VISCOELASTIC PROPERTIES OF RAISINS

The rheological properties of raisins are important in the processing, storage and utilization of the product. Owing to their physicochemical composition and structure, raisins are expected to behave like viscoelastic materials. A rheological model can be developed, based on mechanical testing of raisins. Stress relaxation measurements were performed on compressed Corinth and Sultana raisins, using a Universal Texture Testing Machine. The stress relaxation data of both types of raisins were fitted to a three-element Maxwell (spring and dashpot) model. The raisin flesh (mainly sugars) can be represented by a two-element model with short relaxation times, indicating a viscous behaviour. The elastic component (longer relaxation time) is evident due to the raisin skin. Partial extraction of the raisin sugars tended to increase the elastic character of the product.