DAIRY CHEMISTRY AND BIOCHEMISTRY Crystalline lactose(%) 05955 505 F In practice, the most easily controlled factor is the moisture content of the whey powder, which is determined by the outlet temperature of the dryer (to, Figure 2.13). However, as a result of evaporative cooling, the tempera- ture of the particles in the dryer is lower than the outlet temperature (t Figure 2.13)and the difference between f, and t, increases with increasing moisture content. The sticking temperature for a given whey powder decreases with increasing moisture content (ts, Figure 2. 13)and where the intersect (point TPC, Fi 13)is the maximu product moisture content at which the dryer can be operated without product sticking during drying. The corresponding point on the outlet temperature curve(ToC)represents the maximum dryer outlet temperature which may be used without causing sticking Sweetened condensed milk. Crystallization of lactose occurs in sweetened ondensed milk(SCM)and crystal size must be controlled if a product with a desirable texture is to be produced. As it comes from the evaporators, SCM is almost saturated with lactose When cooled to 15-20 C. 40-60% of the lactose eventually crystallizes as a-lactose hydrate. There are 40-47 parts of lactose per 100 parts of water in SCM, consisting of about 40%- and 60%B-lactose(ex-evaporator). To obtain a smooth texture, crystals with dimensions of less than 10 um are desirable. The optimum temperature
36 DAIRY CHEMISTRY AND BIOCHEMISTRY Crystalline lactose (%) 45 55 65 75 1 I I 3 0 4 8 12 16 Lactic acid added (%) Figure 2.12 Effect of added lactic acid (- - - -) and degree of lactose crystallization (-) on the sticking temperature of whey powder (1.5-3.5% moisture). In practice, the most easily controlled factor is the moisture content of the whey powder, which is determined by the outlet temperature of the dryer (to, Figure 2.13). However, as a result of evaporative cooling, the temperature of the particles in the dryer is lower than the outlet temperature (tp, Figure 2.13) and the difference between to and t, increases with increasing moisture content. The sticking temperature for a given whey powder decreases with increasing moisture content (ts, Figure 2.13) and where the two curves (t, and t,) intersect (point TPC, Figure 2.13) is the maximum product moisture content at which the dryer can be operated without product sticking during drying. The corresponding point on the outlet temperature curve (TOC) represents the maximum dryer outlet temperature which may be used without causing sticking. Sweetened condensed milk. Crystallization of lactose occurs in sweetened condensed milk (SCM) and crystal size must be controlled if a product with a desirable texture is to be produced. As it comes from the evaporators, SCM is almost saturated with lactose. When cooled to 15-20°C, 40-60% of the lactose eventually crystallizes as a-lactose hydrate. There are 40-47 parts of lactose per 100 parts of water in SCM, consisting of about 40% aand 60% /?-lactose (ex-evaporator). To obtain a smooth texture, crystals with dimensions of less than 10 pm are desirable. The optimum temperature
LACTOSE Sticking zone TOC gIree zone 60 Powder moisture(%) igure 2. 13 Influence of moisture content on the temperature of powder in a spray dryer(tp), dryer outlet temperature('o and ng temperature(f, ) The minimum product tempera sticking is at TPC with the corresponding dryer outlet temperature TOC(Modified from Hynd, 1980. for crystallization is 26-36C. Pulverized az-lactose, or preferably lactose glass, is used as seed. Continuous vacuum cooling, combined with seeding, gives the best product Ice-cream. Crystallization of lactose in ice-cream causes a sandy texture. In freshly hardened ice-cream, the equilibrium mixture of a- and B-lactose is in the 'glass' state and is stable as lor onstant. During the freezing of ice-cream, the lactose solution passes through the labile zone so rapidly and at such a low temperature that limited lactose crystallization occurs If ice-cream is warmed or the temperature fluctuates, some ice will melt, and an infinite variety of lactose concentrations will emerge, some of which will be in the labile zone where spontaneous crystallization occurs while others will be in the metastable zone where crystallization can occur if suitable nuclei, e.g. lactose crystals, are present. At the low temperature crystallization pressure is low and extensive crystallization usually does not occur. However, the nuclei formed act as seed for further crystallization
LACTOSE 37 110 - 100 - h 90- * : * E 80- E ?L 2 3 .- ‘ 70- 60 0 1 2 3 4 5 Powder moisture (%) Figure 2.13 Influence of moisture content on the temperature of powder in a spray dryer (t,), dryer outlet temperature (to) and sticking temperature (fJ The minimum product temperatured required to avoid problems with sticking is at TPC with the corresponding dryer outlet temperature TOC. (Modified from Hynd, 1980.) for crystallization is 26-36°C. Pulverized @-lactose, or preferably lactose ‘glass’, is used as seed. Continuous vacuum cooling, combined with seeding, gives the best product. Ice-cream. Crystallization of lactose in ice-cream causes a sandy texture. In freshly hardened ice-cream, the equilibrium mixture of a- and p-lactose is in the ‘glass’ state and is stable as long as the temperature remains low and constant. During the freezing of ice-cream, the lactose solution passes through the labile zone so rapidly and at such a low temperature that limited lactose crystallization occurs. If ice-cream is warmed or the temperature fluctuates, some ice will melt, and an infinite variety of lactose concentrations will emerge, some of which will be in the labile zone where spontaneous crystallization occurs while others will be in the metastable zone where crystallization can occur if suitable nuclei, e.g. lactose crystals, are present. At the low temperature, crystallization pressure is low and extensive crystallization usually does not occur. However, the nuclei formed act as seed for further crystallization