Abstract
Crystalline material can develop on the surface of raisins during storage and transport, affecting the final acceptability of the product. In this work, a mild thermal pretreatment was applied to raisins to melt the pre-existing crystals and the effect of such thermal treatments on the development of crystals over a storage period was investigated. The raisins selected for this study were of the Thompson seedless variety from one Chilean company. The thermal pretreatment of raisins at 50 °C and 70 °C for 20 min in an oven and microwave (800 W) irradiation for 15 s resulted in a reduction in the percentage of crystallized raisins (w/w) from more than 50% in the control samples to less than 10% after 35 days of storage at 15 and 25 °C in a 57% relative humidity environment. The results showed that some textural parameters, such as cohesiveness and chewiness, were not affected by thermal treatment and were independent of storage temperature.
Highlights
IntroductionExposure to a high temperature and water loss during dehydration results in shrinkage, crystallization, and color, flavor and texture alterations of the final product (raisin)
The control of crystallization kinetics of Thompson raisins by thermal pretreatment in hot air and microwave ovens was studied at different storage conditions
The increase in the environmental temperature from 15 ◦C to 25 ◦C favored the crystallization of the raisins without pretreatment
Summary
Exposure to a high temperature and water loss during dehydration results in shrinkage, crystallization, and color, flavor and texture alterations of the final product (raisin) All these changes may adversely affect the quality of the dried product. Truong et al [11] investigated the crystallization and glass transition temperature (Tg) as affected by the sugar component ratio, water fraction, and storage conditions on three Chilean raisins that were examined by means of differential scanning calorimetry (DSC), polarized light microscopy and X-ray diffraction (XRD) techniques. The present study aims to explore the potential advantages of applying microwave technology to volumetrically heat whole raisins as a pretreatment in order to melt the existing crystals that can grow in an uncontrolled way over the time, and to compare it with the conventional air-heating oven processing. Texture and color analyses of pretreated samples were undertaken after one month of storage
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