Dairy Chemistry And Biochemistry

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, 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 60°C while the same
product with 80% pre-crystallization sticks at 78°C (Figure 2.12). Pre-
crystallization of the concentrate feed to the dryer thus permits considerably
higher feed concentrations and drying temperatures.