Rheological properties of raisins. I. Compression test

Thompson seedless raisins were subjected to compressive unidirectional force and deformation at constant crosshead speed was measured. Relaxation behaviour of the berry was also studied. The force-apparent strain relationship is described by a straight line on a semi-logarithmic coordinate system. The correlation coefficient is high, although the spread of points is large and reflects the geometrical, rheological and chemical heterogeneity of raisins. The stress-true strain curves are composed of both a concave upward and a convex upward part. Curves plotted on a double-logarithmic coordinate system yield straight lines with high correlation coefficients. Analysis shows a tendency for the stiffness of a raisin to increase with increasing load. Calculated work of deformation has shown that the probability of breaking of the raisin is related to the unit work applied to compress the sample. At unit work less than 20 mJ/g the probability of breakage is zero. Relaxation curves normalized according to Peleg and Pollak give straight lines with a very high correlation coefficient. Data collected in this work suggest that the compression of a raisin is a process of relocation of concentrated cell sap enclosed in a berry with a stretching wrinkled skin. During relaxation there is little possibility for the viscous concentrated liquid to move. It exerts pressure on the skin and a high unrelaxed stress remains in a raisin. From a rheological point of view a raisin is a viscoelastic body.