《乳品生物化学》（英文版） 2 Lactose

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; non￾mammalian sources are very rare. Milk contains only trace amounts of other sugars, including glucose (50 mg l-’), 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 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-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

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

DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 2.4 Effect of pH on the rate of mutarotation of lactose. At equilibrium: 894x+35(100-x)=554×100 100-x=62.7 Thus, the equilibrium mixture at 20C is composed of 62.7%B-and 37.3% a-lactose. The equilibrium constant, B/a, is 1.68 at 20C. The proportion of lactose in the a-form increases as the temperature is increased and the equilibrium constant consequently decreases. The equilibrium constant is not influenced by pH, but the rate of mutarotation is dependent on both temperature and pH. The change from a- to B-lactose is 51.1, 17.5 and3, 4% complete at 25, 15 and 0C, respectively, in 1 h and is almost instantaneous at about 75C. The rate of mutarotation is slowest at pH 5.0, increasing rapidly at more acid or alkaline values; equilibrium is established in a few minutes at pH 9.0 (Figure 2. 4)

26 DAIRY CHEMISTRY AND BIOCHEMISTRY oL I I I I 2 4 6 8 PH Figure 2.4 Effect of pH on the rate of mutarotation of lactose. At equilibrium: 89.4~ + 35(100 - X) = 55.4 x 100 x = 37.3 100-x = 62.7 Thus, the equilibrium mixture at 20°C is composed of 62.7% 8- and 37.3% a-lactose. The equilibrium constant, P/a, is 1.68 at 20°C. The proportion of lactose in the @-form increases as the temperature is increased and the equilibrium constant consequently decreases. The equilibrium constant is not influenced by pH, but the rate of mutarotation is dependent on both temperature and pH. The change from m- to p-lactose is 51.1, 17.5 and 3.4% complete at 25, 15 and O"C, respectively, in 1 h and is almost instantaneous at about 75°C. The rate of mutarotation is slowest at pH 5.0, increasing rapidly at more acid or alkaline values; equilibrium is established in a few minutes at pH 9.0 (Figure 2.4)

LACTOSE 2.2.4 Significance of mutarotation The a-and B-forms of lactose differ with respect to solubility crystal shape and size pecific rotatio ystal form -hygroscopicity hydration of cry ● sweetness. Many of these characteristics are discussed in the following sections 2.2.5 Solubility of lactose The solubility characteristics of the a- and B- isomers are distinctly different When a-lactose is added in excess to water at 20 C, about 7 g per 100 g water dissolve immediately. Some a-lactose mutarotates to the B anomer to establish the equilibrium ratio 627B: 37. 3a; therefore, the solution becomes unsaturated with respect to a and more a-lactose dissolves. These two processes(mutarotation and solubilization of a-lactose) continue until two criteria are met: 7 g a-lactose in solution and a b/a ratio of 1.6: 1.0. Since the B/z ratio at equilibrium is about 1.6 at 20 C, the final solubility is 7g+(1.6x 7)g=18.2 g per 100 g water When B-lactose is dissolved in water, the initial solubility is -50g per 00 g water at 20C. Some B-lactose mutarotates to a to establish a ratio of 1.6: 1. At equilibrium, the solution would contain 30.8 g B and 19.2 ax/100 ml; therefore, the solution is supersaturated with a-lactose, some which crystallizes, upsetting the equilibrium and leading to further mutaro- tation ofβ→α. These two events.,ie, crystallization of o- -lactose and mutarotation of B, continue until the same two criteria are met, i. e. 7g 2-lactose in solution and a B/a ratio of 1.6: 1. Again, the final solubility is 18.2 g lactose per 100 g water. Since B-lactose is much more soluble than o and mutarotation is slow, it is possible to form more highly concentrated solutions by dissolving B- rather than a-lactose. In either case, the final solubility is the same The solubility of lactose as a function of temperature is summarized in Figure 2.5. The solubility of a lactose is more temperature dependent than that of B-lactose and the solubility curves intersect at 935C. A solution at 60C contains approximately 59 g lactose per 100g water. Suppose that a 50% solution of lactose(30 g B- and 20 g a-)at 60C is cooled to 15C At this temperature, the solution can contain only 7 g a lactose or a total of 18.2 g per 100 g water at equilibrium. Therefore, lactose will crystallize very slowly out of solution as irregularly sized crystals which may give rise to a andy, gritty texture

LACTOSE 27 2.2.4 Signgcance of mutarotation The a- and 8-forms of lactose differ with respect to: 0 solubility; 0 crystal shape and size; 0 hydration of crystal form - hygroscopicity; 0 specific rotation; 0 sweetness. Many of these characteristics are discussed in the following sections. 2.2.5 Solubility of lactose The solubility characteristics of the a- and /?-isomers are distinctly different. When a-lactose is added in excess to water at 20°C, about 7 g per 100 g water dissolve immediately. Some a-lactose mutarotates to the 8 anomer to establish the equilibrium ratio 62.78 : 37.3~; therefore, the solution becomes unsaturated with respect to a and more a-lactose dissolves. These two processes (mutarotation and solubilization of a-lactose) continue until two criteria are met: - 7 g a-lactose in solution and a P/a ratio of 1.6 : 1.0. Since the P/sc ratio at equilibrium is about 1.6 at 20"C, the final solubility is 7 g + (1.6 x 7) g = 18.2 g per 100 g water. When /-lactose is dissolved in water, the initial solubility is -50g per 100 g water at 20°C. Some /?-lactose mutarotates to a to establish a ratio of 1.6: 1. At equilibrium, the solution would contain 30.8 g /? and 19.2 g a/100 ml; therefore, the solution is supersaturated with a-lactose, some of which crystallizes, upsetting the equilibrium and leading to further mutaro￾tation of /? -+ a. These two events, i.e. crystallization of a-lactose and mutarotation of 8, continue until the same two criteria are met, i.e. -7g a-lactose in solution and a P/a ratio of 1.6: 1. Again, the final solubility is - 18.2 g lactose per 100 g water. Since 8-lactose is much more soluble than a and mutarotation is slow, it is possible to form more highly concentrated solutions by dissolving /?- rather than a-lactose. In either case, the final solubility is the same. The solubility of lactose as a function of temperature is summarized in Figure 2.5. The solubility of a-lactose is more temperature dependent than that of /?-lactose and the solubility curves intersect at 93.5"C. A solution at 60°C contains approximately 59g lactose per lOOg water. Suppose that a 50% solution of lactose (- 30 g p- and 20 g a-) at 60°C is cooled to 15°C. At this temperature, the solution can contain only 7 g a-lactose or a total of 18.2 g per 100 g water at equilibrium. Therefore, lactose will crystallize very slowly out of solution as irregularly sized crystals which may give rise to a sandy, gritty texture

DAIRY CHEMISTRY AND BIOCHEMISTRY Final soluhility at equilibrium Initial solubility of B-lactose 435° Initial solubility or a-lactose Temperature,(°C) Figure 2.5 Solubility of lactose in water(modified from Jenness and Patton, 1959) 2.2.6 Crystallization of lactose As discussed in section 2.2.5, the solubility of lactose is temperature dependent and solutions are capable of being highly supersaturated before spontaneous crystallization occurs and even then, crystallization may be slow. In general, supersolubility at any temperature equals the saturation (solubility) value at a temperature 30C higher. The insolubility of lactose, coupled with its capacity to form supersaturated solutions, is of considerable practical importance in the manufacture of concentrated milk products In the absence of nuclei and agitation, solutions of lactose are capable of being highly supersaturated before spontaneous crystallization occurs. Even in such solutions, crystallization occurs with difficulty. Solubility curves for lactose are shown in Figure 2.6 and are divided into unsaturated, metastable and labile zones. Cooling a saturated solution or continued concentration beyond the saturation point, leads to supersaturation and produces a metastable area where crystallization does not occur readily. At higher levels of supersaturation, a labile area is observed where crystallization occurs readily, The pertinent points regarding supersaturation and crystallization Neither nucleation nor crystal growth occurs in the unsaturated region Growth of crystals can occur in both the metastable and labile areas Nucleation occurs in the metastable area only if seeds(centres for cryst growth) are added

Solubility, g anhydrous lactose I100 g water --*- sggggsggg

CLOSE 00 100 8 Figure 2.6 Initial solubility of x lactose and B-lactose, final solubility at equilibrium (line 1) and supersaturation by a factor 1. 6 and 2. 1(au-lactose excluding water of crystall (Modified from Walstra and Jenness, 1984.) Spontaneous crystallization can occur in the labile area without the addition of seeding material The rate of nucleation is slow at low levels of supersaturation and in highly supersaturated solutions owing to the high viscosity of the solution The stability of a lactose 'glass' is due to the low probability of nuclei forming at very high concentrations Once a sufficient number of nuclei have formed crystal growth occurs at a rate influenced by degree of supersaturation; surface area avai ilable for deposition viscosity e agitation, temperatu mutarotation, which is slow at low temperatures a-Hydrate. a- Lactose crystallizes as a monohydrate containing 5% of crystallization and can be prepared by concentrating aqueous 1a solutions to supersaturation and allowing crystallization to occur e

LACTOSE 200 100 29 - 2.1 - 1 Figure 2.6 Initial solubility of a-lactose and b-lactose, final solubility at equilibrium (line l), and supersaturation by a factor 1.6 and 2.1 (r-lactose excluding water of crystallization). (Modified from Walstra and Jenness, 1984.) Spontaneous crystallization can occur in the labile area without the addition of seeding material. The rate of nucleation is slow at low levels of supersaturation and in highly supersaturated solutions owing to the high viscosity of the solution. The stability of a lactose 'glass' is due to the low probability of nuclei forming at very high concentrations. Once a sufficient number of nuclei have formed, crystal growth occurs at rate influenced by: degree of supersaturation; surface area available for deposition; viscosity ; agitation; temperature; mutarotation, which is slow at low temperatures. ?-Hydrate. cc-Lactose crystallizes as a monohydrate containing 5% water of crystallization and can be prepared by concentrating aqueous lactose solutions to supersaturation and allowing crystallization to occur below

DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 2.7 The most common crystal form of a-lactose hydrate 935C. The a-hydrate is the stable solid form at ambient temperatures and in the presence of small amounts of water below 93 5C, all other forms change to it. The a-monohydrate has a specific rotation in water at 20C 89.4. It is soluble only to the extent of 7 g per 100 g water at 20C. It forms a number of crystal shapes, depending on the conditions of crystalli zation; the most common type when fully developed is tomahawk-shaped (Figure 2.7). Crystals are hard and dissolve slowly. In the mouth, crystals less than 10 um are undetectable, but above 16 um they feel gritty or sandy ind at 30 um, a definite gritty texture is perceptible. The term'sandy or andiness is used to describe the defect ndensed milk. ice-cream or processed cheese spreads where, due to poor manufacturing techniques, large lactose crystals are formed -Anhydrous. Anhydrous a-lactose may be prepared by dehydrating hydrate in vacuo at temperatures between 65 and 93 5C; it is stable only in the absence of moisture B-Anhydride. Since B-lactose is less soluble than the a-isomer above 935C, the crystals formed from aqueous solutions at temperatures above 93 5C are B-lactose; these are anhydrous and have a specific rotation of 35 B-Lactose is sweeter than a-lactose, but is not appreciably sweeter than the quilibrium mixture of a-and B-lactose normally found in solution

30 DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 2.7 The most common crystal form of a-lactose hydrate. 93.5"C. The a-hydrate is the stable solid form at ambient temperatures and in the presence of small amounts of water below 93.5"C, all other forms change to it. The a-monohydrate has a specific rotation in water at 20°C of +89.4". It is soluble only to the extent of 7g per 1OOg water at 20°C. It forms a number of crystal shapes, depending on the conditions of crystalli￾zation; the most common type when fully developed is tomahawk-shaped (Figure 2.7). Crystals are hard and dissolve slowly. In the mouth, crystals less than 10 pm are undetectable, but above 16 pm they feel gritty or 'sandy' and at 30pm, a definite gritty texture is perceptible. The term 'sandy' or sandiness is used to describe the defect in condensed milk, ice-cream or processed cheese spreads where, due to poor manufacturing techniques, large lactose crystals are formed. a-Anhydrous. Anhydrous a-lactose may be prepared by dehydrating a-hydrate in V~CUO at temperatures between 65 and 93.5"C; it is stable only in the absence of moisture. B-Anhydride. Since /%lactose is less soluble than the a-isomer above 93.5"C, the crystals formed from aqueous solutions at temperatures above 93.5"C are p-lactose; these are anhydrous and have a specific rotation of 35". /%Lactose is sweeter than a-lactose, but is not appreciably sweeter than the equilibrium mixture of a- and p-lactose normally found in solution