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Lesson 17. LACTOSE: NOMENCLATURE AND STRUCTURE
Module 5. Carbohydrates in milk
LACTOSE: NOMENCLATURE AND STRUCTURE
17.1 Introduction
The characteristic carbohydrate of milk is lactose (4-O-β-D-galactopyranosyl-D-glucopyranose). It is commonly referred as milk sugar. Milk of mammals is the sole source of lactose. Of all the mammalian milk human milk contains highest lactose content (7.0%) while the average lactose content of normal bovine milk is 4.8% which accounts for about 50 to 52% of the total solids in skim milk. Besides lactose small amount of other carbohydrates are found in milk partly in a free form and partly bound to proteins, lipid or phosphate. Cow’s milk contains monosaccharide glucose and galactose in concentrations of about 10mg/10ml. the amount of oligosaccharides is 100 mg/litre. Lactose is a disaccharide which upon hydrolysis will yield one molecule of glucose and one molecule of galactose. This disaccharide would act as a controlling factor in fermented and ripened dairy products. It contributes to the nutritive value of milk and milk products and plays essential role in the body and texture and solubility of certain stored products. It has an essential role in the color and flavour of heated products. Essential role in the color and flavour of the high heated dairy products.
17.2 Nomenclature
Lactose is a disaccharide that yields D-glucose and D-galactose on hydrolysis. It is designated as 4-0-β-galactopyranosyl-D-glucopyranose and occurs in both alpha and beta forms. The predominant carbohydrates encountered in the body are structurally related to the aldotriose glyceraldehyde and to the ketotriose dihydroxyacetone . All carbohydrates contain at least one asymmetrical (chiral) carbon and are, therefore, optically active. In addition, carbohydrates can exist in either of the two conformations, as determined by the orientation of the hydroxyl group about the asymmetric carbon atom. farthest from the carbonyl. With a few exceptions, those carbohydrates that are of physiological significance exist in the D-conformation. The mirror-image conformations, called enantiomers , are in the L-conformation.
17.3 Structure of Lactose
Fig. 17.1 Fischer structure of a lactose
(Source: Jenness and Patton, Principles of Dairy Chemistry, 1959)
Fig.17.2 Haworth structure of a lactose
(Source: Jenness and Patton, Principles of Dairy Chemistry, 1959)
The aldehyde and ketone moieties of the carbohydrates with five and six carbons will spontaneously react with alcohol groups present in neighboring carbons to produce intra molecular hemiacetals or hemiketals, respectively. This results in the formation of five- or six-membered rings. Because the five-membered ring structure resembles the organic molecule furan, derivatives with this structure are termed furanoses. Those with six-membered rings resemble the organic molecule pyran and are termed pyranoses
The rings can open and re-close, allowing rotation to occur about the carbon bearing the reactive carbonyl yielding two distinct configurations (α and β) of the hemiacetals. The carbon about which this rotation occurs is the anomeric carbon and the two forms are termed anomers. Carbohydrates can change spontaneously between the α and β configurations: a process known as mutarotation .