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 mutarotation 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 crystallization; 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