Milk, the lacteal secretion of the mammalian mammary gland is a mixture of substances that are intended for the nutrition of infant mammals. Milk is composed of protein to provide amino acids for the growing infant and energy sources which are fat and sugar. For most mammals, this mixture is adequate for the complete nutrition of the infant.
The primary sugar in milk is lactose. There are two exceptions, in nature, the milk of monotremes (platypus and echidna) and some pacific pinnipeds (fur seals, sea lions and walruses) that produce no lactose in their milk at all. As far as it has been studied all other mammals produce lactose in their milk.
Lactose is separated from milk and, more commonly, whey by crystallization. Pure lactose is used in foods and pharmaceuticals (as a filling material in nearly all pills). It is also used in the production of industrial derivatives ranging from the sugar alcohol lactitol, used in many laxatives to some plastics.
Lactose is a disaccharide composed of glucose and galactose connected with aβ-1→4 glycosidic linkage. Its chemical name is β-D-galactopyranosyl-(1→4)-D-glucose.
The galactose can only have the β-pyranose form and the glucose can be in either the α-pyranose form or the β-pyranose form: hence α-lactose and β-lactose refer to the anomeric form of glucose moiety alone. Both the alpha and beta forms of lactose are poorly soluble in water. The ratio of alpha to beta forms and the solubility of the two forms are dependent on the initial ratio, the temperature, and other components in the dairy product and the process the lactose is involved in. On rapid cooling or rapid dehydration lactose remains in solution forms an amorphous solution or glass. These conditions exist during drying and freezing, as with nonfat dry milk and frozen desserts.
Lactose is one of the least soluble sugars with a solubility of only 21.6 gm/100 ml water (sucrose – 200gm/100 ml). It is this property of lactose (ease of glass formation and low solubility) that causes sandy texture in ice cream. Sandiness can also result from lactose crystallization in condensed milk. One property of Lactose crystallization is that due to the equilibrium properties of α&β lactose the sugar can persist in a supersaturated form for a long time. This is problematic because a product can persist in the super saturated state for long periods without getting sandy and then turn sandy unpredictably during shelf life. Lactose crystals are easily identified under the microscope, they are hatchet shaped, see picture on right.
Like most reducing sugars, on heating lactose, with amino acids present, will participate in the Maillard reaction. This leads, through a very complex series of rearrangements to brown colors and (cooked) flavors. Heating milk also results in the formation of Lactulose (4-0-β-D-galactopyranosyl-D-fructose). Lactulose is slightly sweeter than lactose and considerably more soluble. Up to 1% of this compound can be found in condensed milk. This is an interesting molecule that has been alleged to promote the growth of Bifido bacteria and is thus thought of as a prebiotic.
Lactose is less sweet than other common sugars. It is roughly around 16% as sweet as sucrose, but it is sweeter at higher concentrations than it is at lower ones. Protein can cover the lactose sweetness somewhat, and this explains why whey seems just a bit sweeter than milk. In many formulations with dairy products the minimal sweetness from lactose can be ignored especially when it is at low concentrations. Hydrolyzed lactose, containing glucose and galactose, is considerably sweeter than lactose.
Lactose is difficult to hydrolyze into glucose and galactose. Treatments like heating a solution of lactose to 150⁰C with 0.1M hydrochloric acid that will easily hydrolyze sucrose will hydrolyze lactose only slowly. There are a set of enzymes that can enable the hydrolysis they are collectively called β-galactosidase or simply lactase. These enzymes are widely distributed in nature; in plants, animals, bacteria and fungi. They act on the β-1,4 linkage between the glucose and galactose in lactose. They also can act to link the galactose units to various other hydroxyl-containing compounds such as glycerol. Under the right conditions lactase can link galactose units to form oligosaccharides containing glucose and multiple galactose sugars linked in various ways. Galactooligosaccharides (GOS) are known to occur naturally in milk (especially human milk) and are important in stimulating the growth of Bifido bacteria in the gut (especially in infants).
Lactose is the energy supplying material in dairy fermentations. In homofermentative organisms such as Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus lactose is fermented into galactose and lactic acid, one mole of lactose yields either two or four moles of lactic acid depending on if the bacteria can ferment galactose. Heterofermentative organisms such as Leuconostoc and some lactobacillus organisms follow different pathways to lactic acid production and produce other compounds like ethanol and acidic acid. The relative amounts of these products and by -products depend on the organism. Propionobacterium can ferment lactic acid into propionic acid, acetic acid CO2 and water. This fermentation is important to Swiss type cheeses. Lactic acid can be further fermented to butyric acid and CO2 by certain clostridium bacteria this can be undesirable because of the off flavor and gas produced. There are of course, many fermentative pathways that start with lactose; and this is a separate study in and of itself.