LACTOSE 2. 3 Some physical properties of the two common forms of lactose(modified from Jenness Patton, 1 a-Hydrate B-Anhydride Melting point"(C +35° er(g 100 ml-)at 20'C Heat of combustion(kJ mol -) "Decompose D anhyd.es vary with rate of heating, a-hydrate loses water at 120C. rous basis, both forms mutarotase to +55. 4 4 Some properties of a-and B-lactose are summarized in Table 2.3.Mixed /B crystals, e.g. as B3, can be formed under certain conditions. The relation- ship between the different crystalline forms of lactose is shown in Figure 2.8 Lactose glass. When a lactose solution is dried rapidly, viscosity increases so quickly that crystallization is impossible. A noncrystalline form produced containing a- and p-forms in the ratio at which they exist in solution. Lactose in spray-dried milk exists as a concentrated syrup or amorphous glass which is stable if protected from air, but is very hygro opic and absorbs water rapidly from the atmosphere, becoming sticky 2.2.7 Problems related to lactose crystallization The tendency of lactose to form supersaturated solutions that do not ystallize readily causes problems in many dairy products unless adequate controls are exercised. The problems are due primarily to the formation of large crystals, which cause sandiness, or to the formation of a lactose glass hich leads to hygroscopicity and caking(Figure 2.9) Dried milk and whey. Lactose is the major component of dried milk products: whole- milk powder, skim-milk powder and whey powder C,30, 50 and 70% lactose, respectively. Protein, fat and air are dispersed in a continuous phase of amorphous solid lactose. Consequently, the behav iour of lactose has a major impact on the properties of dried milk products. In freshly made powder, lactose is in an amorphous state with an x/B ntio of 1: 1.6. This amorphous lactose glass is a highly concentrated syrup since there is not sufficient time during drying for crystallization to normally. The glass has a low vapour pressure and is hygroscop up moisture very rapidly when exposed to the atmosphere. On the of moisture, dilution of the lactose occurs and the molecules acquire sufficient mobility and space to arrange themselves into crystals of a-lactose
LACTOSE 31 Table 2.3 Some physical properties of the two common forms of lactose (modified from Jenness and Patton, 1959) Property a-H ydrate 8- Anhydride Melting point" ("C) 202 252 Solubility in water (g 100 rn1-l) at 20°C I 50 Specific gravity (20°C) 1.54 1.59 Specific heat 0.299 0.285 Heat of combustion (kJ mol-') 5687 5946 "Decomposes; values vary with rate of heating, ti-hydrate loses water at 120°C. bValues on anhydrous basis, both forms mutarotate to f55.4". Specific rotaltionb [a]:' + 89.4" +35" Some properties of c(- and !-lactose are summarized in Table 2.3. Mixed a/! crystals, e.g. asp3, can be formed under certain conditions. The relationship between the different crystalline forms of lactose is shown in Figure 2.8. Lactose glass. When a lactose solution is dried rapidly, viscosity increases so quickly that crystallization is impossible. A noncrystalline form is produced containing a- and !-forms in the ratio at which they exist in solution. Lactose in spray-dried milk exists as a concentrated syrup or amorphous glass which is stable if protected from air, but is very hygroscopic and absorbs water rapidly from the atmosphere, becoming sticky. 2.2.7 Problems related to lactose crystallization The tendency of lactose to form supersaturated solutions that do not crystallize readily causes problems in many dairy products unless adequate controls are exercised. The problems are due primarily to the formation of large crystals, which cause sandiness, or to the formation of a lactose glass, which leads to hygroscopicity and caking (Figure 2.9). Dried milk and whey. Lactose is the major component of dried milk products: whole-milk powder, skim-milk powder and whey powder contain c. 30, 50 and 70% lactose, respectively. Protein, fat and air are dispersed in a continuous phase of amorphous solid lactose. Consequently, the behaviour of lactose has a major impact on the properties of dried milk products. In freshly made powder, lactose is in an amorphous state with an a/! ratio of 1 : 1.6. This amorphous lactose glass is a highly concentrated syrup since there is not sufficient time during drying for crystallization to proceed normally. The glass has a low vapour pressure and is hygroscopic, taking up moisture very rapidly when exposed to the atmosphere. On the uptake of moisture, dilution of the lactose occurs and the molecules acquire sufficient mobility and space to arrange themselves into crystals of a-lactose
DAIRY CHEMISTRY AND BIOCHEMISTRY LACTOSE IN SOLUTION a=阝 B]/[al=1.64-00027T Amorphous Lactose T=100°, YacuLIll (lactose.1.H,O) hydrous a Water uptake, T<y stable(s) Dissolve, T< 93.50 Supersaturation in ethanol Compound crystal s B,(anhydrous Figure 2.8 Modifications of lactose(T, temperature in 'C)(from Walstra and Jenness, 1984) monohydrate. These crystals are small, usually with dimensions of less than 1 um. Crevices and cracks exist along the edges of the crystals, into which ther components are expelled. In these spaces, favourable conditions exist for the coagulation of casein because of the close packing of the micelles and the destabilizing action of concentrated salt systems. The fat globule membrane may be damaged by mechanical action, and Maillard browning, involving lactose and amino groups of protein, proceeds rapidly when crystallization has occurred
32 LACTOSE IN SOLUTION a-b [Pl/[al= 1.64-0.0027T L DAIRY CHEMISTRY AND BIOCHEMISTRY .- Amorphous Lactose V a-Hydrate (lactose. I.H,O) Conipound crystal asp3 (anhydrous) [ p] / [a] = 1.25 I T = IOW, presence 1 01 \vay LI I Anhydrous a unstable -1 Water uptake, T < !USo I stable (S) TI 150", presence 01 water vapour Dissolve, T c 93.5' ....................... Silpersatiiration in ethanol Figure 2.8 Modifications of lactose (T temperature in 'C) (from Walstra and Jenness, 1984). monohydrate. These crystals are small, usually with dimensions of less than 1 pm. Crevices and cracks exist along the edges of the crystals, into which other components are expelled. In these spaces, favourable conditions exist for the coagulation of casein because of the close packing of the micelles and the destabilizing action of concentrated salt systems. The fat globule membrane may be damaged by mechanical action, and Maillard browning, involving lactose and amino groups of protein, proceeds rapidly when crystallization has occurred
LACTOSE e. HYGROScOF 8%H2O d-HYDRATE MOLECULAR MOBILITY (1H2O AGGREGATES OF CAKING OF MILK AND WHEY POWDERS Crystallization of lactose in dried milk particles causes 'caking'of the powder into a hard mass. If a considerable portion of lactose in the freshly dried product is in the crystalline state, caking of the powder on contac with water is prevented, thereby improving the dispersibility of the powde Lactose crystallization is achieved by rehydrating freshly dried powder to c.10%water and redrying it, or by removing partly dried powder from the drier and completing drying in a fluidized bed dryer. This process is used commercially for the production of "instantized milk powders. Clustering of the particles into loose, spongy aggregates occurs; these agglomerates are dily wettable and dispersible. They exhibit good capillary action and water readily penetrates the particles, allowing them to sink and disperse, hereas the particles in non-instantized powder float due to their low density which contributes to their inability to overcome surface tension Also, because of the small size of the particles in conventional spray-dried ders, close packing results in the formation of capillary action between the particles, thereby preventing uniform wetting As a result, large masses of material are wetted on the outside forming a barrier of highly concentrated product which prevents internal wetting and results in large undispersed lumps. This problem is overcome by agglomer ation and, in this respect, lactose crystallization is important since it facilitates the formation of large, sponge-like aggregates The state of lactose has a major effect on the properties of spray-dried They powder manufactured by conventional methods, i.e. preheating, con densing to about 50% total solids and drying to less than 4% water. The powder is dusty and very hygroscopic, and when exposed to ambient air it
LACTOSE 33 MILK,WHEY, Rapid drying Concentrated lactose syrup PERMEATE * “LACTOSE GLASS” (Non-crystalline) a-HYDRATE Cryslallizatioir 4 MOLECULAR MOBILITY CAKING OF MILK AND WHEY POWDERS AGGREGATES OF CRYSTALS Figure 2.9 Formation and crystallization of lactose glass. Crystallization of lactose in dried milk particles causes ‘caking’ of the powder into a hard mass. If a considerable portion of lactose in the freshly dried product is in the crystalline state, caking of the powder on contact with water is prevented, thereby improving the dispersibility of the powder. Lactose crystallization is achieved by rehydrating freshly dried powder to c. 10% water and redrying it, or by removing partly dried powder from the drier and completing drying in a fluidized bed dryer. This process is used commercially for the production of ‘instantized’ milk powders. Clustering of the particles into loose, spongy aggregates occurs; these agglomerates are readily wettable and dispersible. They exhibit good capillary action and water readily penetrates the particles, allowing them to sink and disperse, whereas the particles in non-instantized powder float due to their low density which contributes to their inability to overcome surface tension. Also, because of the small size of the particles in conventional spray-dried powders, close packing results in the formation of inadequate space for capillary action between the particles, thereby preventing uniform wetting. As a result, large masses of material are wetted on the outside, forming a barrier of highly concentrated product which prevents internal wetting and results in large undispersed lumps. This problem is overcome by agglomeration and, in this respect, lactose crystallization is important since it facilitates the formation of large, sponge-like aggregates. The state of lactose has a major effect on the properties of spray-dried whey powder manufactured by conventional methods, i.e. preheating, condensing to about 50% total solids and drying to less than 4% water. The powder is dusty and very hygroscopic, and when exposed to ambient air it
DAIRY CHEMISTRY AND BIOCHEMISTRY has a pronounced tendency to cake owing to its very high lactose content (~70%). Problems arising from the crystallization of lactose in milk and whey powders may also be avoided or controlled by pre-crystallizing the lactose Essentially, this involves adding finely divided lactose powder which acts as nuclei on which the supersaturated lactose crystallizes. Addition of 0.5 kg of finely ground lactose to the amount of concentrated product(whole milk, skim milk or whey) containing 1 tonne of lactose will induce the formation of c. 106 crystals ml, about 95% of which will have dimensions less than 10 um and 100%less than 15 um, i.e. too small to cause textural defects Diagrams of spray dryers with instantizers are shown in Figures 2.10 and Feed Hot Cyclone separators Drying ○ Crystalization belt Ⅴ ibrofluidizer Hammer mill I Figure 2. 10 Schematic representation of a low temperature drying plant for whey(modified from Hynd, 1980)
34 DAIRY CHEMISTRY AND BIOCHEMISTRY has a pronounced tendency to cake owing to its very high lactose content ( - 70%). Problems arising from the crystallization of lactose in milk and whey powders may also be avoided or controlled by pre-crystallizing the lactose. Essentially, this involves adding finely divided lactose powder which acts as nuclei on which the supersaturated lactose crystallizes. Addition of 0.5 kg of finely ground lactose to the amount of concentrated product (whole milk, skim milk or whey) containing 1 tonne of lactose will induce the formation of c. lo6 crystals ml- l, about 95% of which will have dimensions less than 10pm and 100% less than 15 pm, i.e. too small to cause textural defects. Diagrams of spray dryers with instantizers are shown in Figures 2.10 and 2.11. Feed-[ I /Hot air Cyclone separators (?/ II I1 b Crystalization belt Vibrofluidizer Hammer mill Product out Figure 2.10 Schematic representation of a low temperature drying plant for whey (modified from Hynd, 1980)
LACTOSE 35 Returned fines Hot air brofluidizer Figure 2.11 Schematic representation of a straight through drying plant for whey(modified rom Hynd, 1980) Thermoplasticity of lactose. Unless certain precautions are taken during the drying of whey or other solutions containing high concentrations lactose, the hot, semi-dry powder may adhere to the metal surfaces of the dryer, forming deposits. This phenomenon is referred to as thermoplasticity The principal factors influencing the temperature at which thermoplasticity occurs(sticking temperature) are the concentrations of lactic acid, amor- phous lactose and moisture in the whey powder Increasing the concentration of lactic acid from 0 to 16% causes a linear decrease in sticking temperature( Figure 2.12). The degree of pre-crystalliza tion of lactose affects sticking temperature: a product containing 45% pre-crystallized lactose has a sticking temperature of 60C while the same product with 80% pre-crystallization sticks at 78C(Figure 2. 12).Pre crystallization of the concentrate feed to the dryer thus permits considerably higher feed concentrations and drying temperatures
LACTOSE 35 Figure 2.11 Schematic representation of a straight through drying plant for whey (modified from Hynd, 1980). Thermoplasticity of lactose. Unless certain precautions are taken during the drying of whey or other solutions containing high concentrations of lactose, the hot, semi-dry powder may adhere to the metal surfaces of the dryer, forming deposits. This phenomenon is referred to as thermoplasticity. The principal factors influencing the temperature at which thermoplasticity occurs (‘sticking temperature’) are the concentrations of lactic acid, amorphous lactose and moisture in the whey powder. Increasing the concentration of lactic acid from 0 to 16% causes a linear decrease in sticking temperature (Figure 2.12). The degree of pre-crystallization of lactose affects sticking temperature: a product containing 45% pre-crystallized lactose has a sticking temperature of 60°C while the same product with 80% pre-crystallization sticks at 78°C (Figure 2.12). Precrystallization of the concentrate feed to the dryer thus permits considerably higher feed concentrations and drying temperatures