The caking of bulk lactose during transportation is an on going global problem. This paper establishes the limits of water activity that must be achieved before the lactose is bagged off to ensure that temperature gradients encountered during cooling or transportation do not cause caking in the bags. This information is summarised in a graph that can be used to predict the final water activity of the product that must be achieved in order for the product not to cake due to known imposed or anticipated temperature gradients. A water activity of below 0.3 is recommended for meeting the typical temperature gradients encountered during transportation if there is no amorphous lactose present on the surface. (C) 2008 Elsevier Ltd. All rights reserved.