Determination of viscoelastic properties of dates (Mazafati Variety)

The knowledge of rheological properties of agricultural fruits can be significantlyhelpful in design and fabrication of the appropriateprocessing machines .In this study, the viscoelastic characteristics of date (Mazafati variety) were investigated. Experimental tests were established in six moisture levels (7.5, 9.0, 10.8, 12.8, 17.59 and 25.5 on dry basis) and temperature of 45ºC which contributed to obtain a suitable model. To achieve the desired moisture levels, various saturated salts were used. A chamber capable of controlling temperature was fabricated and placed around the plunger of texture analyzer to monitor the temperature during the experiments. Tests were conducted using an Instron Universal Testing Machine in the way that, samples were exposed to a sudden load of 1.7 Newton for 300 seconds. Experimental data of creep tests were fitted into Burger four-element model. Results indicated that, creep factor inclines when moisture content increases, meanwhile, its minimum and maximum values were 0.08 mm and 0.26 mm at the moisture contents of 7.5% d.b. and 25.5% d.b., respectively. Consequently, constant values related to the model, were determined and correlated with moisture contents of products using non-linear regression analysis. Analysis of variances and using Tuky test (Pvalue<0.01) on creep parameters revealed that moisture content has significant effect on all firmness values except primary strain. Moreover, the moisture content showed a great impact on delay time. According to the results, rheological properties of date are highly dependent on its moisture content