The modelling of caking in bulk lactose : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Process and Environmental Technology at Massey University
Caking during storage is a serious problem for manufacturers of bulk lactose. This study was carried out to investigate the causes of caking and identify solutions as to how such problems can be eliminated. The mechanisms for caking in crystalline lactose powders were identified. Liquid bridging between adjacent particles was shown to occur in high relative humidity environments (>80% RH). These liquid bridges could form crystalline solid bridges if the material was subsequently dried out. The potential mechanism of amorphous lactose flow and bridging in conditions where the glass transition temperature is exceeded was shown to be insignificant in predominantly crystalline lactose powders (<5% amorphous lactose). The presence of amorphous lactose is still important as the amorphous matrix acts as a sink of moisture, which can be released upon crystallisation. This increases the moisture available in the system which can contribute to caking by the liquid bridging mechanism. Both of these mechanisms involve changes in the local temperature and moisture conditions within the bulk powder. Such changes were known to be caused by moisture migration under the influence of a temperature gradient. A model which describes the transport of moisture in one dimension as a result of temperature gradients was developed and validated. The microscopic scale processes of liquid bridging and amorphous lactose moisture relations were included into this model. The model predictions agreed well with experimental trials for completely crystalline lactose powders. Comparison of model predictions for the case where amorphous lactose was present on the surface of the particles showed some inadequacies exist in the model. These were the rate of amorphous lactose crystallisation and the assumption of instantaneous equilibrium between the crystallising amorphous matrix and the air present in the interstices of the bulk lactose. Using the model it was shown that for expected storage conditions, the product should be stored with a water activity below 0.57 aw if no amorphous lactose is present and below 0.25 aw if it is present. If these prescribed limits are met then the goal of producing caking free lactose powders can be achieved.