LACTOSE it contributes to the nutritive value of milk and its products; however, nany non-Europeans have limited or zero ability to digest lactose in adulthood, leading to a syndrome known as lactose intolerance it affects the texture of certain concentrated and frozen products involved in heat-induced changes in the colour and flavour of highly heated milk products 2.2 Chemical and physical properties of lactose 2.2.1 Structure of lactose Lactose is a disaccharide consisting of galactose and glucose, linked by a B1-4 glycosidic bond (Figure 2. 2). Its systematic name is B-0-D-galac topyranosyl-(1-4)-aX-D-glucopyranose(ez-lactose)or B-0-D-galactopyranosyl- (1-4)-B-D-glucopyranose(B-lactose). The hemiacetal group of the glucose moiety is potentially free(i.e. lactose is a reducing sugar) and may exist as an a-or B-anomer. In the structural formula of the a-form, the hydroxyl group on the C, of glucose is cis to the hydroxyl group at C2(oriented downward 2.2.2 Biosynthesis of lactose Lactose is essentially unique to mammary secretions. It is synthesized from glucose absorbed from blood. One molecule of glucose is isomerized to UDP-galactose via the four-enzyme Leloir pathway(Figure 2.3). UDP-Gal is then linked to another molecule of glucose in a reaction catalysed by the enzyme, lactose synthetase, a two-component enzyme Component A is non-specific galactosyl transferase which transfers the galactose from UDP. Gal to a number of acceptors. in the presence of the b component, which is the whey protein, a-lactalbumin, the transferase becomes highly specific for glucose (its Ky decreases 1000-fold), leading to the synthesis of lactose Thus, a-lactalbumin is an enzyme modifier and its concentration in the milk of several species is directly related to the concentration of lactose in those milks: the milks of some marine mammals contain neither a-lactalbumin nor lactose The presumed significance of this control mechanism is to enable mamma terminate the synthesis of lactose when nec to regulate and control osmotic pressure when there is an infux of NaCl, e.g during mastitis or in late lactation (lactose and NaCl are major determi- nants of the osmotic pressure of milk, which is isotonic with blood, the osmotic pressure of which is essentially constant). The ability to control osmotic pressure is sufficiently important to justify an elaborate control mechanism and the wastage of the enzyme modifierLACTOSE 23 0 it contributes to the nutritive value of milk and its products; however, many non-Europeans have limited or zero ability to digest lactose in adulthood, leading to a syndrome known as lactose intolerance; 0 it affects the texture of certain concentrated and frozen products; 0 it is involved in heat-induced changes in the colour and flavour of highly heated milk products. 2.2 Chemical and physical properties of lactose 2.2.1 Structure of lactose Lactose is a disaccharide consisting of galactose and glucose, linked by a pl-4 glycosidic bond (Figure 2.2). Its systematic name is j3-0-D-galac￾topyranosyl-( 1 -4)-ol-~-glucopyranose (a-lactose) or P-0-D-galactopyranosyl- (1-4)-P-~-glucopyranose (p-lactose). The hemiacetal group of the glucose moiety is potentially free (i.e. lactose is a reducing sugar) and may exist as an a- or p-anomer. In the structural formula of the a-form, the hydroxyl group on the C, of glucose is cis to the hydroxyl group at C, (oriented downward). 2.2.2 Biosynrhesis of lactose Lactose is essentially unique to mammary secretions. It is synthesized from glucose absorbed from blood. One molecule of glucose is isomerized to UDP-galactose via the four-enzyme Leloir pathway (Figure 2.3). UDP-Gal is then linked to another molecule of glucose in a reaction catalysed by the enzyme, lactose synthetase, a two-component enzyme. Component A is a non-specific galactosyl transferase which transfers the galactose from UDP￾Gal to a number of acceptors. In the presence of the B component, which is the whey protein, a-lactalbumin, the transferase becomes highly specific for glucose (its K, decreases 1000-fold), leading to the synthesis of lactose. Thus, r-lactalbumin is an enzyme modifier and its concentration in the milk of several species is directly related to the concentration of lactose in those milks; the milks of some marine mammals contain neither a-lactalbumin nor lactose. The presumed significance of this control mechanism is to enable mammals to terminate the synthesis of lactose when necessary, i.e. to regulate and control osmotic pressure when there is an influx of NaC1, e.g. during mastitis or in late lactation (lactose and NaCl are major determi￾nants of the osmotic pressure of milk, which is isotonic with blood, the osmotic pressure of which is essentially constant). The ability to control osmotic pressure is sufficiently important to justify an elaborate control mechanism and the ‘wastage’ of the enzyme modifier