Study On Drying Processing And Sterilization On Spinach

Spinach(Spinacia oleracea L.) is an annual or biennial herbaceous plant. It is a popular vegetable in China and 90% of which was dried by hot air. It was difficult to kill the micro-organisms, so there were excessive micro-organisms in dried vegetables. In this paper, the changes of micro-organisms, moisture and nutrients of the vegetables were investigated throught the whole drying procedures. The main results were as follows:1. In experiments of sodium hypochlorite, ultrasonic and ozone for spinach sterilization, results showed that the optimal conditions for sodium hypochlorite were at a concentration of 75 mg/L, pH of 5.5, and soaking time of 15min. The optimal conditions for ultrasonic were at a power of 200 W, water temperature of 40℃, time of 20min. The effect of ozone directly treated spinach was better than that immersed spinach into the ozone water. In comparison among sodium hypochlorite, ultrasonic and ozone, ultrasound treatment had the best effcts on spinach sterilization.2. During drying process, dehydration rate of spinach increased with the increase of hot-air temperature and microwave power. Three semi-theoretical thin layer drying models were used to fit the dehydration rate. Page model gave a better fit for spinach dehydration conditions applied. Spinach samples were rehydrated by immersed in distilled water bath at controlled temperatures of 25,45, and 80℃, respectively. The rehydration ratio increased with the increase of water temperature. Hot air-microwave dehydration not only reduced the drying time, but also improved the sensory quality and nutrients retention rate. The total numberof bacterial can be decreased to safe range treated with hot air-microwave dehydration. The optimal conditions for hot air-microwave drying were at a temperature of 50℃, a conversion rate of water 60%, and a microwave power of 540 W.3. Amount of total bacteria was influenced by microwave power and time, decreased significantly as microwave power and microwave time increased. However, high power and long-time processing will destroy the texture structure of spinach and lead to partial burned taste. The optimimal conditions of steam-microwave treatment, namely steam time, microwave power and microwave time, were studied using response surface methodology with three variables and two response functions. Analysis of variance indicated that Log (No/Nt) was significantly influenced by steam time, microwave power and microwave time. There were significant interactions between steam time and microwave power, steam time and microwave time, microwave power and microwave time. The optimal conditions for Log (No/Nt) were at a steam time of 6 min, a microwave power of 540 W and a microwave time of 111 s during steam-microwave treatment. The maximum of Log (No/Nt) was 3.07 Log cfu/g in the optimized condition.