INFLUENCE OF RELATIVE HUMIDITY ON THE PHYSICOCHEMICAL STATE OF LACTOSE IN SPRAY‐DRIED SWEET WHEY POWDERS

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Abstract
The objective of the present study was to evaluate, by means of moisture sorption isotherms and the Scanning Electron Microscope (SEM), the effects of relative humidity during storage on the physicochemical state of lactose, a major constituent of spray‐dried whey powders. Lactose, which is present in whey powders at about 70%/weight levels, can be in either the amorphous or crystalline form depending upon processing conditions during the drying treatment. Two spray‐dried whey powders were examined: a hygroscopic spray‐dried sweet whey powder (HW) in which the lactose was chiefly in the amorphous form, and a nonhygroscopic spray‐dried sweet whey powder (NHW) which contained a mixture of both amorphous and crystalline lactose. Moisture adsorption isotherms at 25°C under vacuum over saturated salt solutions in the water activity (a,) range 0.1l‐0.85 were determined by weighing periodically over a 6‐wk period. The adsorption isotherm for the nonhygroscopic whey (NHW) samples was continuous or smooth over the entire a, range; however, the adsorption isotherm for the hygroscopic whey (HW) samples had a break in the 0.33‐0.44 aw range. Work with the Scanning Electron Microscope (SEM) indicated that in HW samples, lactose crystals were present at an aw of 0.33 and above. Examination of the NHW samples with the SEM indicated the presence of lactose crystals at all aw levels up to 0.75. In both the HW and NHW powders, the amount of lactose crystallization appeared to increase with increasing aw above 0.40, indicating a continuing shift from amorphous to crystalline lactose. This was also confirmed for both whey powders since they both lost weight after an initial rapid gain during the 6‐wk period at an aw of 0.40 and above followed by another weight gain at the higher aw’s.

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