Combined Drying of Apple Cubes by Using of Heat Pump, Vacuum-Microwave, and Intermittent Techniques

Abstract

Apple cubes of 15 mm were dried naturally without adding any chemical preservative using various drying methods namely intermittent hot air-dehumidified air drying with cyclic temperature profile and step-up temperature profile, heat-pump-assisted (HP) drying, convective vacuum-microwave (C/VM) drying, and heat pump vacuum-microwave (HP/VM) drying. The drying kinetics of apple samples dehydrated by different methods was divided into characteristic drying periods and fitted with empirical models, which gave high value of determination coefficient. The application of C/VM in drying of fruits gave the shortest drying time compared to other drying methods (about 50 % of the total drying time). The drying time was affected by effective diffusivity ranging from 3.522×10 −8 to 1.431×10 −6 m 2 /min depending on the drying technique used. It was found that combined drying which apply vacuum microwave (C/VM, HP/VM) gave the lowest values in hardness and chewiness. In addition, HP/VM drying gave the highest retained total polyphenol content, antioxidant activity, and the best appearance quality.

Colour, phenolic content and antioxidant capacity of some fruits dehydrated by a combination of different methods

“…Then, the weight of samples was measured every 60 min. The processing parameters depended on the particular method of drying (Chong, Figiel, Law, & Wojdyło, 2013). During VM drying the weight of samples was measured at 3 min and 4 min intervals for 360 W and 240 W of microwave power respectively.…”

“…Low temperature dehumidified air (LTDA) horizontally flew through the sample during drying process. The processing air temperature and relative humidity were 35°C and 20%, respectively (Chong et al, 2013).…”

“…For instance, cold air tempering period can be inserted between two hot air drying periods. In this study hot air (HA) of temperature 53.94 ± 0.03°C was supplied continuously for the first two hours followed by two hours of cold air (CA) tempering (11.54 ± 0.26°C) and thereafter hot air (HA) drying again until the equilibrium moisture content was achieved (Chong et al, 2013).…”

“…The processing air temperature and relative humidity of the heat pump dryer (iLab LT1000; The University of Nottingham, Selangor, Malaysia) were 35°C and 20%, respectively (Chong et al, 2013). The fruits were placed in a single layer on a perforated tray and pre-dried until the moisture content of dehydrated samples achieved 30%.…”

Evaluation of Energy Aspects of Apple Drying in the Hot-Air and Infrared Dryers

“…The determined values of effective moisture diffusivity (D eff ) for different hot air and IR drying are given in Table 1 (Chong et al, 2013). The values for D eff obtained from IR drying were greater than hot-air drying.…”