Fluid separation from semi-solid food products, like yoghurt, during storage is often considered by consumers as an indication of minor quality. Also in condiments, like processed mustard, customer complaints are frequently due to fluid separation. It is highly complicated to understand the mechanisms of such structural change. It is even more difficult to govern the structural behaviour of such semi-solid food materials without the use of food additives. It is possible, however, to assess the permeability of such materials which determines the rate of fluid separation, if all other conditions are constant. It is shown how permeability can be derived from centrifuge experiments using a simple two-parameter model. (c) 2004 Elsevier Ltd. All rights reserved.