Acid whey is considered a hard to process stream mainly due to failure of lactose to crystallize, apparently attributed to the presence of lactic acid (LA) and calcium (Ca). Understanding the physical state and the thermal behaviour of lactose during crystallization in the presence of LA and Ca is important in order to develop adequate strategies to improve processability of acid whey and thus was the objective of this study. Presence of high concentration of LA (1% w/w) resulted in a decline in lactose crystal yields (similar to 59%). This was alleviated by lowering the LA content (0.2% w/w) which improved the yield (similar to 77%). At high LA content, small amounts of Ca (0.072% w/w) appeared to diminish the negative impact of LA presence by increasing the yield. Pure L solution yielded lactose particles of similar to 84 mu m. Crystal size slightly increased to similar to 91 mu m in the system containing high concentrations of LA along with low Ca concentration and although the presence of low concentrations of both LA (0.2% w/w) and Ca (0.072% w/w) resulted similar to 163 mu m sized crystals. Water molecules in the hydration layer around lactose molecules appeared to play a significant role governing the crystallisation behaviour and characteristics of lactose crystals in the presence of LA and Ca and/or in combination. Hence, the removal of some Ca to concentrations <= 0.072% w/w from acid whey may improve the crystallisation of lactose and thereby improve the processability of acid whey. (C) 2016 Elsevier Ltd. All rights reserved.