Study On The Combined Drying Of Apple Slices

Apple is widely planted in China, it has taken up the first place both in plant area and output for several years, but the fine-processing proportion is very limited, resulting in many apples were wasted or rotted. The consuming of apple products is rising in recent years. The research was conducted to develop a process of apple slices dehydration.Browning often occurs on the cutting surface of apple slices during the production process, anti-browning reagents are usually used to avoid it. Some anti-browning reagents such as NaCl,Vc,Citric acid,Oxalate and sodium metabisulfite show their inhibition abilities in the experiments, whereby the inhibition ability depended largely on their concentration. Because of the toxicity and irritating smell, sodium metabisulfite and oxalate were limited to use in food industry. The combination of NaCl 2% with Vc 0.7% was effective in inhibiting enzymatic browning, while the solid to liquid weight ratio is 1/5, and the soaking time is 20 minutes.Osmotic dehydration (OD) can be considered as an important step prior to drying, since it provides a reduction in energy and a losing in nutrition and also an improvement in product quality. An osmotic process rig was designed and used to study the process of osmotic dehydration of apple slices. Response surface methodology (RSM) was employed to investigate the effects of solution compositions, solution temperature, and osmotic time on osmotic process. The optimized parameters of osmotic dehydration are as follows: solution composition, 25%maltose/45%sucrose; solid to liquid ratio, 1/10; vacuum pressure, 0.04 MPa; solution temperature, 40℃; osmotic time, 100 minutes. Under these parameters, the moisture content can be reduced by 20%.Combined of microwave-vacuum drying (MVD) and air-drying (AD) was use to dehydrate the osmotic dehydrated apple slices, and the best parameters of combined dehydration are obtained by orthogonal experiments. The technical parameters are as follows: vacuum pressure,3.0~4.5 KPa; microwave power, 314 W; the amount of water dehydrated by microwave-vacuum dehydration in advance is about 40%;the temperature of air-drying is 60℃.The comparison was done among apple slices dehydrated by different drying methods. From the results, it can be concluded that apple slices pretreated with osmotic dehydration and then dehydrated by microwave-vacuum and air-drying showed not only better quality, but also less dehydration time and electricity energy.The combination of OD,MVD and AD can be used as a alternative method to dehydrate apple slices with good quality and low cost.