Dairy Science & Technology · Dairy Chemistry · also Milk Carbohydrates and Minerals
Lactose chemistry and crystallisation, the salt system of milk, trace elements and water-soluble vitamins.
In the exams: ICAR AICE subject 34 Unit 1; ICAR AIEEA-PG group 16 (lactose crystallisation and its significance)
Lactose, the milk salts and the water-soluble vitamins make up most of the serum solids of milk. They receive less attention than fat and protein in payment schemes, but they govern several of the most common problems in dairy technology: sandy sweetened condensed milk and ice cream, caking milk and whey powders, browning of heated and stored products, coagulation of evaporated milk during sterilisation, the release of calcium during cheese and yoghurt making, and the nutritional losses that follow heat and light. Lactose is also the basis of a family of derived ingredients, from pharmaceutical lactulose to prebiotic galacto-oligosaccharides, and the cause of the most widespread food intolerance in the world.
This subject covers the structure, anomers and mutarotation of lactose; its solubility and crystallisation and their consequences in concentrated, dried and frozen products; Maillard browning; lactose derivatives; lactose synthesis and intolerance; the salt system of milk and the partition of calcium, phosphate and citrate between the serum and the casein micelles; salt balance and heat stability; trace elements; and the water-soluble vitamins and their losses in processing. Gross composition and the physical properties of milk are in Milk Composition and Physical Properties; the casein micelle, colloidal calcium phosphate and alpha-lactalbumin are in Milk Proteins and Enzymes; and fat-soluble vitamins are in Milk Lipids. This manual refers to those subjects rather than repeating them. Values quoted are typical textbook figures for cow milk and vary with species, feed, season and stage of lactation.
The subject is examined in ICAR AICE-JRF/SRF subject 34 (Dairy Chemistry), whose first unit covers the constituents of milk, and in group 16 of ICAR AIEEA-PG, which names lactose crystallisation and its significance and the chemistry of condensed and dried milks. ASRB NET Dairy Chemistry, food analyst examinations and GATE food technology, which tests carbohydrate chemistry and non-enzymatic browning, cover parts of it. Expect questions on alpha and beta lactose, crystallisation control in sweetened condensed milk and ice cream, the Maillard reaction, salt partition and stabilising salts, and calculations of sucrose ratio, serum solids limits, anomer proportions and mineral molarities.
Lactose is the characteristic carbohydrate of milk, present at about 4.6 to 4.9 per cent in cow milk and about 7 per cent in human milk. It is a disaccharide, 4-O-beta-D-galactopyranosyl-D-glucopyranose: a galactose unit linked by a beta-1,4 glycosidic bond to a glucose unit. The anomeric carbon of the glucose unit is free, so lactose is a reducing sugar: it reduces Fehling's solution and copper and silver reagents, which are the basis of classical analytical methods, and it takes part in the Maillard reaction with amino groups. The glycosidic bond is hydrolysed by the enzyme beta-galactosidase (lactase) and by acid at high temperature, giving glucose and galactose. Lactose is much less sweet than sucrose, commonly quoted at about a fifth to a third of the sweetness, so milk does not taste sweet despite its lactose content. Its solubility in water is also low compared with sucrose, which explains many technological problems. Lactose is found almost only in milk; a few plants contain traces. Its concentration in milk is relatively constant because lactose is the main osmotically active solute of milk, as described in Milk Composition and Physical Properties, and the gland adjusts the volume of milk secreted to the amount of lactose made. Industrially, lactose is crystallised from whey or permeate and is used in infant formula, as a pharmaceutical excipient and filler in tablets and inhalers, in confectionery and bakery products, and as the raw material for derivatives.
Because the glucose unit can close its ring with the hydroxyl at its anomeric carbon pointing in either of two directions, lactose exists as two anomers, alpha-lactose and beta-lactose. They differ in specific optical rotation (about plus 89.4 degrees for alpha and plus 35 degrees for beta), in solubility, in crystal forms and in sweetness, beta being somewhat sweeter and much more soluble initially. In solution, each anomer converts slowly into the other through the open-chain aldehyde form until an equilibrium is reached; this change, observed as a change in optical rotation, is called mutarotation. At equilibrium near 20 degrees Celsius, about 37 per cent of the lactose is alpha and 63 per cent beta, a beta-to-alpha ratio of about 1.6 to 1.7, which falls slightly as temperature rises. The specific rotation of the equilibrium mixture is about plus 55 degrees, which can be checked from the proportions: 0.373 x 89.4 + 0.627 x 35.0 = 55.3. Mutarotation is slow at low temperature, taking hours at room temperature, and fast at high temperature, where it is complete within minutes; it is catalysed by both acid and alkali, with a minimum rate in the mildly acid range. The rate of mutarotation matters because crystallisation from solution removes one anomer, usually alpha, and the other must mutarotate to replace it; slow mutarotation at low temperature can therefore limit the rate of crystallisation. Milk itself contains an equilibrium mixture of the anomers.
Lactose has two solubilities. When alpha-lactose is added to water, it first dissolves to its initial solubility, which is low, about 7 grams per 100 grams of water at about 15 degrees Celsius. As the dissolved alpha-lactose mutarotates to beta-lactose, more alpha can dissolve, and the system approaches the final solubility of the equilibrium mixture, about 17 grams per 100 grams of water at the same temperature, more than twice the initial value. Beta-lactose has a much higher initial solubility, around 50 grams per 100 grams of water at about 15 degrees Celsius, so beta-lactose dissolves fast, after which some alpha crystallises out if the solution is concentrated enough. Both solubilities rise strongly with temperature, so a solution saturated when hot becomes supersaturated on cooling. Lactose solubility is far below that of sucrose, about 200 grams per 100 grams of water at room temperature, and it is further reduced by sucrose and other dissolved solutes that compete for water. A solution containing more lactose than its final solubility is supersaturated, and the degree of supersaturation, the ratio of actual concentration to solubility at that temperature, drives crystallisation. Three zones are described: an undersaturated zone, where crystals dissolve; a metastable zone of moderate supersaturation, where existing crystals grow but new nuclei form only slowly; and a labile zone at high supersaturation, where nuclei form spontaneously and in large numbers. Crystallisation control in dairy products is largely the management of supersaturation and nucleation.
Below about 93.5 degrees Celsius, lactose crystallises from aqueous solution as alpha-lactose monohydrate, which forms hard, relatively insoluble crystals with a characteristic tomahawk shape when grown slowly; this is the form of commercial edible and pharmaceutical lactose. Above about 93.5 degrees Celsius, anhydrous beta-lactose crystallises instead, and roller-dried products can contain beta-lactose. Anhydrous alpha-lactose can be made by careful drying. When a lactose solution is dried very rapidly, as in spray drying, there is no time for crystals to form, and lactose solidifies as an amorphous glass, a disordered solid that contains both anomers. The size of lactose crystals determines whether they can be felt in the mouth: crystals larger than about 15 to 20 micrometres give a sandy or gritty texture. The number and size of crystals depend on how many nuclei form: a few nuclei grow into a few large crystals, while many nuclei share the available lactose and stay small. Industrial lactose crystallisation from concentrated whey or permeate uses slow, controlled cooling with stirring, often with seeding, to grow large, easily separated crystals, which are then washed, dried and milled to the particle size required. In products such as sweetened condensed milk, the aim is the opposite: very many small crystals. Seeding, the addition of a large number of fine crystals at the right degree of supersaturation, together with rapid cooling and agitation, provides the nuclei for this. Impurities such as riboflavin, proteins and salts affect crystal habit and growth rate. Lactose crystals are examined with a polarising microscope, in which they appear bright against a dark background, and crystal size distributions are measured by microscopy or laser diffraction to check seeding and storage performance.
Sweetened condensed milk is milk concentrated by evaporation with added sucrose, typically containing around 8 per cent fat, 20 per cent milk solids-not-fat and 43 to 45 per cent sucrose, with about 26 to 27 per cent water; standards differ between countries. Its keeping quality comes from the high sugar concentration in the water phase, expressed as the sucrose ratio: sucrose divided by sucrose plus water, multiplied by 100. With 44 per cent sucrose and 26.5 per cent water, the sucrose ratio is 44 divided by 70.5, or 62.4 per cent. A sucrose ratio of about 62.5 to 64.5 per cent is commonly recommended: lower values allow microbial growth, while higher values risk sucrose crystallisation. Lactose in such a product, about 10 to 11 per cent, is dissolved in only about 27 per cent water, about 40 grams per 100 grams of water, roughly twice its solubility, which the high sucrose content lowers further. Lactose crystallisation is therefore unavoidable; the only question is crystal size. If crystallisation is left to chance, a few large crystals form and the product becomes sandy and develops sediment. The standard practice is forced crystallisation: the concentrate is cooled rapidly with vigorous stirring to about 30 degrees Celsius, in the zone of high supersaturation, and seeded with very finely ground lactose, so that many nuclei form and the crystals stay below about 10 micrometres. Correct seeding and cooling give a smooth product; later temperature fluctuations during storage can still let crystals grow.
In spray-dried milk and whey powders, lactose is mainly in the amorphous glassy state. Amorphous lactose is hygroscopic and has a glass transition temperature (Tg), the temperature above which the glass becomes a rubbery, sticky material. Tg of dry amorphous lactose is about 100 degrees Celsius but falls steeply as the powder absorbs water, so a powder exposed to humid air can pass from glass to rubber at room temperature. The powder then becomes sticky and cakes, and the lactose crystallises, releasing water that accelerates caking, browning and fat oxidation. Stickiness in the dryer and caking in storage are therefore controlled by keeping the product temperature well below Tg and moisture low. Whey powder, which is about 70 per cent lactose, is especially prone; it is made non-hygroscopic by precrystallising the lactose in the concentrate before drying, so that most lactose enters the powder as alpha-monohydrate crystals. Agglomeration and controlled lactose crystallisation are also used in instant powders. In ice cream, lactose is dissolved in the unfrozen serum. As water freezes, the lactose in the remaining serum becomes highly supersaturated, and if the milk solids-not-fat content is too high or the product undergoes temperature fluctuations, lactose crystallises as large crystals and the ice cream feels sandy. A traditional rule limits milk solids-not-fat to about one seventh of the difference between 100 and the sum of the other solids: maximum MSNF equals (100 minus fat, sugar, stabiliser and other solids) divided by 7. For a mix with 10 per cent fat, 15 per cent sugar and 0.3 per cent stabiliser, the maximum MSNF is 74.7 divided by 7, or about 10.7 per cent. Lactose hydrolysis and stabilisers reduce sandiness. Storage freezers that hold a steady, low temperature also matter, because each warming partly melts ice, dissolves small lactose and ice crystals, and lets larger ones grow when the product cools again.
The Maillard reaction, or non-enzymatic browning, is a series of reactions between reducing sugars and free amino groups. In milk the sugar is lactose and the main amino group is the epsilon-amino group of lysine residues in proteins. In the early stage, the carbonyl group of lactose condenses with the amino group to form a Schiff base, which rearranges into a more stable Amadori product, lactulosyllysine. The early stage causes no colour change, but it blocks lysine, an essential amino acid, so the nutritional value of the protein falls; when blocked lysine is hydrolysed with acid in the laboratory, it yields furosine, which is measured as a marker of early Maillard reaction and heat load. In the advanced stage, Amadori products degrade by several routes into dicarbonyl compounds, hydroxymethylfurfural, reductones and other reactive intermediates; Strecker degradation of amino acids by dicarbonyls produces aldehydes and pyrazines that contribute flavour. In the final stage, these intermediates polymerise into brown nitrogenous pigments called melanoidins. In dairy products the Maillard reaction is responsible for the brown colour and caramel or cooked flavour of in-container sterilised milk, evaporated milk, dulce de leche, khoa, burfi and other heat-concentrated sweets, for the stale flavour that develops in UHT milk and milk powder during storage, and for loss of available lysine in heated and stored products. Some products, such as caramelised milk sweets, depend on it for their character; in others it is a defect.
The rate of the Maillard reaction depends on temperature, time, water activity, pH, the type of sugar and the concentration of reactants. Temperature has the largest effect: the rate rises several-fold for each 10 degree Celsius rise, so high-temperature processes and warm storage accelerate browning. Water activity matters because the reaction needs mobility of reactants but is slowed by dilution: browning is fastest at intermediate water activity, roughly 0.6 to 0.7, typical of concentrated and partly dried products, and slow in very dry powders and dilute liquids. Higher pH accelerates the reaction because more amino groups are unprotonated, and it slows as pH falls. Glucose and galactose react faster than lactose, so lactose-hydrolysed (lactose-free) milks brown more quickly on UHT processing and storage than ordinary milk. Control therefore uses the least heat consistent with safety; direct rather than indirect UHT heating where browning is a problem; cool storage of UHT milk and powders; low moisture in powders, kept below their glass transition; low-pH formulations where possible; and limiting storage time. In milk powders, browning is accompanied by loss of solubility and of lysine. Indicators of Maillard reaction include furosine, hydroxymethylfurfural, colour measurement (a fall in lightness and rise in yellowness on the CIE scale), fluorescence, and loss of available lysine. Lactulose, formed alongside the Maillard reaction by isomerisation, is used with furosine to grade the heat treatment of milk, as described in Milk Proteins and Enzymes.
Lactose is a cheap, abundant by-product of cheese and casein manufacture, and several derivatives with higher value are made from it. Lactulose (4-O-beta-D-galactopyranosyl-D-fructose) is formed when the glucose unit of lactose isomerises to fructose under alkaline conditions, by the Lobry de Bruyn-Alberda van Ekenstein rearrangement; small amounts also form when milk is heated, which is why lactulose is an indicator of heat treatment. Lactulose is not hydrolysed by human intestinal enzymes, so it reaches the colon, where bacteria ferment it; it is used as an osmotic laxative, in the treatment of hepatic encephalopathy and as a bifidogenic (prebiotic) ingredient. Galacto-oligosaccharides (GOS) are chains of two to several galactose units, usually with a terminal glucose, formed by beta-galactosidases that, at high lactose concentrations, transfer galactose to lactose or other sugars instead of to water (transgalactosylation). GOS resist digestion, stimulate bifidobacteria and are added to infant formula, often blended with fructo-oligosaccharides, to imitate some effects of human milk oligosaccharides. Hydrolysis of lactose by beta-galactosidase, from yeasts such as Kluyveromyces for milk at neutral pH or from moulds such as Aspergillus for acid whey, produces glucose and galactose. Hydrolysed products are sweeter, more soluble and suitable for people with lactose intolerance; hydrolysis is used in lactose-free milk, to prevent sandiness in ice cream and condensed products, and to make sweet syrups from whey permeate.
Lactitol is the sugar alcohol made by catalytic hydrogenation of lactose, reducing the glucose unit to sorbitol. It is less sweet than sucrose, poorly absorbed and provides fewer calories, and it does not promote dental caries, so it is used as a bulk sweetener in sugar-free confectionery, chocolate and bakery products and as a laxative. Lactobionic acid is formed by oxidation of the free aldehyde of the glucose unit to a carboxylic acid, chemically, enzymatically or by microorganisms. It is a strong chelator of calcium and iron and an antioxidant, and it is used in solutions for preserving organs for transplantation, in pharmaceuticals and cosmetics, and as a food ingredient where permitted. Other derivatives include tagatose, a low-calorie sweetener made by isomerising galactose; lactosucrose, formed enzymatically from lactose and sucrose; and lactose esters used as emulsifiers. Lactose and whey permeate are also fermentation substrates: lactose-fermenting yeasts such as Kluyveromyces marxianus convert whey permeate to ethanol, and bacteria convert lactose to lactic acid, which is used in food and to make biodegradable polylactic acid. In homofermentative lactic fermentation, one molecule of lactose (molecular mass 342) and one of water give four molecules of lactic acid (molecular mass 90), so 342 grams of lactose can yield up to 360 grams of lactic acid, a factor of about 1.05. These routes turn the large volumes of whey lactose that once polluted rivers into products of value, which is a central theme of whey utilisation.
Lactose is synthesised in the Golgi apparatus of the mammary secretory cells. Glucose from the blood enters the cell; part of it is converted into UDP-galactose, and the enzyme lactose synthase then transfers galactose from UDP-galactose to glucose, forming lactose. Lactose synthase consists of two proteins: beta-1,4-galactosyltransferase, a Golgi enzyme present in many tissues, which normally transfers galactose to N-acetylglucosamine on glycoproteins, and alpha-lactalbumin, the whey protein described in Milk Proteins and Enzymes, which binds to the galactosyltransferase and changes its specificity so that glucose becomes the acceptor. Lactose cannot cross the Golgi and vesicle membranes, so it accumulates inside the secretory vesicles and draws water in by osmosis; the vesicles are then secreted, and the lactose concentration of milk is set by this osmotic balance. Because of this mechanism, the rate of lactose synthesis largely controls milk volume: animals that make more lactose secrete more milk. Glucose supply is therefore critical for milk yield; in ruminants, most glucose is made in the liver from propionate produced in the rumen, which links feeding to milk volume. Species whose milk contains little lactose, such as some seals and sea lions, produce milk very rich in fat and low in water. Lactose concentration falls when the tight junctions between cells open, as in mastitis and late lactation, because lactose leaks into blood and is excreted in urine.
Lactose is digested in the small intestine by lactase (lactase-phlorizin hydrolase), a beta-galactosidase on the brush border, into glucose and galactose, which are absorbed. In most mammals, lactase activity is high in infancy and falls after weaning. In humans, this decline, called lactase non-persistence, occurs in the majority of the world's adults, commonly estimated at about two-thirds, with high prevalence in East Asia, much of Africa and South America and lower prevalence in northern Europe and some pastoral populations. Lactase persistence into adulthood is a genetic trait associated with specific variants upstream of the lactase gene, different in European and in African and Middle Eastern populations, which spread with dairying. In a person with low lactase, undigested lactose reaches the colon, where bacteria ferment it to gases and short-chain fatty acids and draw water osmotically, causing bloating, abdominal pain, flatulence and diarrhoea; this is lactose intolerance. It is distinct from cow's milk protein allergy, an immune reaction described in Milk Proteins and Enzymes, and from galactosaemia, an inherited inability to metabolise galactose that requires strict avoidance of lactose. Diagnosis commonly uses the hydrogen breath test, in which a rise in breath hydrogen after a lactose dose indicates malabsorption. Most people with lactose malabsorption tolerate small amounts, such as the lactose in a cup of milk taken with food, according to a review by the European Food Safety Authority, which put the amount tolerated by most such people at up to about 12 grams of lactose in a single dose; fermented milks and aged cheeses, which contain less lactose, are often well tolerated. Lactose-free and lactose-reduced milks are made by enzymatic hydrolysis, sometimes combined with membrane removal of lactose; legal thresholds for lactose-free claims differ by country.
Milk salts are the inorganic ions and organic acid salts of milk, which should not be confused with ash, the residue left after incineration. Ash, about 0.7 per cent of cow milk, loses citrate and carbonate, which burn, and gains phosphate and sulphate from the organic phosphorus and sulphur of proteins and lipids, so it does not represent the salts as they exist in milk. Typical concentrations in cow milk are: calcium about 1.2 grams per litre (about 30 millimolar), magnesium about 0.11 to 0.12 grams per litre (about 5 millimolar), sodium about 0.5 grams per litre, potassium about 1.5 grams per litre, chloride about 1.0 gram per litre, total phosphorus about 0.9 to 1.0 gram per litre (about 30 millimolar, of which part is inorganic phosphate and part is organic, in the phosphoserine of casein and in phospholipids and phosphate esters), citrate about 1.7 to 1.8 grams per litre (about 9 millimolar), with smaller amounts of sulphate, carbonate and bicarbonate. To convert a mass concentration to millimolar, divide grams per litre by the atomic or molecular mass and multiply by 1,000: 1.2 grams per litre of calcium divided by 40.08 gives about 30 millimolar. Milk is a rich source of calcium in most diets, both because of its concentration and because calcium from milk is well absorbed. Sodium, potassium and chloride rise and fall with udder health, as noted in Milk Composition and Physical Properties.
The salts of milk are distributed between the soluble phase (the serum) and the colloidal phase (the casein micelles). Sodium, potassium and chloride are almost entirely soluble. About two-thirds of the calcium and about a third of the magnesium are colloidal, held in the micelles as colloidal calcium phosphate and bound to casein phosphoserine; the remaining third of calcium is soluble, but most of this is complexed with citrate and phosphate, and only about 2 millimolar, under a tenth of total calcium, is free ionic calcium. About half of the inorganic phosphate is colloidal. Citrate is mostly soluble, about 90 per cent, mainly as calcium citrate complexes. The serum is supersaturated with respect to calcium phosphate; the micelles hold the excess in a stable colloidal form. The partition is not fixed. Lowering pH dissolves colloidal calcium phosphate, as described in Milk Proteins and Enzymes, so acidified milk and acid whey are rich in soluble calcium and phosphate. Heating moves calcium and phosphate from the serum into the colloidal phase and lowers pH; the change is partly reversed slowly on cooling. Cooling moves calcium phosphate into the serum. Concentration by evaporation increases soluble salts and lowers pH, and can exceed the solubility of calcium phosphate, contributing to mineral fouling and to sediment in concentrated products. Ultrafiltration and diafiltration remove soluble salts but retain the colloidal ones. Adding calcium chloride, phosphates or citrates shifts the balance deliberately. Ionic calcium is measured with calcium-selective electrodes. In cheesemaking, the colloidal calcium phosphate left in the curd at draining, which depends on the pH at that point, largely determines the texture of the finished cheese, linking salt partition directly to product quality.
The stability of milk to heat, alcohol and concentration depends on the balance between the divalent cations, calcium and magnesium, which reduce the charge on casein and bridge micelles, and the anions phosphate and citrate, which bind calcium and keep it from acting on the casein. This idea, the salt balance theory, was developed in the early twentieth century to explain why some milks coagulate during the sterilisation of evaporated milk while others do not. Milk with too much ionic calcium relative to phosphate and citrate is unstable; adding phosphate or citrate binds the excess calcium and restores stability, while adding calcium has the opposite effect. The salt balance varies naturally: it changes with the stage of lactation, being least favourable in colostrum and late-lactation milk, with season and feed, and with udder health. Ionic calcium concentration correlates with alcohol stability, which is why the alcohol test is used as a quick guide to the heat stability of raw milk. pH is closely linked to salt balance because it determines the ionisation of phosphate, citrate and casein and the solubility of calcium phosphate; a small fall in pH raises ionic calcium. In practice, heat stability is described by the heat coagulation time and its dependence on pH, covered in Milk Proteins and Enzymes, and salt balance is one of the main factors, together with pH, kappa-casein and beta-lactoglobulin interactions and the concentration of the milk.
Concentration makes the problem much worse, because evaporation raises the concentration of calcium and casein and lowers pH, so evaporated milk sterilised in the can at about 115 to 120 degrees Celsius is far less heat-stable than the original milk. Manufacturers improve stability in several ways. Forewarming, heating the milk before evaporation, typically at a temperature above 100 degrees Celsius for a short time or at a lower temperature for longer, transfers calcium phosphate to the micelles and denatures whey proteins in a way that improves the heat stability of the concentrate. Stabilising salts, mainly disodium or trisodium phosphates and trisodium citrate, are added in small amounts to bind calcium and adjust pH; the amount is decided each day by pilot sterilisation tests, in which small cans of concentrate with different additions are sterilised and examined for coagulation and viscosity, because the requirement varies with the milk. Their use and maximum levels are regulated by Codex standards and national law. Too little stabiliser leads to coagulation in the steriliser; too much can cause excessive softness, browning or later gelation. Similar principles apply to UHT milk, where stabilising salts are sometimes permitted to reduce sediment and fouling, to recombined milks made from powder, and to processed cheese, in which emulsifying salts such as citrates and phosphates sequester calcium from para-casein so that it hydrates and emulsifies the fat. Control of salt balance also matters in coffee whiteners and cream liqueurs, which must withstand hot, acidic coffee or alcohol without feathering or gelling.
Trace elements are minerals present at milligram or microgram per litre levels. Their concentrations in milk vary much more than those of the major salts, because they depend on the animal's diet, the soil on which feed was grown, supplements, and contamination from equipment and cleaning chemicals. Iron is low, about 0.3 to 0.6 milligrams per litre, bound mainly to casein, lactoferrin and the fat globule membrane; milk is a poor source of iron, and infants given cow milk instead of breast milk or iron-fortified formula in the first year are at risk of iron-deficiency anaemia. Copper is about 0.05 to 0.1 milligrams per litre; it is nutritionally minor but technologically important as a strong catalyst of lipid oxidation, as described in Milk Lipids, so contamination from copper or brass fittings causes oxidised flavour. Zinc, about 3 to 5 milligrams per litre, is mostly bound to casein, and milk is a useful dietary source. Selenium varies widely with soil selenium, from very low levels in milk from selenium-deficient regions to much higher levels where soils or supplements supply more; some countries have added selenium to fertilisers or feeds. Iodine varies more than any other element, from tens to several hundred micrograms per litre, depending on iodine supplements in feed and on iodophor teat dips and sanitisers; milk can be a major source of iodine in human diets, and both deficiency and excess are of concern. Cobalt is needed by rumen microbes to make vitamin B12, molybdenum is part of xanthine oxidoreductase, and manganese and fluoride occur in small amounts.
Some trace elements in milk are contaminants with no nutritional role. Lead, cadmium, mercury and arsenic reach milk from contaminated feed, water, soil and the environment near mines, smelters and industrial sites; their levels in milk are generally low, because the mammary gland is a partial barrier, but they are monitored, and maximum limits for lead in milk are set by Codex and national food law. Radionuclides were an important concern after nuclear accidents, when iodine-131, caesium-137 and strontium-90 deposited on pasture passed rapidly into milk; iodine-131, with a half-life of about 8 days, dominates in the first weeks, while caesium and strontium persist much longer. Milk monitoring is part of emergency response, and measures include keeping animals off contaminated pasture and diverting milk to products such as butter and cheese, in which some radionuclides are partly removed, or holding it until short-lived isotopes decay. Within the dairy, trace-element contamination is controlled by using stainless steel for all product-contact surfaces, controlling the quality of water used in processing and for reconstitution, managing iodine from sanitisers by correct concentrations and rinsing, and using approved, clean packaging. Trace elements are measured by atomic absorption spectrometry and, increasingly, by inductively coupled plasma mass spectrometry, which can determine many elements at once at very low concentrations. Fortification with iron, zinc and other minerals is common in infant formula and some milk products, and requires care because added iron and copper promote oxidation. Encapsulated or protein-bound forms of iron are often chosen for this reason.
Milk contains all the water-soluble vitamins, in amounts that make it an important dietary source of some. Riboflavin (vitamin B2), about 1.5 to 1.8 milligrams per litre, is the most notable; milk and dairy products are a major source of riboflavin in many diets, and riboflavin gives whey its greenish-yellow colour. Vitamin B12 (cobalamin), about 3 to 5 micrograms per litre, is synthesised by rumen microbes from cobalt and is particularly valuable in vegetarian diets, since it is absent from plant foods. Thiamine (vitamin B1) is about 0.4 milligrams per litre, pantothenic acid about 3 to 4 milligrams per litre, vitamin B6 about 0.4 to 0.6 milligrams per litre, biotin about 20 to 30 micrograms per litre and folate about 50 micrograms per litre, the folate being bound partly to a folate-binding protein. Niacin is low, under 1 milligram per litre, but milk protein is rich in tryptophan, from which the body can make niacin. Vitamin C (ascorbic acid) is present at about 10 to 20 milligrams per litre in freshly drawn milk but is unstable and falls rapidly with exposure to oxygen and light, so milk is not a reliable source; camel milk contains several times more, as noted in Milk Composition and Physical Properties. Most of these vitamins in ruminant milk come from synthesis by rumen microbes rather than directly from the feed, so their levels vary less with diet than those of the fat-soluble vitamins. Human milk differs in several, being richer in vitamin C and poorer in riboflavin and B12 than cow milk.
The water-soluble vitamins differ widely in stability. Vitamin C is the least stable, destroyed by oxygen, heat, light and copper. Folate and vitamin B12 are sensitive to oxygen and to heat in its presence. Thiamine is heat-sensitive. Riboflavin is stable to heat but very sensitive to light. Losses in heat treatment therefore increase with the intensity of the treatment and with the oxygen content of the milk. Pasteurisation causes only small losses, commonly under 10 per cent for most B vitamins, with somewhat larger losses of vitamin C. UHT treatment causes moderate losses of thiamine, vitamin B6, B12, folate and vitamin C, larger with indirect heating and with dissolved oxygen; deaeration before UHT reduces them. In-container sterilisation, with its long heating at high temperature, causes the largest losses, and boiling milk at home also destroys a substantial part of the labile vitamins. During storage, losses continue: in UHT milk, vitamin C, folate and B12 decline steadily if oxygen is present in the milk or penetrates the package. Light is the main enemy of riboflavin: milk in clear glass or translucent plastic exposed to sunlight or strong retail lighting can lose a large part of its riboflavin within hours, and the excited riboflavin also destroys vitamin C and causes light-induced off-flavour, as described in Milk Composition and Physical Properties. Opaque, light-blocking packaging is the main protection. Evaporation and drying cause further losses depending on temperature and time, which is why infant formulas and some milk powders are fortified after processing.
A student uses this subject in dairy chemistry practicals and in the courses on condensed and dried milks, ice cream and whey processing. You will determine lactose by polarimetry or reducing-sugar methods, ash and calcium, measure the sucrose ratio of condensed milk, observe lactose crystals under the microscope, and see Maillard browning develop in heated milk. Learn the properties of alpha and beta lactose, the crystallisation rules, the salt concentrations and their partition, and the stability of each vitamin, because they explain defects you will be asked to diagnose and calculations you will be asked to do.
Five years into work in a condensed milk, powder, ice cream or UHT plant, you control seeding and cooling to keep condensed milk smooth, keep powders below their glass transition to prevent caking, run pilot sterilisation tests each day to dose stabilising salts, reduce browning and stale flavour in UHT and lactose-free milk by adjusting heat treatment and storage temperature, and specify opaque packaging to protect riboflavin. You also explain lactose-free claims and test results to customers and regulators, and investigate complaints of sediment, grittiness or brown colour.
A senior manager or consultant with twenty-five years of experience decides how to use whey lactose, whether as edible or pharmaceutical lactose, permeate powder, galacto-oligosaccharides or fermentation feedstock, plans investment in lactose-free product lines and membrane processes, sets mineral and stabiliser policies for export products under different national rules, and manages risks such as iodine excess from sanitisers or radionuclide contamination in emergency planning. Such a person also negotiates specifications with infant formula and pharmaceutical customers, whose requirements for lactose purity, particle size and mineral content are much stricter than those of food uses.
A researcher with long experience works on open questions: the kinetics of lactose crystallisation and glass transition in complex dairy matrices, the control of Maillard reaction products and their nutritional effects, enzymatic production of prebiotic oligosaccharides, the physical chemistry of calcium phosphate and its role in heat stability and fouling, and vitamin retention in new processes. Methods include polarimetry, chromatography, calorimetry, X-ray diffraction, calcium-selective electrodes, kinetic modelling and controlled storage trials. Such work informs process design, packaging choices and standards for heat-treated and lactose-free products.
This manual gives typical concentrations and principles; it does not tabulate national standards for condensed milk and powders, permitted stabilising salts and their limits, lactose-free claim thresholds, or maximum limits for contaminants, which differ by country and change. Exact solubility curves of lactose at each temperature, glass transition curves at each moisture content and kinetic constants for the Maillard reaction are given only approximately. Vitamin losses are given as broad ranges because published figures vary widely with process, oxygen and packaging. Human milk oligosaccharides, the detailed technology of whey processing and lactose manufacture, and fat-soluble vitamins are covered in other subjects.
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.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.