RHEOLOGICAL PROPERTIES OF RAISINS .2. EFFECT OF WATER ACTIVITY

Rheological properties of Thompson seedless raisins were measured in relation to the water activity of the berry using the compression-relaxation test. The water sorption isotherm of raisins was determined according to the COST 90 procedure. It has been found that the theological properties of raisins are strongly dependent on water activity. At water activities lower than 0 . 25, the raisin behaves as a brittle body. It is inferred that sugars in dry raisins are amorphous and the properties of the glass are pivotal to the characteristics of the material. In this state the probability of breakage during compression is high. At water activities above 0 . 3, a plasticizing effect of water is manifested. The raisin becomes a viscoelastic body but until a(w) = 0 . 4, the elastic component is not strong because of the high viscosity of the interior of the berry. Further uptake of water and increase of a(w) leads to dilution of the sugar solution and increased elasticity of the skin. The relaxation behaviour of compressed raisins changes around a(w) = 0 . 4. The elasticity of the skirt is sufficiently high to prevent breakage of raisins during the compression test at stresses lower than 0 . 3 MPa. Water activity higher than 0 . 7 weakens the skin and eases of the flow of diluted sugar solution. Deformation of the berry is easy and the probability of breakage increases strongly.