Effects of Vacuum and Normal Pressure Impregnation on Water Loss and Solid Gain of Apple (Malus pumila Mill)

Abstract

The rates of water loss (WL) and solid gain (SG) by osmotic dehydration (OD) are important criteria for evaluating the dehydration efficiency and commercial values of a certain OD technique. For comparing the effect of OD with and without vacuum treatment on the WL and SG of fruits, round apple slices were impregnated in sucrose solution at different concentrations (30–60°Brix) and different temperatures (10–55C) with vacuum (20 kPa, VI) or without vacuum (NVI, atmospheric pressure). The values of WL were significantly (P ≤ 0.05) lower and those of SG higher by VI than by NVI, irrespective of concentration, temperature, time of impregnation or water content of the sample. This study suggests that VI is unfeasible for the OD of apple slices if the pretreatment is for the processing of dried products with the lowest sugar content. Practical Applications Osmotic dehydration (OD) has been extensively studied, and vacuum impregnation (VI) is usually combined with OD to enhance product quality. According to a report, VI increases water loss (WL), but does not increase solid gain (SG) compared with normal OD. While our results suggested vacuum or negative pressure is unfavorable to increasing the WL rate, but favorable to increasing the SG rate of OD, thereby conflicting with former reports to some extent. This study puts forward some practical references for further research efforts on VI and for commercial applications of OD.