EFFECTS OF VACUUM AND NORMAL PRESSURE IMPREGNATION ON WATER LOSS AND SOLID GAIN OF APPLE (MALUS PUMILA MILL) ZHONGFENG WANG 1 , TIAN WEI and MIN ZHANG Department of Biological and Environmental Engineering, Hefei University, Hefei 230601, China 1 Corresponding author. TEL: +86-551-62158450; FAX: +86-551-62158450; EMAIL: wangzhf@126.com Received for Publication February 16, 2014 Accepted for Publication June 2, 2014 doi:10.1111/jfpp.12319 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 impreg- nated 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 impregna- tion (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) com- pared 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 com- mercial applications of OD. INTRODUCTION Osmotic dehydration (OD) is a food-processing technique that partially removes water from fruits, vegetables, meats and other moist foodstuff immersed in a hypertonic solu- tion without high temperatures and long-time drying. OD can remove the moisture of fruits and vegetables by up to 50%, and the energy requirements for subsequent thermal drying can be considerably reduced (Rastogi and Niranjan 1998). Therefore, OD has successfully been applied to many fruits and vegetables, such as apples, bananas, guavas, pine- apples, potatoes and red seaweeds as a pretreatment for further drying (Lombard et al. 2008; Lemus-Mondaca et al. 2009; Azarpazhooh and Ramaswamy 2010; Ruiz-Lopez et al. 2010). Osmotic pretreatment with salt concentration below 5% or sucrose concentration below 30% prior to microwave freeze-drying enhances the quality of dried products with shorter total drying times relative to that of untreated samples (Wang et al. 2010). OD is relatively slow, and pretreatment is therefore time consuming. To accelerate mass transfer, several techniques, such as high pressure (Rastogi and Niranjan 1998), pulsed vacuum (Ito et al. 2007; Lombard et al. 2008), pulsed elec- trical field (Andres et al. 2007; Santacruz-Vaazquez et al. 2008), ohmic heating (Allali et al. 2009), ultrasound (Rodrigues and Fernandes 2007; Garcia-Noguera et al. 2010), microwave (Azarpazhooh and Ramaswamy 2010) and high hydrostatic pressure (Núñez-Mancilla et al. 2014; Verma et al. 2014) have been employed as assistant mea- sures of osmotic pretreatment. OD gives rise to two major simultaneous countercurrent mass transfer processes, namely the diffusion of water from Journal of Food Processing and Preservation ISSN 1745-4549 Journal of Food Processing and Preservation •• (2014) ••–•• © 2014 Wiley Periodicals, Inc. 1