Food Science & Technology · Commodity Processing Technologies · also Dairy Technology (food technology degrees), Milk and Milk Products Processing, Dairy Processing
The dairy course taught inside food technology degrees: milk as a raw material, fluid-milk processing, the main dairy products, and dairy ingredients (powders, whey and milk proteins) used across the food industry. For depth in dairy chemistry, microbiology and plant engineering see the Dairy Technology track; for Indian traditional dairy products and FSSAI dairy standards see Veterinary's milk-products-technology.
In the exams: GATE XE-G and XL-U Section 3 (milk and milk products processing: pasteurisation, sterilisation, cream, butter, ghee, ice cream, cheese, milk powder); ICAR AIEEA-PG Code 18 Unit III; ICAR AICE 35.a Unit 3
Milk is the most thoroughly processed raw material in the food industry. A food technologist meets it everywhere: as liquid milk and cream, as butter, ghee, cheese and yoghurt, as ice cream and condensed milk, and above all as dairy ingredients, skim and whole milk powder, whey proteins, caseinates and lactose, that go into bakery, confectionery, meat products, beverages and infant formula. This manual is the dairy course taught inside food technology degrees. It explains what milk is, how it is received and tested, the main processing operations (separation, standardisation, homogenisation, pasteurisation and sterilisation), the principal products, and the ingredients made from milk and whey, ending with the cleaning that every dairy plant depends on.
It is deliberately a compact overview. Dairy chemistry, microbiology, starter cultures, cheese science, plant engineering and dairy law are taught in depth in the Dairy Technology track of this programme, in subjects such as milk-composition-physical-properties, market-milk-processing, fat-rich-dairy-products, cheese-science-technology, fermented-milks-technology, frozen-dairy-desserts, concentrated-dried-milks, whey-dairy-byproducts and cip-sanitation-hygienic-design; Indian traditional dairy products and FSSAI dairy standards are covered in Veterinary's milk-products-technology. Each chapter here names where to go for depth.
Dairy questions appear in GATE XE-G and XL-U Section 3 (pasteurisation, sterilisation, cream, butter, ghee, ice cream, cheese and milk powder), ICAR AIEEA-PG Code 18 Unit III and ICAR AICE 35.a Unit 3. Values quoted here are typical textbook figures for cow milk; composition varies with species, breed, feed and season, and legal standards for each product differ between countries and must be read from the current regulation.
Cow milk is roughly 87% water and 13% total solids. Typical values are fat about 3.5–4.5%, protein about 3.2–3.5%, lactose about 4.6–4.8% and minerals (ash) about 0.7%; the solids other than fat, the solids-not-fat (SNF), are commonly about 8.5–9%. Buffalo milk, important in South Asia, has much more fat (commonly about 6–8%) and somewhat more protein and SNF; goat and sheep milks differ again. Composition changes with breed, stage of lactation, feeding, season and milking, so standards and payment systems test each supply.
Milk fat occurs as globules, mostly 1–10 µm across, each surrounded by the milk fat globule membrane, a layer of phospholipids and proteins that keeps the globules apart. About four-fifths of milk protein is casein, present as colloidal micelles that also carry much of the milk's calcium phosphate; the remaining fifth is whey (serum) protein, mainly β-lactoglobulin and α-lactalbumin, which stays in solution when casein is precipitated. Lactose, a disaccharide of glucose and galactose, is the main carbohydrate. Milk also carries enzymes such as alkaline phosphatase and lipase, vitamins and somatic cells. The chemistry of each component is covered in depth in the Dairy Technology track.
Several physical properties are used to test milk. Its density is commonly about 1.028–1.034 g/mL at 15–20 °C; added water lowers it and skimming raises it, so the lactometer reading is combined with fat content to estimate SNF. Its freezing point is close to −0.52 to −0.54 °C, set by lactose and soluble salts, and is very constant; added water raises it towards 0 °C, so the cryoscope is the most sensitive test for watering. Fresh milk has a pH of about 6.6–6.8 and a titratable acidity, expressed as lactic acid, commonly about 0.13–0.16%, which comes from the buffering of casein, phosphates and citrate rather than from lactic acid; developed acidity from bacterial growth raises it. Casein micelles precipitate at their isoelectric pH, about 4.6, the basis of acid casein, paneer-type products and yoghurt gels.
Milk's white colour comes from light scattering by casein micelles and fat globules; its creamy tint from carotene in the fat. Viscosity, surface tension and heat stability are other properties that matter in processing and are treated in milk-composition-physical-properties. Heat changes these properties in ways that matter downstream. Heating above about 70 °C begins to denature whey proteins, which then attach to casein micelles and change rennet coagulation, yoghurt texture and powder functionality; strong heating develops cooked flavours and Maillard browning between lactose and proteins. Cooling and storage change the balance of calcium phosphate between the micelles and the serum, which is one reason why cold-stored milk behaves differently in cheese making.
Milk arriving at a collection centre or plant is checked quickly at the reception platform before it is accepted. The platform tests are designed to take minutes and to reject milk that would cause trouble in processing. They include organoleptic examination (appearance, smell, taste where safe), temperature, the clot-on-boiling test, in which a sample is boiled and the formation of clots shows high developed acidity or colostrum, the alcohol test, in which milk mixed with ethanol of a set strength (commonly 68%) flocculates if its proteins are unstable to heat, titratable acidity, sediment, and lactometer and fat readings for composition and adulteration. Milk is also screened for antibiotic residues with rapid tests, because residues inhibit starter cultures and are a public-health hazard, and for adulterants specific to the region. Tanker milk is sampled after thorough mixing.
Microbiological quality is assessed by dye reduction tests such as the methylene blue reduction time (MBRT) and the resazurin test, where the faster the dye is reduced by bacterial metabolism, the poorer the milk, and in the laboratory by standard plate counts and somatic cell counts. Somatic cell count is an index of udder health (mastitis); milk from mastitic udders has altered composition and poorer processing quality. Milk that fails a platform test is rejected or diverted to a use where the defect does not matter, and the reason is recorded so that the supplier can be advised; repeated failures from one source are followed up at the farm.
Fat is measured by the Gerber method, in which sulphuric acid dissolves the proteins and amyl alcohol helps separate the fat in a calibrated butyrometer after centrifuging, or by instrumental infrared analysers calibrated against reference methods. SNF is often estimated from the corrected lactometer reading (CLR) and the fat percentage with an empirical formula. One widely used version, Richmond's formula, gives total solids = 0.25 L + 1.2 F + 0.14 and SNF = 0.25 L + 0.2 F + 0.14, where L is the corrected lactometer reading and F the fat percentage; for L = 28 and F = 4.0 this gives SNF = 7.94% and total solids = 11.94%. Other formulas with different constants are used in different countries and for different lactometers, so the formula must be stated with the result.
Milk is commonly paid for on fat and SNF (or fat and protein), so testing accuracy has direct financial consequences. Adulteration with water, starch, sugar, salt, urea, detergents or vegetable fat is detected by specific chemical tests and by cryoscopy; methods and legal limits are set nationally and are covered in dairy-analysis-instrumental and Veterinary's milk-products-technology.
Clarifiers are centrifuges that remove dirt, cells and sediment from milk. Bactofugation uses a special high-speed centrifuge to remove most bacterial spores, which matters for cheese milk, where spores of Clostridium tyrobutyricum cause late blowing. Cream separators are disc-stack centrifuges that use the difference in density between fat globules and skim milk. Stokes' law describes the rate at which a globule rises: velocity is proportional to the square of the globule diameter and to the density difference, and inversely proportional to the viscosity of the liquid. Doubling the globule diameter therefore increases the rising velocity four times. Separation is done warm, commonly about 40–50 °C or higher, because fat is liquid and viscosity lower; efficient separators leave skim milk with only a few hundredths of a per cent of fat.
The separation is a mass balance on fat. 1000 kg of milk with 4.0% fat separated into cream of 40% fat and skim of 0.05% fat gives cream = 1000 × (4.0 − 0.05) ÷ (40 − 0.05) = 98.9 kg and skim milk = 901.1 kg.
Standardisation adjusts the fat (and sometimes protein) content of milk to a fixed value for the product or the law. It is done in line by separating milk and remixing cream and skim in measured proportions, or in batches by adding skim milk or cream. The Pearson square or a simple fat balance gives the proportions. To bring 1000 kg of milk at 4.5% fat to 3.0% fat with skim milk at 0.05% fat, the skim needed is 1000 × (4.5 − 3.0) ÷ (3.0 − 0.05) = 508.5 kg, giving 1508.5 kg of standardised milk. A common error is to use 3.0 − 0 in the denominator or to apply the percentages to the final rather than the initial mass. Protein standardisation, permitted in some countries, uses ultrafiltration retentate or permeate. Standardisation also sets the protein-to-fat ratio of cheese milk, which controls the fat content of the cheese in its dry matter and therefore whether it meets its standard; the same balance logic is applied with protein instead of fat. In continuous standardisation, in-line density and infrared sensors measure the fat of the cream and skim streams, and control valves remix them automatically to the target, with the surplus cream diverted to butter or cream products.
Homogenisation breaks fat globules into much smaller ones, most below about 1–2 µm, by forcing warm milk through a narrow gap at high pressure, commonly in the range of about 10–25 MPa in the first stage of a two-stage homogeniser; the second stage, at lower pressure, breaks up clusters of new globules. The new globules are coated mainly by casein and whey proteins, which prevents them coalescing. Because Stokes' law makes the rising velocity depend on the square of the diameter, small globules cream so slowly that no cream line forms. Homogenised milk is whiter, tastes richer, forms a softer curd, and is more susceptible to light-induced oxidised flavour and, if raw milk is homogenised, to rapid lipolysis because the new globule surfaces expose fat to lipase; for this reason homogenisation is done immediately before or after pasteurisation. Equipment design is covered in dairy-processing-equipment. Homogenisation also changes how milk behaves as an ingredient. The larger fat surface takes up casein, so homogenised milk gives a softer, weaker rennet curd, which is why cheese milk is usually not homogenised except for some soft cheeses, and homogenised cream whips poorly. In ice cream mix and recombined products, by contrast, homogenisation is essential for a smooth, stable emulsion.
Pasteurisation is a heat treatment that destroys pathogenic microorganisms likely to be present in milk and reduces spoilage organisms, with minimal change to flavour and nutrients. The two classic time–temperature combinations are low-temperature long-time (LTLT or batch) pasteurisation at 63 °C for 30 minutes and high-temperature short-time (HTST) pasteurisation at 72 °C for 15 seconds, in a plate heat exchanger with holding tube and a flow diversion valve that returns under-heated milk. The standards were set to destroy the most heat-resistant non-spore-forming pathogens historically associated with milk, Mycobacterium tuberculosis and later Coxiella burnetii. Alkaline phosphatase, a native milk enzyme slightly more heat-resistant than these pathogens, is inactivated by correct pasteurisation, so a negative phosphatase test shows that milk has been adequately heated. Pasteurised milk must be kept refrigerated; its shelf life is limited by post-pasteurisation contamination and by heat-resistant spore-formers.
Plate heat exchangers save energy by regeneration: cold incoming milk is heated by hot outgoing pasteurised milk. Regeneration efficiency is the temperature rise achieved by regeneration divided by the total rise. Milk entering at 4 °C, regenerated to 60 °C and heated to 72 °C has a regeneration efficiency of (60 − 4) ÷ (72 − 4) = 82.4%.
Ultra-high-temperature (UHT) treatment heats milk continuously to a high temperature, commonly about 135–150 °C, for a few seconds, then cools it and fills it aseptically into sterile packages; the product is commercially sterile and keeps for months at room temperature. Heating is either direct, by steam injection or infusion with flash cooling to remove the added water, or indirect, through plate or tubular heat exchangers. Direct heating gives a milder flavour because the time at high temperature is shorter. UHT milk has a cooked flavour and can show age gelation and sedimentation on long storage, linked to heat-resistant proteases from psychrotrophic bacteria in the raw milk.
Extended shelf life (ESL) milks are given treatments between pasteurisation and UHT, or microfiltration plus pasteurisation, and filled under very clean conditions; they keep for weeks under refrigeration. In-container sterilised milk is filled into bottles or cans and retorted, commonly at about 110–120 °C for some minutes, giving a darker colour and stronger cooked and caramel flavour from Maillard reactions. Thermal process theory is in thermal-processing-canning and market milk processing in depth in market-milk-processing.
Cream is the fat-rich part of milk separated by centrifuge. Its fat content defines the type: single or light creams with lower fat, whipping cream commonly around 35–40% fat, double or heavy creams higher still; legal definitions differ by country. Cream is pasteurised at higher temperatures than milk because fat protects bacteria and the product is more viscous. Whipping depends on fat globules partly crystallised at refrigeration temperature; during whipping they partly coalesce around air bubbles to give a stable foam. Cream for butter making is ripened, sometimes with lactic cultures (cultured or ripened cream butter), and aged cold so that the fat crystallises in a controlled way that sets the texture of the butter. Cream is a good medium for bacteria and absorbs odours readily, so it is handled cold and quickly. Sterilised and UHT creams are made for long-life use, and cream liqueurs and coffee creamers are formulated to resist feathering, the flocculation of cream proteins in hot, acidic coffee. Sour cream and crème fraîche are cultured creams fermented with mesophilic lactic cultures to a thick, mildly acid product.
Butter is made by churning cream. Agitation damages the fat globule membranes and makes partly crystalline globules collide and clump, until the emulsion inverts: from fat-in-water (cream) to water-in-fat (butter). The butter grains separate from buttermilk, which is drained; the grains are washed if required, salted if required, and worked to distribute water as fine droplets and to give a smooth, plastic body. Codex and many national standards require butter to contain at least 80% milk fat and not more than about 16% water, with salt and milk solids making up the rest. Continuous butter-making machines now handle most industrial production.
The theoretical butter yield follows from the fat balance. 100 kg of cream at 40% fat churned into butter with 82% fat gives at most 100 × 0.40 ÷ 0.82 = 48.8 kg of butter, ignoring the small fat loss in buttermilk; the 'overrun' of butter over the fat used is the gain from water, salt and solids. Butter defects include rancidity from lipase, oxidised and fishy flavours, mould on the surface, and crumbly or greasy texture from poor fat crystallisation.
Ghee is clarified butterfat of South Asian origin, made by heating butter or cream to drive off moisture and to develop a characteristic cooked, nutty flavour from reactions between milk solids, lactose and fat, then separating the fat from the browned solids (ghee residue). The main methods are the traditional (desi) method from home-made butter, the creamery butter method, the direct cream method and the pre-stratification method, which saves fuel and reduces free fatty acids by separating the buttermilk layer before final heating. Ghee contains almost no water, so it keeps for months at room temperature; its shelf life is limited by oxidative rancidity, slowed by the natural antioxidants formed in the residue and by good packaging. Anhydrous milk fat (AMF) and butter oil are the industrial counterparts, made by phase inversion and centrifugation with little flavour development and used as ingredients in recombined milk, confectionery and bakery. Indian ghee standards, regional flavour types and adulteration tests are covered in Veterinary's milk-products-technology and the Dairy Technology track's fat-rich-dairy-products.
Cheese is made by coagulating milk, removing much of the whey and ripening the curd. Most cheeses are rennet-coagulated. Chymosin, the main enzyme of calf rennet and now usually produced by fermentation, specifically cleaves κ-casein at the bond between phenylalanine 105 and methionine 106. The released caseinomacropeptide is hydrophilic and leaves with the whey; the remaining para-κ-casein micelles lose their steric stabilisation and, in the presence of calcium, aggregate into a gel. The gel is cut, stirred and heated (cooked), and acid produced by the starter culture helps expel whey (syneresis). Acid-coagulated cheeses such as cottage cheese and quark are set by lactic acid to near the isoelectric pH of casein, and acid-heat coagulated products such as paneer, ricotta and queso blanco are set by adding acid to hot milk or whey.
Cheese yield depends mainly on milk fat and casein and on the moisture retained. The Van Slyke formula estimates cheddar yield as (0.93 F + C − 0.1) × 1.09 ÷ (1 − W), where F and C are the fat and casein percentages of the milk and W the moisture fraction of the cheese: milk with 3.6% fat and 2.5% casein, made into cheese with 37% moisture, gives about 9.9 kg of cheese per 100 kg of milk.
Cheddar curd, after cooking and draining, is cheddared: slabs of curd are piled and turned while acid develops and the curd fuses into a fibrous, chicken-breast texture. The curd is then milled into chips, dry-salted, hooped and pressed, and ripened for months, during which proteolysis by rennet, starter and non-starter bacteria, and lipolysis develop flavour and texture.
Mozzarella is a pasta filata (stretched curd) cheese. When the curd reaches the right acidity, with pH commonly around 5.2–5.3 and part of the calcium removed from the casein, it becomes plastic in hot water and can be stretched and kneaded into a fibrous, elastic mass that melts and stretches on pizza. Too little acid and the curd will not stretch; too much and it becomes soft and loses water. Low-moisture mozzarella for pizza is the largest-volume cheese in some markets. Processed cheese is made by heating natural cheese with emulsifying salts such as citrates and phosphates, which bind calcium and disperse the casein. The science of each cheese family is in cheese-science-technology.
Yoghurt is made by fermenting milk with Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. The two grow in protocooperation: the lactobacillus releases peptides and amino acids that stimulate the streptococcus, and the streptococcus produces formic acid, carbon dioxide and folate that stimulate the lactobacillus. Milk is usually fortified with milk solids to raise total solids and firmness, heated strongly (commonly about 85–95 °C for several minutes) to denature whey proteins, which then associate with casein and improve gel firmness and reduce whey separation, cooled to the incubation temperature (commonly about 42–45 °C) and inoculated. Fermentation lowers the pH to about 4.5–4.6, at which casein gels. Set yoghurt is fermented in the retail cup; stirred yoghurt is fermented in a tank, the gel is broken, cooled and filled, often with fruit preparations. Acetaldehyde is the key flavour compound. Defects include whey separation (syneresis), weak body, excess acidity (post-acidification) and yeast and mould spoilage. Yoghurt must be cooled promptly at the end of fermentation to stop acid production; continued activity in the cold store, post-acidification, makes the product sourer over its shelf life and is controlled by the choice of culture strains.
Fermented milks are made worldwide. Dahi of South Asia is set with mixed mesophilic cultures, often at home, and is the base of lassi and shrikhand. Cultured buttermilk and soured cream are made with mesophilic lactococci and Leuconostoc, which produce diacetyl, the buttery aroma compound. Kefir is fermented with kefir grains, a symbiotic matrix of lactic acid bacteria and yeasts, and contains small amounts of ethanol and carbon dioxide; koumiss is a traditional fermented mare's milk of Central Asia, also mildly alcoholic. Skyr and strained yoghurts such as Greek-style yoghurt and labneh are concentrated by removing whey, raising protein. Probiotic fermented milks carry strains of Lactobacillus and Bifidobacterium species selected for health effects; health claims for them are tightly regulated. Starter culture technology, the microbiology of each product and probiotic science are covered in fermented-milks-technology, starter-cultures-lab and probiotics-functional-dairy, and traditional products of the world in traditional-dairy-products-world. Fermented milks are among the oldest dairy foods because acid preservation was available long before refrigeration, and many still carry regional identities and traditional processes that industry has learned to reproduce with defined cultures.
Ice cream is a frozen foam: air cells, ice crystals and partly coalesced fat globules dispersed in an unfrozen, concentrated sugar solution. A typical mix contains milk fat (commonly about 10–16%, lower in light products), milk solids-not-fat (commonly about 9–12%), sugars (about 12–16%), stabilisers such as locust bean gum, guar or carrageenan, and emulsifiers such as mono- and diglycerides; legal minimums for fat and total solids, and the name ice cream itself, differ between countries. The ingredients are blended, pasteurised, homogenised and aged cold for some hours so that the fat crystallises and stabilisers hydrate. The mix is then frozen in a continuous freezer, a scraped-surface heat exchanger that freezes part of the water while beating in air, and leaves at about −5 to −6 °C. Flavours, fruit and inclusions are added, the product is packaged and hardened quickly at very low temperature, commonly about −25 to −30 °C or colder, and stored cold.
Overrun is the increase in volume due to air, expressed as a percentage of the mix volume. By weight, overrun = (mass of a volume of mix − mass of the same volume of ice cream) ÷ mass of that volume of ice cream × 100. If a litre of mix weighs 1.10 kg and a litre of ice cream 0.55 kg, the overrun is (1.10 − 0.55) ÷ 0.55 × 100 = 100%. Premium ice creams have lower overrun and are denser; economy products have higher overrun. Some regulations set minimum weights per litre.
Quality depends on small ice crystals and air cells. Heat shock, repeated warming and cooling during distribution, melts small crystals and refreezes the water onto larger ones, giving a coarse, icy texture. Sandiness is caused by lactose crystals large enough to feel, from too high a milk SNF or poor storage. Shrinkage, the loss of volume in storage, and poor meltdown are controlled by formulation and stabilisers. Ice cream mix is a good medium for pathogens, so pasteurisation of the mix and hygiene after pasteurisation, especially for Listeria, are critical. Frozen dessert technology in depth is in frozen-dairy-desserts.
Evaporated milk is unsweetened milk concentrated by vacuum evaporation to roughly twice the solids of the original milk, then homogenised, filled into cans and sterilised in a retort (or UHT-treated and aseptically filled). Because the concentrated milk is much less heat-stable than ordinary milk, the milk is first forewarmed (heat-treated before concentration) to improve its stability, and stabilising salts such as phosphates or citrates may be added within legal limits to balance calcium. A pilot sterilisation test on samples is used to set the salt dose for each batch. Defects include coagulation during sterilisation, fat separation, browning from Maillard reactions, and age thickening or gelation during storage. Coffee and tea whiteners, sweetened condensed milk for confectionery and recombined condensed milk made from powder and fat in countries without enough fresh milk are important uses. Evaporated milk standards for fat and total solids, and the permitted stabilising salts, are set nationally, and the products are labelled differently from milk because they must be diluted or used as an ingredient.
Sweetened condensed milk is concentrated milk with a large amount of added sugar, commonly about 40–45% of the product. It is not sterilised; it is preserved by the high sugar concentration in its water phase, which lowers water activity enough to inhibit most microorganisms. The key measure is the sugar ratio, the sugar as a percentage of sugar plus water: a product containing 44% sugar and 26% water has a sugar ratio of 44 ÷ 70 × 100 = 62.9%; values of roughly 62.5% or higher are commonly cited as needed for keeping quality. The concentrated milk is supersaturated with lactose, so during cooling it is seeded with very fine lactose powder and stirred to give many tiny crystals; without seeding, a few large crystals form and the product is sandy. Defects include sandiness, thickening and gelation in storage, browning, and swelling or buttons (mould colonies) where hygiene or sugar ratio is poor. Products and processes are covered in depth in concentrated-dried-milks.
Skim milk powder (SMP) and whole milk powder (WMP) are made by concentrating milk in multiple-effect falling-film evaporators to about 45–50% solids and spray drying it in hot air; modern plants use two- or three-stage drying with integrated and external fluid beds, which dry the powder more gently and agglomerate fine particles. SMP is commonly dried to about 4% moisture or less. The powder yield follows the solids balance: 10 000 kg of skim milk with 9% solids gives 10 000 × 0.09 ÷ 0.96 = 937.5 kg of powder at 4% moisture.
Skim milk powders are classified by the heat treatment given before drying, measured by the whey protein nitrogen index (WPNI), the amount of undenatured whey protein left: low-heat powders retain most whey protein and suit recombined milk and cheese making; high-heat powders, with most whey protein denatured, suit bakery, where undenatured whey proteins would weaken bread dough. Instant powders are agglomerated so they wet and sink readily; whole milk powders are often coated with lecithin to help wetting. Whole milk powder is prone to oxidative rancidity and is often packed under nitrogen.
Infant formula is a breast-milk substitute made to strict compositional standards, set internationally by the Codex standard for infant formula and nationally by each regulator. Starting from cow milk (or other bases), manufacturers adjust it toward the composition of human milk: they reduce and modify protein (adding whey proteins so the whey to casein ratio is closer to human milk), add lactose, replace much of the milk fat with vegetable oil blends to supply essential fatty acids, and add vitamins, minerals and, in many products, long-chain polyunsaturated fatty acids such as DHA. It is made either by wet mixing and spray drying or by dry blending of ingredients.
Powdered infant formula is not sterile. Cronobacter sakazakii and Salmonella are the critical hazards, because they can survive in dry powders and cause severe illness in newborns, so production uses rigorous zoning and environmental monitoring, and guidance advises reconstitution with water hot enough to reduce the risk. Labelling and marketing of infant formula are also regulated, in many countries following the International Code of Marketing of Breast-milk Substitutes.
Whey is the liquid left after casein has been removed as cheese or casein. Sweet whey comes from rennet-coagulated cheeses and has a pH near that of milk; acid whey comes from acid-coagulated products such as cottage cheese, Greek-style yoghurt and acid casein and has a pH near 4.5. Whey is mostly water, with about 4.5–5% lactose, a fraction of a per cent of whey protein and minerals. It has a very high biochemical oxygen demand, so disposing of it untreated pollutes water; this was once the main whey problem and is now the reason it is processed into ingredients. Whey is concentrated and dried as whey powder, demineralised by electrodialysis or ion exchange for infant formula, and fractionated. Because whey is produced in very large volumes wherever cheese is made, small cheese plants that cannot afford membrane plants often sell liquid whey to larger processors, feed it to pigs, or digest it to biogas. Acid whey from strained yoghurt is a particular disposal problem because its low pH and high mineral content make it less suitable for conventional whey products.
Membrane processing transformed whey utilisation. Ultrafiltration retains proteins and lets lactose and minerals pass, giving whey protein concentrates (WPC), commonly about 35–80% protein on a dry basis; adding diafiltration or ion exchange gives whey protein isolates (WPI) of about 90% protein or more. The permeate is a source of lactose, which is crystallised, refined and dried for pharmaceuticals, infant formula and confectionery. Whey proteins are valued for nutrition (high in leucine and other essential amino acids) and function: solubility, gelation and foaming. Individual proteins such as lactoferrin and α-lactalbumin are separated for specialised products.
From skim milk, acid casein is made by acidifying to pH 4.6, washing and drying; rennet casein by coagulating with rennet. Caseinates, such as sodium and calcium caseinate, are made by neutralising acid casein with alkali and spray drying; they are excellent emulsifiers used in coffee whiteners, processed meats and nutrition products. Milk protein concentrates (MPC) are made by ultrafiltration of skim milk and contain casein and whey protein in their natural ratio. Membrane processes and dairy ingredients are covered in depth in membrane-processing-dairy-ingredients and whey-dairy-byproducts.
Dairy plants are cleaned mostly by cleaning-in-place (CIP): detergent solutions are circulated through tanks, pipes, heat exchangers and fillers without dismantling them. The soil in a dairy plant consists of fat, protein, lactose and, on heated surfaces, milkstone, a deposit of calcium phosphate with denatured protein that builds up on pasteuriser and UHT plates. A typical CIP sequence is a pre-rinse with water to remove loose soil, an alkaline wash (sodium hydroxide, commonly about 0.5–1.5% at about 70–80 °C) that saponifies fat and dissolves protein, an intermediate rinse, an acid wash (nitric or phosphoric acid, commonly about 0.5–1%) that dissolves milkstone and other mineral deposits, a final rinse, and a sanitising step, with chemical sanitisers or hot water, shortly before production. Concentrations, temperatures and times differ by plant and by soil and are set by validation. Many dairy plants use recovery CIP systems that collect and reuse the detergent solutions, topping them up to strength, to save chemicals, water and energy; single-use systems, which discard solutions after each cycle, are used where cross-contamination risk is high, such as for heavily soiled or allergen-containing lines.
Cleaning effectiveness depends on four factors, often remembered as TACT: time, mechanical action (turbulent flow, commonly a velocity of at least about 1.5 m/s in pipes, and spray coverage in tanks), chemical concentration, and temperature. Weakness in one can be partly made up by more of another. Hygienic design, smooth stainless steel surfaces, no dead legs, drainable lines and properly designed seals, makes cleaning possible; without it, biofilms survive. Cleaning is verified by visual inspection, conductivity and temperature records of each CIP cycle, ATP bioluminescence swabs, and microbiological tests of surfaces and rinse water. Chemical residues are prevented by validated final rinsing and by physical separation of CIP circuits from product lines. Detergent and sanitiser chemistry, CIP system design and hygienic design are covered in cip-sanitation-hygienic-design, and food safety management systems in food-safety-management-systems. Records of every CIP cycle are kept so that a product lot can be traced to the cleaning of the equipment that made it. A missed or failed cycle is treated as a deviation, the product made afterwards is held, and the cause is corrected before production resumes; regulators and customers review these records during audits.
A student uses this subject to answer the dairy questions found in every food technology paper: milk composition, the platform tests, why the phosphatase test proves pasteurisation, what homogenisation does, how butter forms, how rennet coagulates milk, which bacteria make yoghurt, and how milk powders are classified. In the laboratory the same student runs Gerber fat tests and lactometer readings, makes butter, yoghurt and paneer, measures overrun on a batch of ice cream, and works the mass balances for standardisation, separation and powder yield.
Five years into a food plant that uses dairy ingredients, or a dairy plant itself, a technologist chooses and specifies the right milk powder, whey protein or caseinate for each product, checks certificates against heat class and microbiological limits, runs pasteurisers and CIP systems, investigates defects such as sandy condensed milk, icy ice cream or weak yoghurt, and knows when to call on a dairy specialist for problems that need the depth of the Dairy Technology track. The same person trains operators in sampling and platform tests and keeps the records that auditors check.
After twenty-five years a senior manager decides how a business sources and uses dairy: liquid milk versus powders, whether to invest in membrane plants to turn whey into protein ingredients, how to manage price swings in milk fat and protein, how to meet infant formula and allergen rules in several markets, and how to respond to contamination incidents in powders or ready-to-eat dairy products. The manager also decides when to build in-house dairy capability or buy ready-made dairy ingredients, and how to keep supply secure across seasons and countries.
A researcher with long industry experience works on the science behind dairy ingredient performance: casein micelle structure and heat stability, whey protein denaturation and functionality, membrane fractionation and novel ingredients, powder physics such as caking and solubility, pathogen survival in dry dairy products, and the composition of infant formula relative to human milk. Such work underpins the specifications and process changes adopted by the industry and regulators. Collaboration with plants turns laboratory findings into specifications, test methods and process controls that work at industrial scale.
This is a compact overview for food technologists. It does not give national compositional standards for each dairy product, permitted stabilisers and their levels, or legal pasteurisation and UHT requirements beyond the classic LTLT and HTST conditions, because these differ between countries; the current regulation must be read. Dairy chemistry, microbiology, starter cultures, cheese varieties, plant and equipment design, membrane processing, dairy analysis and dairy law are treated in depth in the Dairy Technology track; Indian traditional dairy products (khoa, paneer, chhana, shrikhand and others) and FSSAI dairy standards are in Veterinary's milk-products-technology. SNF formulas other than Richmond's and adulterant tests are mentioned but not detailed.
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. Compositions and process conditions are typical textbook values for cow milk that vary with species, breed and plant; product standards differ by country.
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