered in chapter 10. Cultured butter has a fuller flavour and some protection against microbial spoilage from its low pH, but it oxidises more readily than sweet-cream butter, especially when salted, so it is usually unsalted or lightly salted. Because ripening cream takes time and produces acidic buttermilk that is harder to use, alternative processes add starter distillate (rich in diacetyl) and lactic acid concentrate to butter made from sweet cream, giving cultured flavour with sweet buttermilk. In ghee making, ripened cream or curd-based butter gives the characteristic flavour of traditional ghee.
Churning converts cream, an oil-in-water emulsion, into butter, a water-in-oil system, by destabilising the fat globules. Cream for batch churning usually contains about 35 to 40 per cent fat and is churned cold, at temperatures adjusted by season so that a suitable proportion of the fat is crystalline. The widely accepted explanation combines the action of air with partial coalescence, as described for fat globules in Milk Lipids. Agitation beats air into the cream, forming a foam. Fat globules containing fat crystals attach to the air bubbles, and the membranes of attached globules spread at the air-water interface, releasing some liquid fat. As bubbles collapse and new ones form, globules are brought together and clump through partial coalescence, held by fat crystals that pierce the thin films between them and by the liquid fat released. The clumps grow into visible butter grains, the foam collapses, and the remaining aqueous phase separates as buttermilk; at this point phase inversion has occurred. Churning efficiency depends on the fat content of the cream, temperature (warmer cream churns faster but gives soft butter and higher fat losses), the proportion of solid fat, the degree of agitation and the size of the globules. Fat lost in buttermilk is commonly kept below about 0.5 per cent. Continuous butter makers use cream of higher fat content, about 40 to 50 per cent, and intense agitation to make butter grains in seconds.
Butter is legally defined in most countries by minimum fat and maximum water contents; the Codex standard requires at least 80 per cent milk fat, at most 16 per cent water and at most 2 per cent non-fat milk solids, and national standards are similar. After churning, the butter grains are washed if required, salted if required and worked. Working kneads the grains into a coherent mass and disperses the water into fine droplets, mostly a few micrometres across, whose small size and even distribution protect butter from microbial growth, because bacteria in a droplet are confined to its limited nutrients. Salt, commonly about 1.5 to 2 per cent in salted butter, dissolves in these droplets; what matters for preservation is salt in the moisture, salt divided by (salt plus water) times 100, which for 1.8 per cent salt and 16 per cent water is about 10 per cent. The microstructure of butter is a continuous phase of liquid and crystalline fat in which are embedded intact or partly damaged fat globules, a network of fat crystals, water droplets and a few air cells. The proportion of solid fat and the crystal network determine hardness and spreadability, which is why physical ripening of the cream and working are used to control them. Butter yield is described by overrun: (weight of butter minus weight of fat in the cream) divided by weight of fat, times 100. Cream containing 40 kilograms of fat that yields 48.8 kilograms of butter has an overrun of 22 per cent. Common defects related to chemistry and structure include leaky butter (poorly dispersed water), mottled colour (uneven salt), greasy or crumbly texture (wrong crystallisation) and oxidised or rancid flavours.
Ghee is clarified milk fat made by heating butter or cream to remove almost all water, so that the non-fat solids brown and settle as ghee residue, from which the fat is decanted or filtered. Four methods are described in Indian texts. In the traditional (desi) method, milk or cream is fermented to curd and churned into butter (makkhan), which is heated in an open pan. In the creamery butter method, unsalted creamery butter is melted and heated. In the direct cream method, washed cream of high fat content is heated directly, which saves churning but gives more residue and needs longer heating. In the pre-stratification method, melted butter is held at about 80 degrees Celsius until it separates into layers; the lower aqueous layer, containing much of the non-fat solids, is drained off before final heating, which saves fuel, reduces residue and gives lower acidity. The final clarification temperature, commonly about 110 to 120 degrees Celsius, strongly influences flavour, colour and keeping quality. Ghee is about 99.5 per cent or more fat, with moisture normally below about 0.5 per cent, and small amounts of free fatty acids, phospholipids, carotenoids, fat-soluble vitamins, cholesterol and charred protein. Its phospholipid content depends on the method and temperature: it is higher in ghee from the direct cream method and at higher clarification temperatures, because more phospholipid passes from the residue into the fat. Cow ghee is yellow from carotene; buffalo ghee is white with a greenish tinge.
Because ghee is a high-value product often adulterated, its identity is checked by analytical constants. The butyro-refractometer reading at 40 degrees Celsius is typically about 40 to 45, lower than most vegetable oils and body fats; the Reichert-Meissl value, reflecting butyric acid, is high, commonly in the twenties to low thirties for cow and buffalo ghee; the Polenske value is low, about 1 to 2; the saponification value is high, about 220 to 232; and the iodine value is low, about 26 to 38. Values vary with species, season, feed and region, which is why some national standards set different limits for different regions or seasons; their use in detecting adulteration is covered in Dairy Analysis and Instrumental Methods. The flavour of ghee comes from compounds formed during clarification: free fatty acids, carbonyls (aldehydes and methyl ketones), lactones formed from hydroxy fatty acids on heating, and Maillard reaction products formed between lactose and proteins in the browning residue. Ghee from ripened cream or curd has a stronger, slightly acidic flavour; ghee clarified at higher temperature has a more pronounced cooked and nutty flavour. Ghee deteriorates mainly by oxidation, giving tallowy and oxidised flavours, and by hydrolytic rancidity if moisture remains; the factors that govern keeping quality, including the antioxidant effect of residue phospholipids and Maillard products and the influence of metals, light and packaging, are described in Milk Lipids. On cooling, ghee crystallises; slow cooling at moderate temperatures gives the large grains valued in traditional markets.
Concentrated milks are made by removing water, mainly by vacuum evaporation, and are either sterilised in the container (evaporated milk), preserved by added sugar (sweetened condensed milk) or used as intermediates for powders and other products. The degree of concentration is described by the concentration factor, the total solids of the concentrate divided by that of the milk: concentrating milk from 12.5 to 25 per cent total solids is a factor of 2.0, and removes 500 kilograms of water from each 1,000 kilograms of milk. Concentration changes milk chemistry in predictable ways. Soluble salts become more concentrated and calcium phosphate becomes more supersaturated and partly precipitates onto the micelles, releasing hydrogen ions, so the pH falls, typically by a few tenths of a unit; ionic calcium rises. Viscosity rises steeply with total solids. Heat stability falls, which is why evaporated milk needs forewarming and stabilising salts, as described in Lactose, Milk Salts and Vitamins. Whey proteins denature during preheating and evaporation, and Maillard reactions begin. In sweetened condensed milk, lactose becomes supersaturated and must be crystallised under control, as explained in Lactose, Milk Salts and Vitamins, and the high sucrose concentration lowers water activity enough to prevent most microbial growth. Evaporated milk is homogenised to prevent fat separation during storage and then sterilised in the can, or UHT-treated and aseptically packed.
Concentrated milks continue to change after manufacture. Sweetened condensed milk undergoes age thickening, a gradual increase in viscosity that can end in gelation; it is faster at higher storage temperatures, with higher preheating temperatures and in concentrates with more solids, and is linked to changes in casein micelles and salt balance. Its other defects include sandiness from large lactose crystals, sucrose crystallisation when the sucrose ratio is too high, browning, fat separation, and, if contaminated, mould colonies called buttons and gas from sugar-tolerant yeasts that blow the cans. Evaporated milk can develop sediment of calcium phosphate and citrate, fat separation if poorly homogenised, gelation and thickening, browning and stale flavour from Maillard reactions, which proceed faster at warm storage temperatures, and a loss of vitamins. In both products, colour and flavour change most in hot climates, so storage at moderate temperatures and rotation of stock are important. Because the extent of these changes depends on the raw milk, the preheating and concentration conditions, the stabiliser addition and the storage temperature, manufacturers run accelerated storage tests at elevated temperatures to predict shelf life. Recombined concentrated milks, made from skim milk powder and fat, behave somewhat differently because the heat history of the powder affects whey protein denaturation and heat stability; medium- or high-heat powders, classified by the whey protein nitrogen index described in Milk Proteins and Enzymes, are generally preferred for recombined evaporated milk.
Milk powders are made by spray drying or, less often today, roller drying of concentrated milk. In spray drying, the concentrate is atomised into fine droplets in hot air; water evaporates within seconds, and each droplet becomes a particle. Because drying is so fast, lactose is left as an amorphous glass, as described in Lactose, Milk Salts and Vitamins, forming a continuous matrix in which proteins, casein micelles and fat globules are embedded. Spray-dried particles are roughly spherical, typically tens of micrometres across, and contain air pockets called vacuoles, formed from air incorporated during atomisation and from expansion of the droplet; vacuoles lower particle density and influence bulk density. In whole milk powder, part of the fat is free fat, not enclosed in intact globule membranes or in the matrix, and lies at or near the particle surface, where it is accessible to solvents and to oxygen. Roller-dried powder, made on a heated drum, forms flakes with more free fat, more heat damage to proteins and lower solubility, but better functionality in some uses such as chocolate making. Powder composition is regulated by standards; skim milk powder typically has less than about 5 per cent moisture and whole milk powder about 26 per cent or more fat, and powders are classified by heat treatment using the whey protein nitrogen index described in Milk Proteins and Enzymes, which determines their suitability for recombined, bakery or cheese uses.
Solubility describes how completely a powder reconstitutes. It is measured by the insolubility index, the volume of sediment from a reconstituted sample after centrifugation under standard conditions; high values indicate heat damage, excessive drying temperatures, high moisture or long storage in poor conditions. Instant properties describe how quickly a powder reconstitutes and consist of four steps: wettability (penetration of water into the powder mass), sinkability (sinking of particles below the surface), dispersibility (separation into individual particles without lumps) and solubility (dissolving of the soluble components). Fine powders tend to form lumps because water wets the surface and forms a sticky layer that blocks further penetration. Instant powders are made by agglomeration, in which fine particles are made to stick together into larger, porous clusters, either in the dryer (straight-through) or by rewetting and redrying. Whole milk powder is additionally lecithinated, coated with a small amount of lecithin, an amphiphilic phospholipid that makes the fat-covered surface wettable by water. During storage, powders can cake when moisture lowers the glass transition temperature of amorphous lactose below storage temperature, causing lactose crystallisation, as explained in Lactose, Milk Salts and Vitamins; they can brown and lose lysine through Maillard reactions; and whole milk powder can oxidise, which is why it is packed in containers flushed with nitrogen or a nitrogen-carbon dioxide mixture to a low residual oxygen content. Solubility also declines with long storage at high temperature and humidity.
Rennet coagulation is the basis of most cheese manufacture. It proceeds in three overlapping phases. In the primary, enzymatic phase, chymosin or another coagulant cleaves the Phe105-Met106 bond of kappa-casein, releasing the caseinomacropeptide into the whey and leaving para-kappa-casein on the micelles, as described in Milk Proteins and Enzymes. In the secondary, aggregation phase, the para-casein micelles, having lost their steric and part of their electrostatic stabilisation, aggregate once most of the kappa-casein has been hydrolysed; aggregation needs calcium ions and proceeds only above about 15 to 20 degrees Celsius. In the tertiary phase, the aggregates form a continuous gel network that continues to strengthen, and later, when the gel is cut, the network contracts and expels whey (syneresis). The rennet coagulation time is measured visually as the time when flocculation first appears in a film of milk on the wall of a rotating bottle or tube, or instrumentally by changes in viscosity, light scattering or elasticity. For a given milk, the clotting time is inversely proportional to the coagulant concentration, a relationship known as Holter's rule (clotting time multiplied by enzyme concentration is approximately constant); so if 1 millilitre of rennet clots a vat in 20 minutes, 1.6 millilitres would clot it in about 12.5 minutes. Gel firmness at cutting determines curd losses and yield, so cheesemakers judge it carefully.
Many factors alter rennet coagulation. Lower pH speeds both phases, because chymosin acts faster at slightly acid pH and lower pH reduces micelle charge; this is one reason starter acid development before renneting matters. Higher temperature speeds the secondary phase, with coagulation fastest near 40 degrees Celsius, but cheesemakers usually set milk at about 30 to 35 degrees Celsius to obtain a manageable firming rate. Calcium chloride, added at small rates to cheese milk, shortens coagulation time and firms the gel, especially in milk that has been cold-stored or pasteurised. Cold storage of milk releases beta-casein and calcium phosphate into the serum and slows coagulation; high heat treatment, through whey protein-kappa-casein complexes, slows it and weakens the curd; high casein content and kappa-casein B speed it and firm the curd, as noted in Milk Proteins and Enzymes. Coagulants differ in their proteolytic properties. Calf rennet consists mainly of chymosin with some bovine pepsin; fermentation-produced chymosin, made by microorganisms carrying the chymosin gene, is now the most widely used; microbial coagulants from moulds such as Rhizomucor miehei, Rhizomucor pusillus and Cryphonectria parasitica are cheaper but more proteolytic; camel chymosin cleaves kappa-casein very specifically with little general proteolysis; and plant coagulants such as those from cardoon flowers are used in some traditional cheeses. The ratio of clotting to general proteolytic activity (the C/P ratio) is a key property: high general proteolysis lowers yield and can cause bitterness. Cheese yield can be predicted from fat and casein; Van Slyke's formula, Y = (0.93F + C - 0.1) x 1.09 / (1 - W), with F and C in per cent and W the moisture fraction, gives about 9.9 kilograms of Cheddar from 100 kilograms of milk with 3.6 per cent fat and 2.5 per cent casein at 37 per cent moisture.
Cheese ripening is the set of biochemical changes that turn fresh curd into mature cheese. They are grouped as glycolysis (metabolism of lactose, lactate and citrate), proteolysis and lipolysis, followed by catabolism of the products into flavour compounds. Most lactose leaves with the whey; the residual lactose in the curd is fermented to L-lactate by the starter within the first days, and in hard cheeses its rapid removal limits later growth of undesirable organisms. Non-starter lactic acid bacteria can convert L-lactate to D-lactate (racemisation), and calcium D-lactate can crystallise as white specks on Cheddar. Lactate is further metabolised in particular cheeses. In Swiss-type cheeses, Propionibacterium freudenreichii converts lactate to propionate, acetate, carbon dioxide and water; the carbon dioxide forms the eyes and propionate contributes the sweet, nutty flavour. In surface-mould cheeses such as Camembert, Penicillium camemberti oxidises lactate to carbon dioxide and water, raising the pH at the surface and creating a pH gradient that softens the cheese from the outside in. In some hard cheeses, Clostridium tyrobutyricum ferments lactate to butyrate, carbon dioxide and hydrogen, causing late blowing and off-flavour; it is controlled by milk hygiene, bactofugation, nitrate where permitted or lysozyme. Citrate is metabolised by citrate-fermenting lactococci and leuconostocs to diacetyl and carbon dioxide, which gives openness in Dutch-type cheeses.
Proteolysis is the most complex and important ripening change in most cheeses. The agents are residual coagulant, which hydrolyses alpha-s1-casein at Phe23-Phe24 early in ripening, producing alpha-s1-I-casein and softening the texture; plasmin, which hydrolyses beta-casein to gamma-caseins and is especially active in cheeses cooked at high temperatures, where the coagulant is largely inactivated; the cell-envelope proteinases and intracellular peptidases of the starter bacteria, released when cells lyse, which produce small peptides and free amino acids; and enzymes of non-starter and secondary flora. Proteolysis is followed by measuring nitrogen soluble at pH 4.6 or in water, nitrogen soluble in 12 per cent trichloroacetic acid and in phosphotungstic acid (small peptides and free amino acids), by electrophoresis and by chromatography of peptides. Free amino acids are catabolised by transamination, decarboxylation, deamination and side-chain reactions into aldehydes, alcohols, acids, sulphur compounds such as methanethiol from methionine, and amines; decarboxylation can produce biogenic amines such as histamine and tyramine in some cheeses. Bitterness results from accumulation of hydrophobic peptides when they are produced faster than they are degraded, often from excessive coagulant or unsuitable starters. Lipolysis in cheese, and its contribution to flavour in blue, Italian and some hard cheeses, is described in Milk Lipids: free fatty acids contribute directly and are converted to methyl ketones, lactones and esters. The balance of all these reactions, governed by moisture, salt, pH and temperature, gives each cheese variety its flavour and texture.
Lactic acid bacteria convert lactose to lactic acid, lowering pH, coagulating casein near its isoelectric point and preserving the product. Lactose enters the cell by two main systems. Lactococci use a phosphoenolpyruvate-dependent phosphotransferase system, which phosphorylates lactose as it enters; lactose 6-phosphate is split into glucose and galactose 6-phosphate, the latter metabolised by the tagatose pathway. Streptococcus thermophilus and Lactobacillus delbrueckii subspecies bulgaricus use a permease and hydrolyse lactose inside the cell by beta-galactosidase; many strains use only the glucose half and excrete galactose, which remains in yoghurt and can contribute to browning in some products. Homofermentative bacteria convert hexoses through glycolysis almost entirely to lactic acid, two molecules per hexose. Heterofermentative bacteria such as Leuconostoc use the phosphoketolase pathway and produce, from each hexose, one lactic acid, one carbon dioxide and one ethanol or acetate. In yoghurt, the two cultures grow in protocooperation: the lactobacillus hydrolyses casein to peptides and amino acids that stimulate the streptococcus, while the streptococcus produces formic acid, carbon dioxide and folate that stimulate the lactobacillus. Yoghurt is usually fermented to a pH of about 4.4 to 4.6. Some strains produce exopolysaccharides that increase viscosity and reduce syneresis. In kefir and koumiss, yeasts add ethanol and carbon dioxide to the lactic fermentation.
The key flavour of yoghurt is acetaldehyde, produced mainly by Lactobacillus bulgaricus, from threonine by threonine aldolase and from pyruvate; yoghurt typically contains acetaldehyde in the range of several to a few tens of milligrams per kilogram, together with lactic acid, diacetyl, acetone and 2-butanone. The key flavour of cultured butter, cultured buttermilk and some fresh cheeses is diacetyl, produced from citrate by Lactococcus lactis biovar diacetylactis and Leuconostoc species. Citrate is converted to oxaloacetate and then pyruvate; surplus pyruvate is condensed to alpha-acetolactate, which decarboxylates oxidatively and non-enzymatically to diacetyl, a reaction favoured by low pH and oxygen, or enzymatically to acetoin. Diacetyl can be reduced by diacetyl reductase to acetoin and then butanediol, which have little flavour, so diacetyl levels fall if cultures are held too long or too warm; cooling and aeration help preserve it. A balance between diacetyl and acetaldehyde is important: excess acetaldehyde relative to diacetyl gives a green or yoghurt-like flavour defect in buttermilk and cultured cream, and some Leuconostoc strains reduce acetaldehyde to ethanol, correcting it. Other fermentation flavour compounds include acetic acid from heterofermenters, ethanol, carbon dioxide (which gives a fresh, sparkling note in kefir), and compounds from protein and fat breakdown during longer fermentations. Defects include excessive acidity and bitterness from over-fermentation or proteolytic strains, yeasty and fruity flavours from contaminating yeasts, and lack of flavour from cultures with low citrate metabolism.
Ice cream is a frozen foam in which air cells are dispersed in a partly frozen matrix. Its structure has four phases. Ice crystals, formed from water during freezing, are typically a few tens of micrometres in fresh ice cream; small crystals give smoothness, while growth of crystals during storage gives an icy, coarse texture. Air cells, incorporated during freezing in the barrel freezer, are of similar size and give lightness; the amount of air is expressed as overrun, the increase in volume over the mix: overrun equals (volume of ice cream minus volume of mix) divided by volume of mix, times 100. One litre of mix whipped to two litres of ice cream has 100 per cent overrun; by weight, if a litre of mix weighs 1.10 kilograms and a litre of the ice cream 0.55 kilograms, the overrun is also 100 per cent. A network of partially coalesced fat globules, formed by shear during freezing, surrounds and stabilises the air cells and gives dryness, stiffness and slow melting. The unfrozen serum phase is a concentrated solution of sugars, salts and proteins, freeze-concentrated as water turns to ice; it remains liquid even at low storage temperatures, and its viscosity and the sweetener composition determine how much water is frozen at a given temperature and how fast ice crystals grow. Lactose crystallisation in this serum causes sandiness, described in Lactose, Milk Salts and Vitamins. Mix composition, the freezing process and storage temperature together govern the size and stability of these structural elements.
Stabilisers are hydrocolloids added at a few tenths of a per cent, such as locust bean gum, guar gum, sodium carboxymethyl cellulose, sodium alginate, xanthan gum and gelatin. They increase the viscosity of the unfrozen serum and form weak networks that slow the migration of water and the growth of ice crystals during temperature fluctuations (heat shock), giving smoother texture, better body and melt resistance. Carrageenan is often added as a secondary stabiliser because it interacts with casein to prevent the wheying-off, or serum separation, that some primary stabilisers cause in the mix. Emulsifiers, such as mono- and diglycerides and polysorbate 80, act in an apparently paradoxical way: instead of stabilising the fat emulsion, they partly displace milk proteins from the surface of the fat globules during ageing, making the globules less stable to shear, so that during freezing they undergo more partial coalescence. The resulting fat network gives drier extrusion from the freezer, stiffer body, smoother texture and slower melting; too much emulsifier causes excessive destabilisation and a buttery texture. The mix is aged, held cold for some hours after homogenisation and cooling, so that fat crystallises inside the globules, emulsifiers adsorb and displace protein, and stabilisers hydrate fully. During storage, ice crystals coarsen by recrystallisation, especially at higher and fluctuating temperatures; low and stable storage temperatures, adequate stabiliser and appropriate sweeteners limit this. Fat content and type, protein, sweeteners and the balance of stabilisers and emulsifiers are formulated together to give the desired structure.
Khoa (khoya) is a partly dehydrated whole milk product made by boiling milk in an open pan with continuous stirring and scraping until it reaches a semi-solid consistency; it is the base of many Indian sweets such as burfi, peda and gulab jamun. Buffalo milk is preferred because its higher fat and solids give a higher yield and a softer, smoother, more granular product, whereas cow milk khoa is harder and often sticky. Three types are recognised by consistency: pindi (hard, used for burfi and peda), dhap (soft, for gulab jamun) and danedar (granular, made by adding a little acid or sour whey to produce grains). During manufacture, water evaporates until total solids reach about 65 to 70 per cent; the yield follows from mass balance, so 100 kilograms of milk with 16 per cent total solids gives about 22.9 kilograms of khoa at 70 per cent solids if no solids are lost. The intense heating denatures the whey proteins, which become part of the protein network with casein, causes lactose to concentrate far beyond its solubility so that it is present partly in crystalline and partly in amorphous form, and drives the Maillard reaction, which gives khoa its light brown colour and cooked, caramelised flavour. Fat partly separates and is distributed through the mass, and free fat increases. Khoa has a high water activity for a concentrated product and spoils quickly through mould and bacterial growth and lipid oxidation, so it is cooled quickly and stored cold; its keeping quality is improved by hygienic handling and packaging.
Chhana and paneer are acid-heat coagulated products: milk is heated to a high temperature and coagulated with an acid, such as citric acid, lactic acid or sour whey, and the curd is separated from the whey. Heating before coagulation denatures the whey proteins, which then co-precipitate with casein, so these products retain much more of the milk protein than rennet cheese, and the yield per litre is higher. Acid coagulation at high temperature works at a pH somewhat above the isoelectric point of casein because heat assists aggregation. Chhana, the base of Bengali sweets such as rasogolla and sandesh, is usually made from cow milk, coagulated at about 80 degrees Celsius and drained without pressing; cow milk gives a soft, smooth, spongy texture suited to these sweets, whereas buffalo milk chhana is harder and greasy. Paneer is made from buffalo milk or a mixture standardised to a suitable ratio of fat to solids-not-fat, heated to about 90 degrees Celsius, cooled slightly and coagulated with a dilute citric acid solution at about 70 degrees Celsius, then pressed into blocks and chilled in water. Its firm, sliceable, close texture, which holds its shape on frying, depends on the coagulation temperature and pH, the calcium content and the pressing. Cow milk paneer tends to be soft and crumbly, which can be improved by adding calcium chloride. Indian standards, for example, limit the moisture of paneer to 70 per cent and require at least 50 per cent fat in its dry matter. Both products have high moisture and short shelf lives, governed by microbial spoilage and fat oxidation.
A student uses this subject to understand what is happening in the vat, churn, evaporator and freezer during product practicals. You will neutralise cream and calculate the neutraliser, make butter and calculate overrun and salt in moisture, prepare ghee by different methods and measure its constants, make khoa and paneer and calculate yields, measure rennet clotting time, follow pH and acidity in yoghurt, and calculate ice cream overrun. Learn the mechanisms behind churning, coagulation, ripening and flavour formation, because examiners ask why a product behaves as it does as often as they ask what it contains.
Five years into work in a dairy plant, you diagnose product defects by their chemistry: age gelation in UHT milk traced to psychrotrophic proteases, sandy condensed milk traced to faulty seeding, rancid or oxidised ghee traced to moisture or copper, weak curd traced to cold-stored or over-heated milk, bitter cheese traced to coagulant choice, green flavour in buttermilk traced to culture balance, and coarse ice cream traced to heat shock. You adjust neutralisation, stabiliser dosing, coagulant rates and ageing times accordingly and explain the reasons to production staff.
A senior manager or technical director with twenty-five years of experience applies product chemistry to decisions on raw milk specifications for UHT and cheese plants, the choice between direct and indirect UHT, investment in pre-stratification for ghee or in agglomeration for instant powders, the selection of coagulants and cultures, and the formulation of ice cream and dairy desserts for different climates and markets. Such a person also sets shelf-life claims based on accelerated storage trials and answers customer and regulatory questions about product composition and defects.
A researcher with long experience works on the mechanisms that still are not fully understood: the molecular basis of age gelation, the structure and stability of protein-fat-lactose systems in powders, flavour formation pathways in cheese and fermented milks, the role of emulsifiers and stabilisers at interfaces, and the chemistry of traditional products such as khoa and paneer, including shelf-life extension. Methods include proteomics and peptidomics, volatile analysis by gas chromatography-mass spectrometry, microscopy and rheology, and kinetic modelling of storage changes.
This manual explains the chemistry of each product; manufacturing procedures, equipment and process parameters are treated in the product-technology subjects. National standards for each product are given only as examples and must be checked against current regulations. Infant formula chemistry, processed cheese, dairy desserts other than ice cream, and whey products are covered only in passing. Ghee constants vary with species, season and region and are given as typical ranges; the detailed analytical methods and adulteration tests are in Dairy Analysis and Instrumental Methods. Quantitative kinetics of age gelation, age thickening and Maillard browning in products are not given because published values vary widely with composition and storage conditions.
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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