RHEOLOGICAL PROPERTIES OF RAISINS .1. COMPRESSION TEST

Thompson seedless raisins were subjected to compressive unidirectional force and deformation at constant crosshead speed was measured. Relax ation behaviour of the berry was also studied. The force-apparent stra in relationship is described by a straight line on a semi-logarithmic coordinate system. The correlation coefficient is high, although the s pread 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 plo tted on a double-logarithmic coordinate system yield straight lines wi th high correlation coefficients. Analysis shows a tendency for the st iffness of a raisin to increase with increasing load. Calculated work of deformation has shown that the probability of breaking of the raisi n 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 w ith a very high correlation coefficient. Data collected in this work s uggest that the compression of a raisin is a process of relocation of concentrated cell sap enclosed in a berry with a stretching wrinkled s kin. During relaxation there is little possibility for the viscous con centrated liquid to move. It exerts pressure on the skin and a high un relaxed stress remains in a raisin. From a rheological point of view a raisin is a viscoelastic body.