Effects of ultrasound on glass transition temperature of freeze-dried pear (Pyrus pyrifolia) using DMA thermal analysis

The effect of ultrasound pretreatment at various povier (360W, 600W and 960W, frequency 20 kHz) on the glass transition temperature of freeze dried pear (Pyrus pyrifolia) has been studied. DMA temperature plots were divided into four sections (A- glassy region, B - transition region, C-Rubbex-y plateau region and D - terminal region) with the aim to analyze their properties changed with sonication. Under the same freeze drying condition, with the increase in ultrasonic power, dried pear showed higher glass transition in terms of storage modulus, loss modulus and loss tangent peak. Also a decrease in a(w) (0.31-0.23) and in moisture content (0.12-0.08 g/g d.b.) has been observed. Samples pretreated with ultrasound showed a better texture profile and much porous structure compared to control one. The results from the study indicated that, ultrasound pretreatment prior to freeze drying can improve the stability during storage of freeze dried pear. (C) 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.