Dairy Science & Technology · Dairy Processing & Product Technology · also Condensed and Dried Milk, Dairy Powders Technology
Evaporated and sweetened condensed milk, milk powders and their functional properties, storage and defects.
In the exams: ICAR AICE subject 37 Units 1 and 3 and subject 34 Unit 6; ICAR AIEEA-PG group 16 (changes in manufacture and storage of milk powder)
Concentrating and drying milk removes most of its water, turning a bulky, perishable liquid into products that can be stored for months and shipped across the world. Evaporated milk and sweetened condensed milk are the classic concentrated products; skim, whole and instant milk powders, dairy whiteners and fat-filled powders are the dried ones. Together they move a large share of world dairy trade and supply milk to regions that cannot produce enough of their own.
This manual covers evaporated milk and its heat stability; sweetened condensed milk, the sugar ratio and lactose crystallisation; defects of condensed milks; spray drying and powder properties; roller drying; whole, skim and instant powders; heat classification by WPNI; agglomeration and instantising; water activity, sorption and the glass transition; storage, caking and browning; dairy whiteners and filled powders; and fire and explosion safety. The engineering of evaporators and dryers — evaporator design and steam economy, atomisers, air handling and fluid beds — is treated in evaporation-drying-dairy, and milk reception, standardisation and heat treatment in market-milk-processing; this manual refers to them rather than repeating them.
ICAR AIEEA-PG Dairy Science asks for physico-chemical changes during manufacture and storage of milk powder and lactose crystallisation; ICAR AICE-JRF/SRF Dairy Chemistry (subject 34, Unit 6) covers heat stability, concentrated and dried milks, instant powder and infant food, and Dairy Technology (subject 37, Units 1 and 3) covers concentrated and dried products, advances in drying, glass transition, water activity and sorption. Standards, permitted stabilisers and labelling differ by country; values here are textbook or named-standard examples and must be checked against the current regulation.
Evaporated milk is milk concentrated by removing part of its water, homogenised and sterilised so that it keeps at ambient temperature without added sugar. It is concentrated roughly two to two-and-a-half times. Standards set minimum fat and milk solids: the U.S. standard, for example, requires at least 6.5 per cent milk fat, at least 16.5 per cent milk solids-not-fat and at least 23 per cent total milk solids, while the Codex standard for evaporated milks sets its own minimums and also defines skimmed, partly skimmed and high-fat versions. Evaporated milk is used in cooking, in tea and coffee, for infant feeding in the past, and as a convenient long-life milk in hot climates. Evaporators and the engineering of concentration are covered in evaporation-drying-dairy; this chapter covers the product.
Milk is standardised to the target ratio of fat to SNF, then given a heat treatment called forewarming or preheating, commonly at temperatures of around 90–120 °C for times from seconds to minutes depending on the system. Forewarming destroys micro-organisms and enzymes, but its main purpose is to improve the heat stability of the concentrate so that it survives sterilisation without coagulating: denaturation of whey proteins and their association with casein micelles, and shifts in the calcium phosphate balance, make the concentrated milk more stable. The milk is then concentrated under vacuum in a falling-film evaporator at a low boiling temperature, homogenised to prevent fat separation during long storage, cooled, and tested for heat stability.
The concentrate is filled into cans and sterilised in a retort, often at around 115–120 °C for a period of minutes, with rotation to improve heat transfer; continuous hydrostatic sterilisers are used in large plants. An alternative is UHT treatment of the concentrate followed by aseptic packaging, which gives a lighter colour and less cooked flavour. Before production, pilot sterilisation tests on small samples with graded additions of stabilising salts are run to find the dose that gives the right viscosity without coagulation.
Concentrating milk raises the concentration of calcium ions and proteins and lowers pH slightly, all of which reduce heat stability. The heat stability of milk depends on its salt balance: calcium and magnesium ions destabilise casein, while phosphates and citrates bind them and stabilise it. Seasonal and lactational variation in this balance means that some milks coagulate on sterilisation. Plants therefore add small, controlled amounts of stabilising salts, such as disodium phosphate or trisodium citrate, within legal limits, to restore balance; too much salt gives a thin, unstable product that may gel later, too little gives coagulation. Heat stability is measured as the heat coagulation time (the time to visible coagulation at a set temperature, commonly about 120 °C for concentrates or 140 °C for milk) and depends strongly on pH; milk shows characteristic heat coagulation time–pH profiles. The age of the milk, its somatic cell count and its whey protein content also affect stability, so evaporated milk plants are particular about raw milk quality.
In many importing countries evaporated milk is made not from fresh milk but by recombination: skim milk powder, anhydrous milk fat and water are mixed, homogenised, filled and sterilised. The powder must be a heat-stable, high-heat grade, because low-heat powder with much native whey protein usually coagulates during sterilisation of the concentrate. Recombination plants test each powder lot for heat stability before use, adjust stabilising salts with pilot sterilisation, and control water quality, since calcium and magnesium in hard water reduce heat stability. The finished product must meet the same standards as evaporated milk from fresh milk and is labelled according to national rules.
Sweetened condensed milk (SCM) is concentrated milk with added sugar, normally sucrose, at a level high enough that the osmotic pressure of the water phase prevents microbial growth. It is not sterilised after filling; its safety and keeping quality depend on the heat treatment of the milk before concentration, the high sugar concentration in the water, hygienic filling into clean containers, and exclusion of air. Standards set minimum fat and milk solids, for example at least 8 per cent milk fat and at least 28 per cent total milk solids in the U.S. and Codex standards, with skimmed and partly skimmed versions. SCM is used in confectionery, bakery, desserts and beverages, especially in tropical countries.
The preserving effect depends on the concentration of sugar in the water phase, expressed as the sugar ratio (or sugar-in-water ratio): sugar ratio = sugar ÷ (sugar + water) × 100. A typical target is around 62.5–64.5 per cent. Below this range, osmophilic yeasts and moulds can grow; above it, sucrose may crystallise and cause sandiness. Example: SCM with 8 per cent fat, 20 per cent MSNF, 45 per cent sugar and 27 per cent water has a sugar ratio of 45 ÷ 72 × 100 = 62.5 per cent. Mass balances give the sugar needed: from milk of known composition, the SCM mass follows from the fat target, the water-plus-sugar mass from subtracting milk solids, and the sugar from the sugar ratio.
Milk is standardised, heat-treated (which also affects viscosity and age thickening), and sugar is added either as syrup before or during evaporation or as solid sugar. The mixture is concentrated under vacuum to the target solids. Because SCM contains so little water, lactose is supersaturated: the amount dissolved in the water phase far exceeds its solubility at room temperature, so lactose will crystallise. The aim is to control crystallisation so that it produces a very large number of very small crystals, which are not felt in the mouth, rather than a few large crystals, which give a sandy, gritty texture and can settle to the bottom of the can. This is done by forced crystallisation: the concentrate is cooled rapidly with stirring to the temperature of maximum crystallisation rate and seeded with very finely ground lactose powder, which provides millions of nuclei. Crystals smaller than about 10 µm are not detected as grittiness, while larger ones increasingly are. The cooled, crystallised product is then filled into cans or tubes under hygienic conditions.
The viscosity of sweetened condensed milk must be high enough to keep lactose crystals and fat suspended but low enough to pour and to remain stable for months. Viscosity rises with protein content, with the severity of the heat treatment of the milk before evaporation, with concentration, and with storage time and temperature. Age thickening — a slow increase in viscosity, sometimes ending in a gel — is a major quality problem in hot climates. It is controlled by choosing the forewarming conditions (some heat treatments promote thickening, others limit it), adding sugar before or after part of the concentration, adjusting salt balance with small amounts of stabilising salts where permitted, and storing the product cool. Producers test viscosity after manufacture and after accelerated storage to predict behaviour.
Evaporated milk defects include coagulation (curdling) during sterilisation, from poor heat stability, insufficient forewarming, high acidity or salt imbalance; thin or thick consistency from wrong stabiliser dose or forewarming; age gelation, a gradual thickening to a gel during storage, linked to protein interactions and accelerated by low forewarming and high storage temperature; fat separation (a cream plug at the top of the can) from inadequate homogenisation; browning and caramelised flavour from Maillard reactions during sterilisation and warm storage; sediment of calcium citrate or calcium phosphate crystals; and swelled cans from under-sterilisation or leaking seams that let micro-organisms in. Cooked flavour is normal in canned evaporated milk but should not be excessive.
SCM defects include sandiness from large lactose crystals (poor seeding or cooling) or sucrose crystallisation (sugar ratio too high); lactose sediment settling at the bottom; age thickening, in which viscosity rises during storage until the product will not pour, associated with heat treatment of the milk, high storage temperature and high protein; 'buttons', which are small, reddish-brown, cheesy-tasting lumps formed by moulds growing on the surface where air was trapped; blowing or gas formation from osmophilic yeasts when the sugar ratio is too low or hygiene is poor; browning and caramel flavour from Maillard reactions during warm storage, especially in tropical climates; fat separation; and rancid or oxidised flavours from lipolysis or oxidation. Most defects can be traced to raw milk quality, the heat treatment, the sugar ratio, crystallisation control, filling hygiene and storage temperature.
Spray drying is the dominant method of making milk powder. Milk is standardised, heat-treated (to a degree chosen for the powder's heat class), concentrated in a multiple-effect evaporator to around 45–50 per cent total solids, and pumped to the dryer. Concentrating first is essential because removing water by evaporation is far cheaper than removing it in the dryer. In the drying chamber, the concentrate is atomised into fine droplets by a rotary atomiser (a spinning wheel) or by high-pressure nozzles, and meets a stream of hot, filtered air (inlet temperatures commonly around 180–220 °C or higher in modern multistage dryers). Water evaporates within seconds; because evaporation cools the droplets, the particle temperature stays well below the air temperature, near the wet-bulb temperature at first, which limits heat damage. The air leaves at an outlet temperature commonly around 70–95 °C, and the outlet temperature is the main control on final powder moisture. Powder is separated from the air in cyclones and bag filters. The engineering of dryers, atomisers and air handling is covered in evaporation-drying-dairy.
In single-stage drying, all the water is removed in the chamber, which requires a high outlet temperature and gives fine, dusty powder with more heat damage. In two-stage drying, the powder leaves the chamber with a higher moisture and is finished in an external vibrating fluid bed, where it is dried gently and cooled; this allows lower outlet temperatures, better energy efficiency, less heat damage and the possibility of agglomeration. Three-stage dryers have an integrated fluid bed in the base of the chamber and an external fluid bed for final drying and cooling; they produce agglomerated powders with good functional properties at high efficiency. Fines from cyclones are returned to the atomisation zone or the fluid bed to build agglomerates.
The properties that matter to customers include moisture content (typically a few per cent; Codex sets a maximum for each type), bulk density, particle size distribution, solubility (often measured as the insolubility index, the volume of sediment after reconstitution and centrifugation under standard conditions), wettability, dispersibility and sinkability (instant properties), scorched particles (burnt particles from deposits in the dryer), free fat in whole milk powder (fat not protected by protein, which affects wettability, oxidation and flowability), occluded air inside particles, heat class, colour, flavour and microbiological quality. Bulk density depends on particle size and shape, occluded air and the degree of agglomeration; dense, non-agglomerated powders are cheaper to ship, while agglomerated powders reconstitute better. Solids concentration, atomisation method and drying temperatures are the main levers for these properties.
Drying calculations rest on the solids balance: the solids in the feed equal the solids in the powder, assuming no losses. If a concentrate of 48 per cent total solids is dried to a powder of 3.5 per cent moisture (96.5 per cent solids), each 100 kg of concentrate gives 48 ÷ 0.965 = 49.74 kg of powder and 50.26 kg of water must be evaporated, roughly one kilogram of water per kilogram of powder. Starting from milk instead, skim milk of 9 per cent solids gives 9 ÷ 0.96 = 9.38 kg of 96 per cent-solids powder per 100 kg, with most of the water removed in the evaporator rather than the dryer. The evaporator's steam economy (kilograms of water evaporated per kilogram of steam) rises with the number of effects and with vapour recompression, and the spray dryer's specific energy is several times higher per kilogram of water than a multiple-effect evaporator's; this is why plants concentrate as far as viscosity allows before drying. Moisture can be stated on a wet basis (water ÷ total mass) or a dry basis (water ÷ dry solids): 4 per cent wet basis is 4 ÷ 96 × 100 = 4.17 per cent dry basis.
Roller (drum) drying was the first industrial method of making milk powder. Milk, sometimes pre-concentrated, is applied as a thin film to the surface of one or two internally steam-heated rotating drums. The film dries in a few seconds during less than one revolution and is scraped off by a knife as a sheet, which is then milled and sieved. Double-drum dryers feed milk into the nip between two drums rotating towards each other; single-drum dryers use dip, splash or applicator-roll feeding. Atmospheric drum dryers are most common; vacuum drum dryers, which dry at lower temperature, give better quality but are more expensive.
Because the milk film is in contact with a surface above 100 °C, roller-dried powder suffers more heat damage than spray-dried powder: more whey protein denaturation, more Maillard browning, a cooked or caramel flavour, and a much higher insolubility index, so it reconstitutes poorly into milk. The flakes have irregular shapes, low occluded air and high bulk density, and in whole milk powder much of the fat is free fat, released from damaged globules. These properties are undesirable for drinking milk but useful in some applications: high free fat and cooked flavour are valued in milk chocolate, where roller-dried whole milk powder gives a characteristic flavour and helps reduce viscosity of chocolate mass, and in some bakery, confectionery and animal-feed uses. Roller drying uses less energy per kilogram of water than single-stage spray drying, is simple and compact, but has lower capacity and is limited by quality. Today it is a niche process.
Milk powders are classified by fat content. Codex defines whole milk powder (milk fat at least 26 and less than 42 per cent), partly skimmed milk powder (more than 1.5 and less than 26 per cent fat), skimmed milk powder (at most 1.5 per cent fat), and high-fat powders, each with a maximum moisture and a minimum milk protein content in the milk solids-not-fat. National standards follow similar lines. Skimmed milk powder is the major traded dairy commodity alongside butter; it is used for recombination, bakery, confectionery, processed meats, chocolate, infant formula ingredients and animal feed. Whole milk powder is used for direct consumption by reconstitution in many developing markets, for recombined milk, and in chocolate and confectionery. Buttermilk powder, whey powders, milk protein concentrate and dairy permeate powders are related products made on the same plant.
Whole milk powder is more difficult than skim. Its fat is prone to oxidation, so it is often packed under nitrogen or carbon dioxide with low residual oxygen; antioxidants and a high heat treatment of the milk (which produces protective sulphydryl compounds) may be used. Free fat on the particle surface makes the powder hydrophobic and poorly wettable, sticky and prone to oxidation; it rises with poor homogenisation of the concentrate, mechanical damage and high temperatures. For good reconstitution, whole milk powder is often lecithinated: a small amount of lecithin dissolved in butter oil is sprayed onto the agglomerated powder in the fluid bed, making the particle surface more hydrophilic so the powder wets and sinks quickly in cold water.
'Instant' powders reconstitute quickly and completely with little stirring. Four properties are distinguished: wettability (the ability of particles to absorb water, overcoming surface tension), sinkability (the ability to sink rather than float, which depends on particle density and occluded air), dispersibility (the ability to separate into individual particles in water without lumping) and solubility (the ability to dissolve completely, which depends on heat damage). Agglomeration produces porous, larger particles with many capillaries that take up water fast; lecithination improves wettability of whole milk powder. Instant powders are tested by standard methods that time wetting and measure dispersed material. They have lower bulk density than non-agglomerated powders, which increases packaging and transport volume.
The same evaporator and dryer lines make many other powders. Buttermilk powder, from sweet-cream buttermilk, carries milk fat globule membrane phospholipids that make it a valued emulsifier. Whey powders and demineralised whey powders are made from cheese whey; because whey is rich in lactose, it is usually pre-crystallised after concentration to reduce stickiness and hygroscopicity before drying. Milk protein concentrates and isolates, made by ultrafiltration and diafiltration of skim milk, have much higher protein and lower lactose, dry differently and must be managed for solubility, which can fall during storage. Permeate powders, the lactose-rich by-product of protein concentration, behave like whey powders. Infant formula base powders are made on dedicated, highly hygienic lines with strict control of Cronobacter and Salmonella, and are covered in the infant formula subject. Each powder has its own specification for moisture, protein, fat, ash, solubility, heat class or functionality, and microbiological limits.
The heat treatment given to milk before drying (and to a lesser extent during drying) determines how much of its whey protein is denatured, and that determines how the powder behaves in use. Low-heat powders, with most whey protein still native, are needed for recombined fluid milk, cheese making from recombined milk, and where good solubility and a fresh flavour are wanted. High-heat powders, with most whey protein denatured, give better water binding, loaf volume and texture in bakery, and some heat-stable high-heat powders are made for recombined evaporated milk, which must survive sterilisation. Medium-heat powders are used in confectionery, ice cream and other foods. Buyers specify the heat class, so producers must control preheating time and temperature precisely.
Heat class is most commonly measured by the whey protein nitrogen index (WPNI): the amount of undenatured whey protein nitrogen, in milligrams per gram of powder. In the method, the reconstituted powder is saturated with salt to precipitate casein and denatured whey protein, the filtrate containing native whey proteins is acidified, and the turbidity produced is compared with a standard curve. A widely used classification for skim milk powder (from the American Dairy Products Institute) is: low-heat, at least 6.0 mg/g; medium-heat, 1.51 to 5.99 mg/g; high-heat, at most 1.5 mg/g. Other indices are used too, including the cysteine number and the casein number, and heat-stability tests for powders destined for recombined evaporated milk. Because WPNI measures the result of the whole heat history, it also reflects the initial whey protein content of the milk, which varies with season.
In practice, a recombining plant making pasteurised or UHT milk buys low-heat skim milk powder because its native whey protein gives a flavour and colour close to fresh milk and it causes less fouling and sediment. A bakery buys high-heat powder because denatured whey protein no longer interferes with gluten and gives better loaf volume. A confectionery or ice cream plant often uses medium-heat powder. A plant making recombined evaporated milk buys heat-stable high-heat powder and checks it with a heat-stability test, since WPNI alone does not guarantee stability. Mixing up heat classes is a common cause of processing problems, so heat class is printed on bags and checked at intake.
Agglomeration joins fine powder particles into larger, porous clusters held together by sticky surface bridges. Fine primary particles wet poorly because water penetrates slowly between them and lumps form with dry powder inside; agglomerates, being larger, more porous and with capillaries between their component particles, wet and sink quickly. Agglomeration needs the particle surfaces to be sticky for a short time — moist or warm enough that amorphous lactose at the surface is above its glass transition — followed by drying and cooling to fix the bridges.
Straight-through (on-line) agglomeration is done within the dryer: fines from the cyclones are blown back into the atomisation cloud, where they collide with moist droplets and stick, and the agglomerates are further built and dried in the integrated and external fluid beds of a two- or three-stage dryer. Re-wet (off-line) agglomeration takes a finished fine powder, wets its surface with steam or atomised water in a separate unit, lets particles collide and stick, and then dries and cools them in a fluid bed. Straight-through agglomeration is cheaper and standard in modern plants; re-wet processes give strong, well-defined agglomerates and are used for some speciality products.
For whole milk powder, agglomeration alone is not enough, because free fat on the surface makes particles water-repellent. Lecithin, a surface-active phospholipid, is dissolved in butter oil and sprayed in a thin layer onto the agglomerated powder in the fluid bed; its hydrophilic groups improve wetting in cold water. The amount used is small and regulated. Instantised whole milk powder is tested for wettability and dispersibility in water at a specified temperature, and must still meet oxidation and flavour standards, since lecithin can contribute off-flavours and oxidation if poorly handled. Over-agglomeration produces large, fragile agglomerates that break during handling and give fines and dust again, so the process is tuned to a target particle size distribution.
Water activity (aw) is the ratio of the vapour pressure of water in a food to that of pure water at the same temperature, and governs microbial growth, chemical reactions and physical changes. Milk powders have low water activity (commonly around 0.1–0.3 at their normal moisture), so micro-organisms cannot grow. The relationship between moisture content and water activity at constant temperature is the sorption isotherm, which for milk powders has a sigmoid shape. Models such as BET (useful at low aw for estimating the monolayer moisture) and GAB (over a wider range) describe it. Powders take up moisture from humid air until their aw equals the relative humidity, so packaging with a good moisture barrier is essential; moisture is also quoted on a wet basis (water ÷ total mass) or a dry basis (water ÷ dry solids), and the two must not be confused.
In spray drying, water is removed so fast that lactose cannot crystallise and is left in an amorphous, glassy state. Amorphous lactose is hygroscopic and metastable. Its glass transition temperature (Tg) — the temperature at which it changes from a hard glass to a soft, rubbery, sticky state — is high when dry (around 100 °C for pure amorphous lactose) but falls steeply as moisture is absorbed, because water acts as a plasticiser; the Gordon–Taylor equation describes this. When the storage temperature exceeds Tg (stickiness becomes significant when the difference T − Tg reaches roughly 10–20 °C), molecular mobility rises: particles become sticky and cake, and the amorphous lactose crystallises. Crystallisation releases the water that was held in the amorphous phase, which raises the moisture of the rest of the powder and accelerates further caking, Maillard browning and loss of quality. This is why the critical water activity for milk powders at room temperature is relatively low, and why powders must be kept dry and not too warm.
The glass transition concept explains many practical observations: powder sticks to dryer walls when outlet temperatures are too high for its moisture; high-lactose powders such as whey and permeate are harder to dry and need pre-crystallisation of lactose before drying; agglomeration relies on brief stickiness; and stored powders cake in humid warehouses. Pre-crystallising lactose (as done for whey powders) reduces hygroscopicity and caking. For whole and skim milk powders, keeping moisture low and storage temperature moderate keeps the lactose glassy and the powder free-flowing.
Water activity is measured with instruments that equilibrate a small sample in a sealed chamber and read the relative humidity of the headspace, by chilled-mirror dew point or by capacitance sensors, at a controlled temperature. Moisture content is measured by oven drying under standard conditions, by vacuum oven, by Karl Fischer titration, or by rapid infrared and near-infrared methods calibrated against reference methods. Sorption isotherms are built by equilibrating samples over saturated salt solutions of known relative humidity and weighing them, or with automated dynamic vapour sorption instruments. These data let a plant set moisture specifications that keep the powder below its critical water activity at the expected storage temperature.
Milk powders keep for a year or more under good conditions, but they change. The main changes are moisture uptake and caking (from humid storage or poor packaging, through lactose glass transition and crystallisation), Maillard browning (reaction of lactose with lysine and other amino groups, giving brown colour, stale and cardboard flavours, loss of available lysine and reduced solubility), oxidation of fat in whole milk powder (giving tallowy and oxidised flavours), and loss of solubility and functional properties. The rates of these reactions rise with temperature and with water activity: Maillard browning has a maximum at intermediate water activity, so even small increases in moisture in powder speed it up considerably. Insects and rodents are a risk in poorly managed warehouses, and odour uptake from nearby materials can taint powder.
Powders are packed in multiwall paper bags with a polyethylene liner, in larger bulk bags, or for retail and infant products in cans or laminated pouches flushed with nitrogen or a nitrogen–carbon dioxide mixture to reduce residual oxygen, which is critical for whole milk powder. Warehouses should be cool, dry and well ventilated, with pallets off the floor and away from walls, stock rotated first-in, first-out, and temperature and humidity monitored. Tropical storage and long sea transport are demanding, so exporters choose packaging, powder moisture and inert-gas packing accordingly. Shelf life is set from storage studies that follow moisture, colour, solubility, free fatty acids or peroxide value, flavour and microbiology over time, sometimes accelerated at higher temperature.
Routine quality testing of milk powders covers moisture, fat, protein (and protein in solids-not-fat), titratable acidity of the reconstituted milk, insolubility index, scorched particles (compared against standard discs after filtering reconstituted milk), bulk density, particle size, heat class (WPNI), instant properties for instant powders, free fat and oxidation indices for whole milk powder, and microbiological criteria including total count, coliforms or Enterobacteriaceae, spore counts (important for powders used in infant formula, recombined evaporated milk and UHT products) and pathogens. Spore specifications, especially for thermophilic and heat-resistant spores, are increasingly demanded by buyers and drive the use of bactofugation or microfiltration of the milk and careful evaporator hygiene, since thermophilic spore-formers can grow in the warm sections of long evaporator runs.
Dairy whitener is a product common in South Asia: a spray-dried powder made from milk with added sugar (sucrose) and sometimes other ingredients, used mainly for whitening tea and coffee. It is defined in Indian food standards by minimum milk fat and milk protein and maximum sugar and moisture, and the exact values should be taken from the current regulation. The added sugar makes the powder sweeter and more hygroscopic, so moisture control, packaging and storage are particularly important; sugar also lowers the glass transition and raises the risk of caking. Whiteners must disperse readily in hot tea without feathering (coagulating into flecks), which depends on heat stability of the protein and the buffering of the product.
Fat-filled milk powders are made by blending skim milk (or skim milk powder and water) with vegetable fat, homogenising and spray drying, to produce a powder resembling whole milk powder but cheaper; they are widely sold in parts of Africa, the Middle East and Asia and must be labelled as fat-filled rather than as milk powder. Formulation uses simple mass balances: to make a powder of 26 per cent fat from skim milk powder of 1 per cent fat and vegetable fat, for example, about 74.75 parts of skim powder and 25.25 parts of fat are combined (ignoring moisture). Coffee creamers (non-dairy creamers) are made from vegetable fat, glucose syrup solids, sodium caseinate and emulsifiers and stabilising salts; feathering in hot acidic coffee is their main technical problem, controlled with buffering salts such as dipotassium phosphate. Labelling rules distinguish 'milk powder', 'dairy whitener', 'fat-filled milk powder' and 'non-dairy creamer', and many countries restrict use of the word 'milk'.
Spray dryers and powder handling systems carry a real risk of fire and dust explosion. Milk powder is combustible. Deposits of powder that build up on hot surfaces in the dryer — around the air inlet, in the chamber, in ducts and in fluid beds — can self-heat because of slow exothermic oxidation and Maillard reactions; if heat is generated faster than it is lost, a smouldering nest forms. This can then ignite surrounding powder, and if a cloud of dispersed powder meets the burning material inside the chamber, cyclone or bag filter, a dust explosion can result. The minimum ignition temperature of a powder layer is lower for thicker layers and for powders with high fat or sugar, and is reduced further by long exposure, so deposits that are left undisturbed for hours are the most dangerous.
Prevention focuses on avoiding deposits and ignition sources: good dryer design with smooth surfaces and correct air flow; avoiding too high inlet and outlet temperatures for the product; regular inspection and cleaning; monitoring of outlet temperature and carbon monoxide in the exhaust air (a rise in CO is an early warning of smouldering); control of static electricity; spark detection; and strict permit-to-work for hot work. Protection limits the damage if an explosion happens: explosion vents (rupture panels) on chambers, cyclones and bag filters that release pressure safely to the outside; explosion suppression systems that inject extinguishing agent within milliseconds; isolation valves that stop flame travelling between units; sprinkler or water-deluge systems; and building design. In Europe, dust-explosion hazard zones are classified under ATEX rules, with corresponding equipment requirements; other countries have equivalent codes. Operators must be trained to recognise early signs, and shutdown procedures must avoid stirring up dust in the presence of burning material.
Plants have written emergency procedures for dryer fires. Typical steps are to stop the product feed, keep or reduce the air flow according to the procedure (stopping fans suddenly can let powder settle onto burning material, while continuing can feed it), shut down heating, isolate the affected unit with isolation valves, activate fixed extinguishing or deluge systems, evacuate personnel from danger zones and call the fire service. Doors and inspection hatches are not opened while burning material may be present, because the inrush of air and the disturbance of powder can trigger an explosion. After an incident, the dryer is inspected for deposits, the cause is investigated and corrective actions are taken before restarting.
A student uses this subject to explain why milk is forewarmed before evaporation, calculate sugar ratios and powder yields, distinguish WPNI classes and explain caking through the glass transition. It supports AIEEA-PG group 16 and AICE Dairy Chemistry Unit 6 questions and powder-plant training.
A powder plant or condensery officer five years in runs preheating to hit a heat class, sets evaporator and dryer conditions for moisture and bulk density, controls agglomeration and lecithination, runs pilot sterilisation tests for evaporated milk, sets the sugar ratio and seeding for condensed milk, and follows dryer fire-safety routines. This subject gives the reasoning and calculations for each task.
A plant head decides on dryer investment (multistage, capacity), product mix between SMP, WMP and specialities, packaging for export markets, and safety systems for explosion protection. They answer to customers for heat class and functional specifications and to regulators for labelling of whiteners and fat-filled products.
A professor or research director works on glass transition and sorption, Maillard chemistry in powders, heat stability mechanisms, agglomeration science and powder safety. They teach and examine AICE Dairy Chemistry and Dairy Technology on these topics and advise industry on product development.
This manual does not yet give: current FSSAI standards for condensed milks, milk powders and dairy whitener; full Codex moisture and protein limits for each powder type; detailed evaporator and dryer design (in evaporation-drying-dairy); quantitative Maillard kinetics; full standard methods for WPNI, insolubility index and instant properties; and legal limits on stabilising salts, lecithin and antioxidants, which differ by country. Temperatures, solids contents and other values are representative ranges, not plant settings.
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