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Effect of Humidity on Moisture Content and Forms of Lactose in Dried Whey

Department of Dairy Industry, Cornell University, Ithaca, New York

ABSTRACT

1. Dried wheys in which crystalline beta lactose is present as a solid phase contain less water when in equilibrium with an atmosphere of constant relative humidity below 65 per cent than do dried wheys in which alpha hydrate is the solid phase. The difference is largely accounted for by the molecule of water of crystallization present in the alpha hydrate crystals.
   
2. Stabilized dried wheys do not absorb excessive amounts of water until the relative humidity exceeds 40–50 per cent.
   
3. Temperature exerts no marked effect upon the equilibrium moisture content of samples exposed to constant relative humidity. The equilibrium is attained more rapidly at the higher temperatures.
   
4. If an appreciable amount of lactose in the glass state is present at relative humidities between 30 to 50 per cent the whey will first absorb and then reject water. This process is accompanied by the crystallization of the solid form of lactose present. In this way crystallization of beta lactose at room temperatures may occur.
   
5. At relative humidities of 65 per cent and above the crystalline beta lactose in dried wheys undergoes conversion to crystalline alpha hydrate.
   
6. The lower the percentage of lactose in the dry matter the greater the equilibrium moisture content of the dried whey at constant relative humidity.