Dairy Science & Technology · Dairy Processing & Product Technology · also Market Milk, Fluid Milk Processing, Liquid Milk Technology
From reception to the consumer pack: clarification, separation, standardisation, homogenisation, pasteurisation, ESL and UHT, and special milks.
In the exams: ICAR AICE subject 37 Unit 1; ICAR AIEEA-PG group 17 Unit I (processing of fluid milk, centrifugal separation and bactofugation, HACCP in fluid milk processing); GATE XL-U Section 3
Market milk, or fluid milk, is milk processed and packed for drinking. It is the largest single product of most dairy industries and the first product most dairy technologists are responsible for. The work looks simple — receive, separate, standardise, homogenise, heat, cool, pack, distribute — but each step has chemistry, microbiology, engineering and law behind it, and mistakes reach consumers within days.
This manual follows the milk through the plant. It begins with reception, testing and payment, and the chilling and storage that decide raw milk quality. It then covers centrifugal clarification, bactofugation and cream separation; standardisation calculations by Pearson square and algebra; homogenisation; pasteurisation and the HTST plant in detail; extended shelf life milk; UHT processing by direct and indirect systems; in-container sterilisation; toned, recombined, flavoured, fortified and lactose-free milks; HACCP for fluid milk; and distribution and retail. The engineering of heat exchangers and pumps is treated at depth in the dairy engineering subjects and, in general form, in the agricultural engineering track's food-dairy-engineering; here the focus is on how the market-milk technologist uses them.
Exams that examine this subject include ICAR AIEEA-PG Dairy Technology (processing of fluid milk, centrifugal separation and bactofugation, HACCP in fluid milk processing), ICAR AICE-JRF/SRF Dairy Technology (technology of milk) and GATE XL Food Technology (pasteurisation and sterilisation). Legal time-temperature combinations, composition classes and cooling limits differ by country; values given here are textbook or named-regulation examples and must be checked against the current local rules.
Market milk is milk processed and packed for drinking. Its quality is fixed largely before it reaches the plant, so reception is the first control point. Milk arrives in cans from village collection centres or, increasingly, in insulated road tankers from bulk milk coolers. At the reception dock the plant checks that the milk is cool, clean and normal, records its quantity and takes representative samples for composition and quality tests. Can milk is tipped into a weigh bowl through a strainer and its weight recorded; tanker milk is measured by weighbridge or by flow meter fitted with an air eliminator, because air in the line makes a volumetric meter over-read. Weighing is preferred for payment because milk density varies with composition and temperature. Empty cans go through a can washer; tankers are cleaned in place after unloading.
Sampling must be representative. Milk in a can or tanker compartment is mixed (plunged or agitated) before a sample is taken, because cream rises quickly in still milk and a sample from the top over-reads fat. Samples for fat testing are preserved where required and held cool. A clean sample is also kept for any later dispute.
Platform tests are quick tests that decide acceptance at the dock. The organoleptic test (smell, appearance, temperature) rejects milk that is sour, off-flavoured, discoloured or warm. The clot-on-boiling (COB) test heats a small sample: milk that clots has developed acid (or is abnormal, such as colostrum) and will not survive pasteurisation. The alcohol test mixes equal volumes of milk and ethanol (commonly 68 per cent; some plants use 72 or 75 per cent for milk destined for high-heat processing): flocculation shows poor heat stability from developed acidity, high salt imbalance or colostrum. Titratable acidity, expressed as per cent lactic acid, is measured by titration with 0.1 N sodium hydroxide using phenolphthalein; fresh cow milk is commonly about 0.13–0.15 per cent and buffalo milk somewhat higher, but the 'natural' acidity comes from milk proteins and phosphates, not from lactic acid. The sediment test shows visible dirt. Dye-reduction tests (methylene blue reduction time and resazurin) estimate bacterial activity: the faster the dye is reduced, the higher the microbial load. Rapid residue tests screen for antibiotics, and many plants also screen for adulterants such as added water, neutralisers, starch, urea, sugar and detergent.
Fat is measured by the Gerber method (sulphuric acid digestion, amyl alcohol, centrifugation in a calibrated butyrometer) or by infrared milk analysers calibrated against reference methods. Solids-not-fat (SNF) can be estimated from the corrected lactometer reading (CLR) and fat with a formula; the best known is Richmond's: SNF % = CLR ÷ 4 + 0.2 × fat % + 0.14 (lactometer reading in Quevenne degrees, corrected to the calibration temperature). Other national formulas use different constants, so a calculation must state which formula is meant. Added water lowers the lactometer reading and raises the freezing point towards 0 °C; freezing-point depression measured by cryoscope is the standard check for added water.
Milk is paid for on composition rather than volume in most organised dairies. A two-axis system pays separate rates for kilograms of fat and kilograms of SNF: payment = kg milk × (fat % ÷ 100 × fat rate + SNF % ÷ 100 × SNF rate). Some countries pay on fat and protein (component pricing) with adjustments for somatic cell count, bacterial count and other quality criteria. Quality-based payment is the most effective lever a plant has to raise raw milk quality, because farmers respond to price.
Milk leaves the udder at about 37 °C, close to the optimum for many bacteria. Its natural antimicrobial systems hold growth back for a few hours at most. Prompt cooling is therefore the single most effective way to keep raw milk quality: below about 4 °C most mesophiles barely grow, although psychrotrophic bacteria, notably Pseudomonas species, continue to multiply slowly. Rules set time and temperature limits that differ by jurisdiction. In the European Union, milk collected daily must be cooled to 8 °C or below, or to 6 °C or below if not collected daily. The U.S. Grade A Pasteurized Milk Ordinance requires farm milk to be cooled to 10 °C (50 °F) or less within four hours of the start of the first milking and to 7 °C (45 °F) or less within two hours after milking is completed. Other countries have their own limits, and where farms are small, village bulk milk coolers serve groups of producers.
Cooling equipment includes direct-expansion bulk tanks, in which the refrigerant evaporates in the tank wall, and ice-bank tanks, which build ice on evaporator coils during off-peak hours and circulate ice water to cool the milk quickly at milking time. Plate coolers using well water or chilled water pre-cool milk before the tank. The energy to remove is Q = m × cp × ΔT, with the specific heat of milk about 3.9 kJ/kg K; this sets the refrigeration capacity needed for each milking.
At the plant, raw milk is held in insulated silos with gentle agitation to prevent creaming, at a temperature commonly at or below 4–5 °C, and processed as soon as practicable. Long storage, even cold, lets psychrotrophs reach high numbers. Most are destroyed by pasteurisation, but many produce proteases and lipases that are very heat-stable and survive even UHT treatment. In pasteurised milk they cause bitterness and off-flavours; in UHT milk they cause bitterness and gelation during storage. For that reason total bacterial count at reception and the age of raw milk at processing are watched closely. Thermisation (commonly 57–68 °C for about 15 seconds) is used where milk must be held for a day or more before processing; the milk is then re-chilled and later pasteurised normally.
Where refrigeration is not available within a few hours of milking, Codex guidelines describe activation of the lactoperoxidase system as a temporary preservation method. Milk naturally contains the enzyme lactoperoxidase and thiocyanate, but too little hydrogen peroxide. Adding small, specified amounts of thiocyanate (as sodium thiocyanate) and a hydrogen peroxide source (sodium percarbonate) produces hypothiocyanite, which inhibits bacteria and delays souring by several hours, depending on temperature. It is not a substitute for cooling or pasteurisation, does not make bad milk good, and its use is governed by national rules and by the conditions in the Codex guideline, including restrictions on trade in treated milk.
A particle in a liquid settles (or rises) at a rate given by Stokes' law: v = d² (ρp − ρl) g ÷ (18 η), where d is the particle diameter, ρp and ρl the densities of particle and liquid, g the acceleration and η the viscosity. Fat globules are lighter than skim milk and rise; dirt, cells and bacterial spores are denser and settle. In a centrifuge, g is replaced by the centrifugal acceleration r ω², many thousands of times gravity, and the stack of conical discs in the bowl shortens the distance each particle travels. Because velocity rises with the square of diameter, large particles separate easily and the smallest fat globules (below about 1 µm) are largely lost to the skim. Warming milk lowers viscosity and so speeds separation: separators commonly operate warm (around 40–55 °C, depending on design), while cold separation is possible at lower capacity or efficiency.
A clarifier is a centrifuge with one outlet for milk that removes dirt, leucocytes, udder cells and other dense particles into the bowl's sediment space, from which it is discharged periodically (self-cleaning). Clarification improves appearance and removes some bacteria attached to particles, but it does not make milk safe; pasteurisation still does that. A bactofuge is a high-speed clarifier designed to remove bacteria, and especially bacterial spores, which are denser than milk serum. The spore-rich fraction (bactofugate) is a small stream that is discarded or sterilised and returned. Bactofugation is used for cheese milk (against late blowing), for extended-shelf-life milk and for milk powder destined for infant formula. Single-stage and two-stage bactofuges are used; efficiency depends on design and is highest for the largest, densest spores. Microfiltration does a similar job by membrane rather than by centrifugal force.
A separator has two outlets: cream and skim milk. The fat content of the cream is set by throttling the cream outlet, or by a regulating screw on the outlet; reducing cream flow raises its fat content. Separator efficiency is judged by the fat left in the skim milk: a well-adjusted warm separator leaves a few hundredths of a per cent of fat (commonly about 0.04–0.07 per cent). Skim fat rises with cold milk, overloading, worn bowls, air in the milk, high acidity, fat globule damage and too low a cream fat setting relative to the design. A mass balance links the streams: milk = cream + skim, and milk × Fm = cream × Fc + skim × Fs, so cream yield = milk × (Fm − Fs) ÷ (Fc − Fs). Fat recovery in cream (often called separation efficiency) = cream × Fc ÷ (milk × Fm) × 100. Modern plants combine separation with automatic in-line standardisation, remixing part of the cream into the skim to give milk of exact fat content.
Standardisation adjusts the fat (and often the SNF or protein) of milk to a legal or commercial target. Indian standards, for example, define classes of milk by minimum fat and SNF: toned milk is commonly specified at 3.0 per cent fat and 8.5 per cent SNF, double-toned at 1.5 per cent fat and 9.0 per cent SNF, standardised milk at 4.5 per cent fat and 8.5 per cent SNF, and full-cream milk at 6.0 per cent fat and 9.0 per cent SNF; current values must be checked in the FSSAI regulation. Other countries define whole, semi-skimmed (reduced-fat) and skimmed milk with their own limits. Over-standardising gives away valuable fat; under-standardising breaks the law. Standardisation is done in batches in tanks or continuously in-line using flow and density or infrared measurements.
The Pearson square solves mixing problems for one component. Write the target percentage in the centre of a square, the two ingredient percentages at the left corners, and subtract diagonally (larger minus smaller, ignoring signs). The numbers on the right corners are the parts of each ingredient. Example: mix milk of 6.0 per cent fat with skim milk of 0.1 per cent fat to make 3.0 per cent milk. Diagonal differences: 3.0 − 0.1 = 2.9 parts of 6.0 per cent milk, and 6.0 − 3.0 = 3.0 parts of skim, total 5.9 parts. For 1,000 kg of product, 6 per cent milk = 2.9 ÷ 5.9 × 1000 = 491.5 kg and skim = 508.5 kg. The square works for any single component (fat, SNF, sugar), but it cannot satisfy two targets at once.
When both fat and SNF must be met, or three ingredients are used, write a mass balance for each component plus a total mass balance and solve the simultaneous equations. For toned milk made from buffalo milk, skim milk powder and water, with x, y and z the masses of each: x + y + z = total; fat: Fb·x + Fp·y = target fat × total; SNF: Sb·x + Sp·y = target SNF × total (water contributes neither). Solving gives the quantities. Recombined milk is made in the same way from skim milk powder, a fat source such as butter oil or anhydrous milk fat, and water; reconstituted milk from whole or skim milk powder and water alone. In practice, allowance is made for powder moisture, for losses, and for the small fat content of skim milk powder. Answers should always be checked by substituting back into the balances.
Raw milk fat globules average a few micrometres in diameter, and they rise to form a cream layer, helped by cold agglutination (clustering by immunoglobulins). Homogenisation forces milk through a narrow gap at high pressure, breaking the globules down to mostly below 1 µm. Because rising velocity varies with the square of diameter, small globules rise far more slowly and the cream line effectively disappears during shelf life. The fat surface area increases several-fold; the original membrane cannot cover it, so casein micelles and whey proteins adsorb onto the new surface, which is why homogenised fat globules behave partly like casein particles.
The mechanism is mainly turbulence: intense eddies in the gap and just beyond it tear the globules apart. Shear at the valve surfaces and cavitation (collapse of vapour bubbles) are also involved, and older texts present them as separate theories. A homogeniser is a high-pressure positive-displacement piston pump, commonly with three pistons (a triplex pump) to smooth the flow, followed by a homogenising valve. Two-stage homogenisation is usual: the first stage does most of the size reduction at high pressure (for milk often in the range of about 10–25 MPa, depending on product and temperature), and the second stage at a much lower pressure breaks up the clusters of small globules that form after the first.
Homogenised milk has no cream line, a whiter colour (more light scattering), a fuller mouthfeel and a slightly higher viscosity. It is more sensitive to light-induced off-flavour, because more fat surface is exposed, and it foams differently. Its curd tension in the stomach is lower, which was once promoted for infant feeding. Homogenisation of raw milk is avoided, or followed immediately by pasteurisation, because breaking the globule membrane exposes the fat to the native milk lipase and rancidity develops within minutes. Homogenisation is normally done warm (about 55–80 °C) because cold fat is partly crystalline and does not break up well. Partial (stream) homogenisation homogenises only the cream fraction and remixes it with skim, which saves energy because only a small flow passes the homogeniser.
Efficiency is checked by how much fat rises. The U.S. Pasteurized Milk Ordinance defines homogenised milk as milk in which, after 48 hours of quiescent storage at 7 °C, the fat in the top 100 mL of a quart (or proportionate volumes of other containers) differs by no more than 10 per cent from the fat in the remainder. A related index, sometimes called the homogenisation index, compares the fat in the top and bottom layers after storage: index = (fat top − fat bottom) ÷ fat top × 100; the formula used must be stated. The NIZO method centrifuges a sample under set conditions and measures the fat in the lower part. Laser diffraction gives the full globule size distribution, often reported as d(3,2) or d(4,3) mean diameter.
Pasteurisation is a heat treatment that destroys vegetative pathogens in milk and reduces spoilage organisms, with minimal change to flavour and nutrients. The classic combinations are low-temperature long-time (LTLT, batch or vat pasteurisation) at 63 °C for 30 minutes and high-temperature short-time (HTST) at 72 °C for 15 seconds; the U.S. figures are 145 °F (62.8 °C) for 30 minutes and 161 °F (71.7 °C) for 15 seconds. The standards were set to destroy Coxiella burnetii, the agent of Q fever, which is the most heat-resistant non-spore-forming pathogen commonly found in milk; Mycobacterium tuberculosis was the earlier target. Higher-heat shorter-time combinations (for example 89 °C for 1 second or 100 °C for 0.01 second in the PMO table) are permitted and used for products with added sugar or solids, where higher temperatures are often required. Pasteurisation does not destroy bacterial spores or the heat-stable enzymes of psychrotrophs, so pasteurised milk must be kept refrigerated.
The alkaline phosphatase test verifies pasteurisation. Alkaline phosphatase is slightly more heat-resistant than the target pathogens and is inactivated by correct pasteurisation, so a positive result shows under-pasteurisation or contamination with raw milk. Modern reference methods are fluorimetric, with an action limit set in regulation. Lactoperoxidase survives HTST but is inactivated at higher temperatures (around 80 °C for a few seconds), so a negative peroxidase test indicates a higher heat treatment.
An HTST pasteuriser is a plate heat exchanger in sections, with a balance tank, a timing (metering) pump, a holding tube, a flow diversion valve and controls. Cold raw milk from the constant-level balance tank enters the regeneration section, where it is warmed by hot pasteurised milk flowing on the other side of the plates, which in turn is cooled. It then passes to a heating section (hot water), reaches the pasteurisation temperature and flows through the holding tube, sized so that the fastest particle is held for at least the legal time. At the end of the holding tube a temperature sensor controls the flow diversion valve: if the temperature is below the setting, the valve diverts milk back to the balance tank; otherwise it passes forward to regeneration and then to a cooling section (chilled water or glycol) and on to storage. A recorder-controller documents temperature and valve position.
Regeneration efficiency = (temperature of raw milk leaving regeneration − raw milk inlet temperature) ÷ (pasteurisation temperature − raw milk inlet temperature) × 100. Modern plants reach 90 per cent or more, saving most of the heating and cooling energy. To protect against leaks, the pasteurised milk in the regenerator is kept at a higher pressure than the raw milk, so that any pinhole leaks pasteurised milk into raw rather than the reverse; the PMO specifies this pressure relationship and how it is maintained.
Holding time is set by the volume of the holding tube and the flow rate, but particles do not all move at the mean velocity. In laminar flow in a pipe the fastest particle at the centre moves at twice the mean velocity; in turbulent flow it moves at roughly 1.2 times the mean. Regulations require the holding time to be met by the fastest particle, and the U.S. rules base it on the timing-pump capacity and test the actual holding time with a salt-conductivity method. To size a tube: mean velocity = flow ÷ cross-sectional area; fastest-particle velocity = factor × mean velocity; length = fastest velocity × holding time. Example: 10,000 L/h through a 50 mm internal diameter tube gives a mean velocity of 1.41 m/s; with a laminar factor of 2 the fastest velocity is 2.83 m/s and 15 seconds needs 42.4 m. The tube slopes upwards in the direction of flow so that air is expelled and the tube stays full.
Extended shelf life (ESL) milk is chilled milk designed to keep for considerably longer than conventional pasteurised milk, typically several weeks unopened under refrigeration, while tasting closer to pasteurised milk than UHT milk does. There is no single international definition: some countries define it by process, others only by the shelf life claimed. Its shelf life depends on three things together: a lower initial count of heat-resistant spores, a process that removes or destroys more organisms than HTST, and filling and packaging that prevent recontamination.
The main process routes are: higher heat treatment, commonly in the range of about 120–130 °C for a few seconds by direct or indirect heating, which destroys most vegetative cells and many spores without the full heat load of UHT; microfiltration of skim milk through ceramic membranes (pore size around 1.4 µm) to remove bacteria and spores, with the cream heat-treated separately and remixed, followed by pasteurisation; and bactofugation combined with pasteurisation. Each route has a different effect on flavour, whey protein denaturation and spore removal, and on which organisms limit shelf life.
ESL milk is filled under ultra-clean conditions: filling machines with sterile air, sterilised or decontaminated packaging and enclosed filling zones. Packages are commonly gable-top cartons or plastic bottles, often with light-barrier properties. ESL milk is not commercially sterile and must stay refrigerated. Spoilage, when it happens, is usually caused by psychrotrophic spore-formers such as Bacillus and Paenibacillus species, or by post-process contaminants if hygiene fails. Shelf-life claims must be validated by storage trials at and above the expected retail temperature. Consumers should be told that the shelf life after opening is that of ordinary milk.
Ultra-high-temperature (UHT) processing heats milk continuously to a temperature typically in the range of 135–150 °C for a few seconds, cools it, and fills it aseptically into sterile packages. The result is commercially sterile milk that keeps for months at ambient temperature. Commercial sterility means freedom from organisms that can grow under normal storage conditions, not absolute sterility. The European Union defines UHT treatment as at least 135 °C for a suitable holding time combined with aseptic packaging; other jurisdictions use different wording.
Two indices describe UHT processes. The bacteriological effect B* is set so that B* = 1 corresponds to a nine-log reduction of thermophilic spores, equivalent to 10.1 seconds at 135 °C with a z-value of 10.5 °C; a process needs B* of at least 1. The chemical effect C* is set so that C* = 1 corresponds to 3 per cent destruction of thiamine, equivalent to 30.5 seconds at 135 °C with a z-value of 31.4 °C; a good process keeps C* low. B* for a holding time t (seconds) at temperature T is t ÷ 10.1 × 10^((T − 135) ÷ 10.5). Because the z-value for spore kill is much smaller than that for chemical change, raising the temperature and shortening the time gives more kill for less chemical damage: this is the logic of high-temperature short-time processing pushed to its limit. Canning practice uses F0, the equivalent time in minutes at 121.1 °C with z = 10 °C: F0 = t (min) × 10^((T − 121.1) ÷ 10).
In direct systems, steam is mixed with the milk. In steam injection, culinary steam is injected into the milk stream; in steam infusion, milk falls as a film or droplets through a steam-filled vessel. Heating is almost instantaneous. After holding, the milk enters a vacuum (flash) vessel where it boils, cooling instantly and losing water vapour equal to the steam condensed into it, so the composition is restored; the flash also removes some volatile off-flavours and dissolved oxygen. Direct systems give the lowest heat load for a given B*, hence less cooked flavour, less whey protein denaturation and less lactulose. Their drawbacks are higher energy use (poorer heat recovery), the need for high-quality culinary steam and more complex control.
Indirect systems heat through a wall in plate, tubular or scraped-surface heat exchangers. They recover heat efficiently by regeneration but heat and cool more slowly, so the total heat load and cooked flavour are greater. Fouling, the deposition of protein and minerals on hot surfaces, limits run length, especially in plate units; tubular units tolerate fouling and some particles better. The choice between direct and indirect is a balance of product quality, energy cost and capital cost.
UHT milk is filled aseptically: the package material (commonly a paperboard-polyethylene-aluminium laminate) is sterilised, typically with hydrogen peroxide followed by heat to remove it, and filled in a sterile zone. Quality problems during storage include sedimentation, fat separation, age gelation (thickening associated with heat-stable proteases from psychrotrophs or with physico-chemical changes), Maillard browning, cooked and stale flavours, and bitterness. Lactulose, furosine and the degree of whey protein denaturation are used to distinguish UHT from in-container sterilised milk and to assess heat load. Commercial sterility is checked by incubating samples of packs, at temperatures chosen to detect mesophilic and thermophilic spoilage, then examining them for swelling, pH change and microbial growth.
Before UHT and aseptic packaging, shelf-stable milk was made by sterilising it in its container. Milk is standardised, homogenised and often preheated, filled into glass bottles, cans or heat-resistant plastic bottles, sealed and then heated in a retort, commonly at about 110–120 °C for a period of tens of minutes, followed by cooling. Batch retorts are used for small outputs; continuous hydrostatic towers, in which containers travel through legs of water that balance the steam pressure in the sterilising zone, are used in large plants. Rotary retorts that agitate the containers improve heat transfer and shorten the process. Some processes use a two-stage approach: a continuous UHT pre-sterilisation of the milk followed by a shorter in-container sterilisation, which reduces the heat load.
The process must achieve commercial sterility at the slowest-heating point of the container, so heat penetration tests are used to establish the schedule. The lethality is calculated as F0, the equivalent minutes at 121.1 °C (z = 10 °C).
In-container sterilised milk has a much higher heat load than UHT milk. It has a creamy-brown colour, a pronounced cooked or caramel flavour that some consumers like, more loss of heat-sensitive vitamins, extensive whey protein denaturation and more lactulose. It keeps for months unopened at ambient temperature. In some markets sterilised milk in glass bottles is a traditional product with loyal buyers; elsewhere UHT has largely replaced it. Evaporated milk is also usually sterilised in its can after concentration, and its heat stability before sterilisation is controlled by preheating and by stabilising salts.
Toned milk was developed in India to extend the supply of high-fat buffalo milk: buffalo milk is diluted with reconstituted skim milk powder (or with skim milk and water) to reduce fat while keeping SNF near the legal minimum. Double-toned milk has still lower fat. Both are standardised by mass balance. Reconstituted milk is milk made by dispersing milk powder in water; recombined milk is made by combining skim milk powder, milk fat (usually anhydrous milk fat or butter oil) and water, then homogenising so that the fat is emulsified. Recombination is important in countries that import milk powder and fat to supply their market in seasons or regions of shortage. Powder must be dissolved properly (mixing, hydration time, often warm water and deaeration) to avoid a chalky or powdery taste and sediment. Labelling rules usually require reconstituted or recombined milk to be named as such.
Flavoured milk adds sugar or other sweeteners, flavour and often colour; chocolate milk adds cocoa powder, which settles unless a stabiliser such as carrageenan forms a weak gel network with milk protein that suspends the particles. Flavoured milks may need higher pasteurisation temperatures because added solids protect micro-organisms, and many are UHT-treated. Fortified milk has vitamins and sometimes minerals added. Vitamins A and D are the most common additions, especially to reduced-fat milk, which loses fat-soluble vitamin A with the fat; many countries have voluntary or mandatory fortification programmes with specified levels. Vitamins are added as a premix, usually before pasteurisation, and losses during processing and storage, especially of vitamin A by light, are allowed for.
Lactose-free and lactose-reduced milks are for people with lactose maldigestion. The enzyme lactase (beta-galactosidase) hydrolyses lactose to glucose and galactose. Neutral lactases from yeasts such as Kluyveromyces lactis suit milk at its natural pH. The enzyme can be added before heat treatment and held to allow hydrolysis, or, for UHT milk, a sterile enzyme can be dosed into the milk after sterilisation so that hydrolysis continues in the pack. Membrane processes can remove part of the lactose first, which reduces the sweetness that hydrolysis otherwise causes, since glucose and galactose are sweeter together than lactose. Hydrolysed milk browns more readily by the Maillard reaction because glucose and galactose are more reactive reducing sugars. The residual lactose permitted for a 'lactose-free' claim differs by country and must be checked in local rules.
Other special milks include reduced-fat and skimmed milks, protein-enriched milks made with ultrafiltration or added milk protein, milks from animals selected for A2 beta-casein, organic milk, filtered milks with longer shelf life, and milks with added omega-3 fatty acids or plant sterols. Each makes a claim that must be supported and labelled according to national law.
HACCP (hazard analysis and critical control points) is the internationally recognised system, set out by Codex, for controlling food safety hazards. It rests on prerequisite programmes (good manufacturing and hygiene practice, cleaning and sanitation, pest control, water quality, maintenance, training, supplier control) and on seven principles: conduct a hazard analysis; determine critical control points (CCPs); establish critical limits; establish monitoring; establish corrective actions; establish verification procedures; and establish documentation and records. Codex describes twelve steps, starting with assembling the team, describing the product and intended use, drawing and confirming the flow diagram on site, and then applying the seven principles.
For pasteurised milk the significant hazards include biological hazards (vegetative pathogens such as Salmonella, Listeria monocytogenes, pathogenic Escherichia coli, Campylobacter and Staphylococcus aureus from raw milk; recontamination after pasteurisation), chemical hazards (veterinary drug residues, cleaning chemicals, mycotoxin M1, contaminants) and physical hazards (foreign matter). The control measures differ: residues are controlled mainly by supplier programmes and testing at reception, cleaning chemicals by CIP design and verification, and pathogens by pasteurisation and by preventing recontamination.
Pasteurisation is the classic CCP in fluid milk: it is the step that eliminates vegetative pathogens, and no later step does so. Its critical limits are the legal time-temperature combination (for HTST, for example, at least 72 °C for 15 seconds or the national equivalent). Monitoring is continuous recording of temperature at the end of the holding tube and of flow diversion valve position, with the holding time assured by the timing pump and holding tube design. Corrective action is automatic diversion of under-temperature milk, plus holding and evaluation of any product made during a deviation and repair of the cause. Verification includes daily checks of recording thermometers against an indicating thermometer, periodic calibration, holding-time tests, alkaline phosphatase tests on product, and review of records.
Other steps may be CCPs depending on the plant's analysis: raw milk receipt with residue screening, metal detection where a physical hazard is significant, and, for some products, the cooling step. Post-pasteurisation contamination, the main cause of Listeria and spoilage problems in pasteurised milk, is usually controlled through prerequisite programmes and hygienic design (closed systems, CIP, environmental monitoring of filling areas) rather than a single CCP. Food safety management standards such as ISO 22000 and FSSC 22000 build on HACCP with management-system requirements.
Fluid milk is sold in many formats: low-density polyethylene pouches (common in South Asia because they are cheap), glass bottles (returnable in some markets), gable-top paperboard cartons, high-density polyethylene and PET bottles, and aseptic laminated cartons for UHT milk. Bulk vending from chilled dispensers also exists. Package choice affects cost, shelf life, recyclability and light protection. Light is a serious problem: riboflavin in milk absorbs light and acts as a photosensitiser, leading to oxidation of methionine (giving a burnt, cabbage-like 'sunlight' flavour) and of lipids, and destroying riboflavin and vitamin A. Pigmented or opaque packaging, and keeping milk away from bright retail lighting, reduce these losses. Oxidised flavour can also come from copper contamination. Packages carry legally required information such as name, composition class, net quantity, date mark, storage instructions and processor details.
Pasteurised and ESL milk depend on an unbroken cold chain from the filler to the consumer's refrigerator. Milk leaves the plant chilled, travels in insulated or refrigerated vehicles, is held in chilled stores and displayed in refrigerated cabinets. Every rise in temperature shortens shelf life, because psychrotrophs grow faster; temperature abuse at retail or in the consumer's car is a common reason for early spoilage complaints. Plants set the date mark from storage trials at the expected temperature, and distribution is planned on a first-in, first-out basis. UHT milk does not need refrigeration before opening, which makes it valuable where cold chains are weak, but once opened it is perishable.
Route planning balances freshness against cost: frequent small deliveries keep stock fresh but raise transport cost. Returns of unsold or out-of-date milk, crate and can logistics, and the handling of complaints are part of distribution management. In many developing markets a large share of milk still moves through informal channels (vendors selling raw milk), and organised dairies compete on safety, convenience and consistent quality.
A student uses this subject in the dairy plant training semester and in every exam: doing Pearson square and algebraic standardisation for toned milk, separator and regeneration calculations, explaining why the flow diversion valve and the regenerator pressure relationship exist, and running platform tests at reception. It is the most-asked processing subject in ICAR AIEEA-PG group 17.
A shift or quality officer five years in runs reception and decides which tankers to accept, keeps the HTST pasteuriser within its legal limits and records, checks holding-time and phosphatase results, sets the standardisation for each batch, investigates short shelf-life complaints by tracing post-pasteurisation contamination or cold-chain abuse, and helps launch a flavoured or lactose-free line. This subject gives the cause-and-effect knowledge and the calculations for each of those jobs.
A plant head decides whether to invest in ESL or UHT capacity, direct or indirect heating, bactofugation, partial homogenisation or more regeneration to save energy; sets raw milk payment and quality policy; and answers to regulators and buyers for the HACCP system and recalls. This subject frames those decisions in terms of product quality, safety, energy and cost.
A professor or research director works on the science behind the processes: heat-stable enzymes and spore formers that limit shelf life, kinetics of heat-induced changes, membrane and non-thermal alternatives to heat, light-induced oxidation, and the modelling of holding tubes and heat exchangers. They set and moderate exam questions on these topics and advise standards bodies on time-temperature requirements and definitions such as ESL and lactose-free.
This manual does not yet give: detailed design of plate and tubular heat exchangers (see the dairy engineering subjects); CIP programmes in detail; the full PMO equipment and testing requirements; exact current legal limits for each country (milk classes, cooling, phosphatase action limits, lactose-free thresholds, fortification levels), which must be read in the current regulation; detailed microbiology of spoilage organisms; and packaging material science. Temperature and pressure ranges for separation, homogenisation, ESL and UHT are representative textbook values that vary with equipment and product. The Indian milk classes are given as commonly cited and should be checked against the current FSSAI text.
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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