Effect of osmotic dehydration on viscoelastic properties of apple and banana

The rheological behavior of osmotically dehydrated apple and banana was exa mined under uniaxial compression and relaxation tests of cylindrical specim ens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging fr om 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding str ain were correlated to the moisture content using simple mathematical equat ions. It was shown that both parameters increase as water was removed and t heir values are significantly higher than the corresponding values for untr eated air dried samples. The effect of moisture content on compressive beha vior of osmotically dried materials was introduced through its effect on th e four model parameters: the maximum stress (sigma(max)), the maximum strai n (epsilon(max)), the elastic parameter (E) and the viscoelastic exponent ( p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment inc reased the remaining force and it decreased the relaxation time of dehydrat ed samples. The osmotic dehydration and therefore the sugar gain tend to in crease the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.