2 Lactose 2.1 Introduction Lactose is the principal carbohydrate in the milks of all mammals; non mammalian sources are very rare. Milk contains only trace amounts of other sugars, including glucose(50 mg"), fructose, glucosamine, galac- tosamine, neuraminic acid and neutral and acidic oligosaccharides The concentration of lactose in milk varies widely between species(Table 2.1). The lactose content of cows'milk varies with the breed of cow, individuality factors, udder infection and especially stage of lactation. The concentration of lactose decreases progressively and significantly during lactation(Figure 2.1); this behaviour contrasts with the lactational trend for lipids and proteins, which, after decreasing during early lactation, increase strongly during the second half of lactation. Mastitis causes an increased level of NaCl in milk and depresses the secretion of lactose. Lactose, along with sodium, potassium and chloride ions, plays a major role in maintaining the osmotic pressure in the mammary system. Thus, any increase or decrease in lactose content (a secreted constituent, i.e. formed within the mammary gland) is compensated for by an increase or decrease in the soluble salt(excreted) constituents. This osmotic relationship partly explains why certain milks with a high lactose content have a low ash content and vice versa(table 2.2) Similarly, there is an inverse relationship between the concentration of lactose and chloride. which is the basis of Koestler's chloride-lactose test Table 2.1 Concentration(%)of lactose in the milks of selected species Lactose Species Lactose California sea lion 0.0 Mouse(house) Black bear og(domestic ka dee Goat 4.1 Rhesus monkey 7.0 lue whale Elephant(Indian) Red deer 2.6 Sheep 4.8 Zebra rey seal 2.6 Water bufalo 8 Green monkey 10.2 Rat(Norwegian) 2.6
2 Lactose 2.1 Introduction Lactose is the principal carbohydrate in the milks of all mammals; nonmammalian sources are very rare. Milk contains only trace amounts of other sugars, including glucose (50 mg l-’), fructose, glucosamine, galactosamine, neuraminic acid and neutral and acidic oligosaccharides. The concentration of lactose in milk varies widely between species (Table 2.1). The lactose content of cows’ milk varies with the breed of cow, individuality factors, udder infection and especially stage of lactation. The concentration of lactose decreases progressively and significantly during lactation (Figure 2.1); this behaviour contrasts with the lactational trends for lipids and proteins, which, after decreasing during early lactation, increase strongly during the second half of lactation. Mastitis causes an increased level of NaCl in milk and depresses the secretion of lactose. Lactose, along with sodium, potassium and chloride ions, plays a major role in maintaining the osmotic pressure in the mammary system. Thus, any increase or decrease in lactose content (a secreted constituent, i.e. formed within the mammary gland) is compensated for by an increase or decrease in the soluble salt (excreted) constituents. This osmotic relationship partly explains why certain milks with a high lactose content have a low ash content and vice versa (Table 2.2). Similarly, there is an inverse relationship between the concentration of lactose and chloride, which is the basis of Koestler’s chloride-lactose test Table 2.1 Concentration (%) of lactose in the milks of selected species Species Lactose Species Lactose Species Lactose California sea lion Hooded seal Black bear Dolphin Echidna Blue whale Rabbit Red deer Grey seal Rat (Norwegian) 0.0 0.0 0.4 0.6 0.9 1.3 2.1 2.6 2.6 2.6 Mouse (house) Guinea-pig Dog (domestic) Sika deer Goat Elephant (Indian) cow Sheep Water buffalo 3.0 3.0 3.1 3.4 4.1 4.7 4.8 4.8 4.8 Cat (domestic) Pig Horse Chimpanzee Rhesus monkey Human Donkey Zebra Green monkey 4.8 5.5 6.2 7.0 7.0 7.0 7.4 7.4 10.2
DAIRY CHEMISTRY AND BIOCHEMISTRY Week Figure 2.1 Changes in the concentrations of fat(A), protein(O)and lactose(O)in milk during Table 2.2 Average concentration (%)of lactose and ash in the milks of some mammals Water 874 69 87.6 Reindeer for abnormal milk %o Chloride x 100 Koestler number a Koestler number less than 2 indicates normal milk while a value greater Lactose plays an important role in milk and milk products it is an essential constituent in the production of fermented dairy reducTS
22 DAIRY CHEMISTRY AND BIOCHEMISTRY 5 3 0 10 20 30 40 50 60 Week Figure 2.1 Changes in the concentrations of fat (A), protein (0) and lactose (0) in milk during lactation. Table 2.2 Average concentration (%) of lactose and ash in the milks of some mammals Species Water Lactose Ash Human 87.4 6.9 0.21 cow 87.2 4.9 0.70 Goat 87.0 4.2 0.86 Camel 87.6 3.26 0.70 Mare 89.0 6.14 0.51 Reindeer 63.3 2.5 1.40 for abnormal milk: YO Chloride x 100 Koestler number = YO Lactose A Koestler number less than 2 indicates normal milk while a value greater than 3 is considered abnormal. Lactose plays an important role in milk and milk products: products; 0 it is an essential constituent in the production of fermented dairy
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 modifier
LACTOSE 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-galactopyranosyl-( 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 UDPGal 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 determinants 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
DAIRY CHEMISTRY AND BIOCHEMISTRY C C-H (1-4) 6 CH2 OH Anomeric carbon B Lactose O-B-D-Galactopyranosyl-(1-4]-d-DGlucopyranose aLactose Galactose0→4 O.B-D-Galactopyranosyl1-4-B-D-Glucopyranose: BLactose (c) re 2.2 Structural formulae of a- and B-lactose. (a) Fischer projection,(b) Haworth ection and(c)conformational formula
DAIRY CHEMISTRY AND BIOCHEMISTRY H B H-C-OH HO-C-H HO-C-H H-C I H-C ' CHzOH I CHzOH -$ OH O-&D-CPLPetopyrPnaPyl~i~)-@-D-Glucopy~naPe : @.Lactose 4 OH OH [xy n n2 3 0 HO 3 H HO OH H Figure 2.2 Structural formulae of a- and p-lactose. (a) Fischer projection, (b) Haworth projection and (c) conformational formula
LACTOSE GLUCOSE Glucose-6-phosphate ATP P-P UDP UDP-glucose Glucose-l-phosphate pyrophosphorylase UdP glucose-4-epimerase UTP ATP UDP-galactose LA CTOSE Figure 2.3 Pathway for lactose synthesis 2.2.3 Lactose equilibrium in solution The configuration around the CI of glucose(i.e. the anomeric C)is not table and can readily change(mutarotase)from the a- to the B-form and vice versa when the sugar is in solution as a consequence of the fact that the hemiacetal form is in equilibrium with the open chain aldehyde form which can be converted into either of the two isomeric forms( Figure 2.2) When either isomer is dissolved in water, there is a gradual change from one form to the other until equilibrium is established, i.e. mutarotation These changes may be followed by measuring the change in optical rotation with time until, at equilibrium, the specific rotation is +55.4 The composition of the mixture at equilibrium may be calculated Specific rotation [a]2 x-form β-form Equilibrium mixture +554° Let equilibrium mixture 100 Let x% of the lactose be in the a-form Then(100-x)% is the B-form
LACTOSE 25 Glucose- 1 -phoSPhE UDP gliiccisr-4-rpinier.osr gnlncros~llrr~~l~?.\:fr,.cl.vr *LACTOSE cr-I~/ctnlDu/ttil? Glucose Figure 2.3 Pathway for lactose synthesis. 2.2.3 Lactose equilibrium in solution The configuration around the C, of glucose (i.e. the anomeric C) is not stable and can readily change (mutarotate) from the x- to the /?-form and vice versa when the sugar is in solution as a consequence of the fact that the hemiacetal form is in equilibrium with the open chain aldehyde form which can be converted into either of the two isomeric forms (Figure 2.2). When either isomer is dissolved in water, there is a gradual change from one form to the other until equilibrium is established, i.e. mutarotation. These changes may be followed by measuring the change in optical rotation with time until, at equilibrium, the specific rotation is + 55.4". The composition of the mixture at equilibrium may be calculated as follows: Specific rotation [NIP a-form + 89.4" p-form + 35.0" Equilibrium mixture + 55.4" Let equilibrium mixture = 100 Let x% of the lactose be in the cr-form Then (100 - x)% is the p-form