Changes in Viscoelastic Properties of Thompson Seedless Grapes During Ripening

In order to investigate the viscoelastic properties of Thompson seedless grapes in ripening، relaxation tests were carried out on whole berry، flesh and skin of the fruit. Samples were harvested weekly from a grapevine for seven consecutive weeks and relaxation test was done using a texture analyzer after sample preparation. The first term of generalized Maxwell model with three elements was determined using a series of assumed data and then the experimental data were evaluated by the model. The results showed that the magnitude of initial stress (stress at time zero) required to create a fixed strain on whole fruit decreases during ripening period when ºBrix of the fruit increases from 16. 0±0. 45 to 22. 5±0. 25، but the relaxation time remains almost constant at the range of 41. 681 to 45. 630 s. Based on results، elastic behavior of the skin enhances and firmness of flesh increases during fruit growth and ripening. According to the results، the skin as compared with the flesh plays a main role on the behavior of viscoelastic alterations of whole fruit. Also on the basis of results obtained by non-linear regression analysis، amount of elastic components of the model raises for flesh and declines for skin and whole berry through ripening. The generalized Maxwell with three elements was the best in fitting experimental data with highest determination coefficients.