When water freezes, its volume expands causing severe stresses and strains. Two models have been proposed to predict these effects. In the isotropic stress model, a frozen shell forms and squeezes the unfrozen material inwards, compressing the core. In the isotropic expansion model, the frozen shell expands outwards and cause tensile stress in the core. These models give very different predictions of stress, strains and mechanical damage in the food. Experimental observation using a photographic method shows that the size change of roughly spherical potatoes during gentle freezing is best explained by the isotropic expansion model. (C) 2009 Elsevier Ltd. All rights reserved.