CHEMISTRY OF MILK

Module 5. Carbohydrates in milk

Lesson 17
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

The two monosaccharides of lactose are linked through the aldehyde group of D-galactose. Thus the aldehyde portion of lactose is on glucose residue. The configuration of the D-galactose residue is of the beta form, which was shown by using the enzyme β- D- galactosidase that hydrolyses both lactose and a methyl β - D- galactopyranoside but not the α anomer. The D-galactose in the lactose molecule is in the beta form was also shown by its synthesis from D-glucose and D-galactose. The point of union of the two monosaccharides was established through products of hydrolysis or methylated 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 .

When drawn in the Fischer projection, the αconfiguration places the hydroxyl attached to the anomeric carbon to the right,towards the ring. When drawn in the Haworth projection, the α configuration places the hydroxyl downward. The spatial relationships of the atoms of the furanose and pyranose ring structures are more correctly described by the two conformations identified as the chair form and the boat form. The chair form is the more stable of the two. Constituents of the ring that project above or below the plane of the ring are axial and those that project parallel to the plane are equatorial. In the chair conformation, the orientation of the hydroxyl group about the anomeric carbon of α-D-glucose is axial and equatorial in β-D-glucose. Covalent bonds between the anomeric hydroxyl of a cyclic sugar and the hydroxyl of a second sugar (or another alcohol containing compound) are termed glycosidic bonds, and the resultant molecules are glycosides. The linkage of two monosaccharides to form disaccharides involves a glycosidic bond. Several physiologically important disaccharides are sucrose, lactose and maltose.