Milk Carbohydrate (Lactose)
This page describes the properties of milk carbohydrate. There is a brief introduction to General Carbohydrate Chemistry, followed by sections on Milk Carbohydrate (Lactose) Chemistry, Lactose Physical Properties, and the Influence of Heat Treatments on Lactose Properties. For more details on lactose properties see references by Fox and McSweeney (1998), Holsinger (1988, 1997), and O'Brien (1995, 1997).
General Carbohydrate Chemistry
Carbohydrates are made up of molecules called saccharides. Simple saccharides contain 1 or 2 molecules and are called monosaccharides or disaccharides, or, more commonly, sugars. Oligosaccharides and polysaccharides are chains that contain a few to many sugar molecules and may be referred to as starches.
The monosaccharides important in food and health are glucose (sometimes called dextrose), fructose, and galactose. The disaccharides are sucrose (glucose + fructose), lactose (glucose + galactose), and maltose (glucose + glucose). The 2 sugar molecules in disaccharides are bonded together and this bond must be broken before the sugars can be used by the body for energy and other body functions. Starches are long chains of glucose that can be straight or have branches, and there are several ways in which the molecules within starches can bond to each other. Starches from different sources, such as wheat or corn, have unique functional properties. The structure of starches can be modified to improve their functional properties and increase their use in foods.
Milk Carbohydrate Chemistry
Milk contains approximately 4.9% carbohydrate that is predominately lactose with trace amounts of monosaccharides and oligosaccharides. Lactose is a disaccharide of glucose and galactose. The structure of lactose is:
Lactose Physical Properties
Lactose is dissolved in the serum (whey) phase of fluid milk. Lactose dissolved in solution is found in 2 forms, called the α-anomer and ß-anomer, that can convert back and forth between each other. The solubility of the 2 anomers is temperature dependent and therefore the equilibrium concentration of the 2 forms will be different at different temperatures. At room temperature (70°F, 20°C) the equilibrium ratio is approximately 37% α- and 63% ß-lactose. At temperatures above 200°F (93.5°C) the ß-anomer is less soluble so there is a higher ratio of α- to ß-lactose. The type of anomer present does not affect the nutritional properties of lactose.
Lactose crystallization occurs when the concentration of lactose exceeds its solubility. The physical properties of lactose crystals are dependent on the crystal type and can greatly influence their use in foods. Temperature affects the equilibrium ratio of the α- and ß-lactose anomers, as described above. Lactose crystals formed at temperatures below 70°F (20°C) are mainly α-lactose crystals. The α-monohydrate lactose crystals are very hard and form, for example, when ice cream goes through numerous warming and freezing cycles. This results in an undesirable gritty, sandy texture in the ice cream. Gums are often used in ice cream to inhibit lactose crystallization. The crystal form of ß-lactose is sweeter and more soluble than the α-monohydrate lactose and may be preferred in some bakery applications. When a lactose solution is rapidly dried it does not have time to crystallize and forms a type of glass. Lactose glass exists in milk powders and causes clumping. The clumping is desirable because it results in a milk powder that dissolves instantly in water.
Influence of Heat Treatments on Lactose Properties
The normal pasteurization conditions used for fluid milk have no significant effect on lactose. The higher temperatures used for ultra high temperature (UHT) pasteurization of extended shelf life products and spray drying can cause browning and isomerization reactions, which may affect product quality and nutritional properties. The browning reaction, called the Maillard reaction, occurs between the lactose and protein in milk and produces undesirable flavors and color, and decreases the available content of the amino acid lysine in milk protein. The isomerization reaction is a molecular rearrangement of lactose to lactulose. Lactulose is produced for use by the pharmaceutical industry in pill production.