Viscoelastic characteristics of Granny Smith apple during glucose osmotic dehydration

Viscoelastic properties of apple tissue during osmotic dehydration in a 25.0% w/w glucose aqueous solution at 20 °C were analyzed in a dynamic rheometer using oscillatory shear and creep-recovery tests. Both storage (G′) and loss (G″) moduli for fresh and osmotically dehydrated tissues showed a weaker dependence with angular frequency. All samples had a viscoelastic solid behavior with G′ dominating the viscoelastic response, but in general both moduli decreased with time of osmosis. For the time scale of the experiments, a generalized Kelvin model with six elements properly predicted the creep compliance response, with a correlation coefficient >0.999 for all samples. In general, initial (J0) and decay compliances (J1 and J2) and steady-state fluidity (ηN) significantly increased as treatment proceeded. Between 40 and 90 or 120 min osmosis, many rheological parameters (G′ at frequencies in the range 1–100 s−1; loss tangent at 0.1 s−1, J1, retardation times (λ1 and λ2), relative contribution of each type of compliance, ηN, plasticity, and overall compliance) showed a turning point, fluctuations or the greatest changes in the evolution along treatment. This behavior was related with shrinkage and posterior round shape recuperation and swelling of the cells, as seen in previous light and environmental scanning electron microscopy studies.