Mechanics of deformation of a fruit subjected to hydrostatic pressure.

Abstract The degree of deformation of a food material under a compression force, applied by fingers or more objectively by an instrument, is an estimate of its firmness. “Firmness” in food materials, as “hardness” in engineering materials, is a loosely defined term and yet it is one of the most important physical properties of foods used as a measure of quality for acquisition of raw material, process control during manufacturing, and assessment of quality of the finished product. The firmness evaluation techniques available to agriculture and the food industry are mainly subjective. There is a need for understanding what is being measured when pressure is applied, for example, on a whole fruit and what are the relationships between the applied forces and deformations and the internal structure and constituents of the fruit. The purpose of this paper is to explore the existence of such relationships. The apple is chosen as the material for investigation. The overall deformation of an apple to any imposed forces is dependent on the mechanical properties of the heterogeneous medium of the apple. The heterogeneous medium of the apple generally is a three phase system consisting of a viscous liquid, a soft solid substance of cellular structure and air. In this paper, an analysis has been developed for the prediction of the overall deformation and the stress distribution in an apple under hydrostatic pressure. The analysis is based upon the linear viscoelastic theory and the assumption that the apple flesh displays linear viscoelastic behaviour and the skin behaves elastically. The predicted deformations have been compared with those observed experimentally. Considering the nature of the material and the difficulty of eliminating the systematic error involved in the experiments, the comparison seems to be reasonably good.