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

• Pear (Pyrus pyrifolia) was treated at different ultrasonic power before freeze drying.
• DMA thermal analysis of freeze dried pear was carried out.
• Modulus plots were effective to analyze the properties of dried pear.
• Glass transition temperature of freeze dried pear has been found to increase due to ultrasound treatment.

The effect of ultrasound pretreatment at various power (360 W, 600 W and 960 W, 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 – Rubbery 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 aw (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.