Combined Radio Frequency With Hot Water Blanching For Sweet Potato And Its Effects On Hot Air Drying

China is the world's largest potato producer and its planting area and production rank first in the world.Drying is one of the main methods of postharvest processing of sweet potatoes,which can effectively extend the shelf life of sweet potatoes and reduce the decay loss.Fruits and vegetables usually need to be blanched before drying.The blanching treatment can inactivate the enzymes that cause the deterioration of the quality of the fruits and vegetables,such as peroxidase(POD),so as to reduce the loss of the product quality.Currently,the most commonly used blanching methods are hot water and steam blanching,which have disadvantages such as long time-consuming,poor quality,and heavy environmental pollution.As a new type of physical heating method,radio frequency(RF)technology has the advantages of fast and volumetric heating,and is often combined with traditional heating methods.It has potential advantages and application prospects in the blanching application of harvested vegetables.However,there is no research reported on the combination of RF and hot water heating for blanching sweet potato.Its enzyme-inactivating and color-protecting effects are still unclear.The effect of blanching on the drying characteristics and the quality of dried products in the subsequent drying process remains to be revealed.This thesis took sweet potatoes as the research object,used the 6 k W,27.12 MHz free-oscillation parallel plate RF heating system and developed combined RF with hot water blanching process;then studied the influence of combined RF with hot water blanching(RFB+HWB)on the subsequent hot air drying characteristics and the quality of dried sweet potato products,providing a theoretical basis for the development of the entire blanching-drying process in the fruit and vegetable industry.The main findings are as follows:(1)The best RF processing parameters were determined as the electrode gap of 90 mm,the sample thickness of 60 mm,and combined RF heating of 4.5 min and hot water heating(90?)of 6 min.When reaching the same target enzyme-inactivating level(<10%),the total time of RFB+HWB was 10.5 min,which was 30%shorter than that of hot water blanching(HWB).In addition,RFB+HWB could better protect the color and texture of the sample.(2)The drying rates decreased continuously as the moisture content decreased for all samples.The heating rate of the sample in the same treatment increased with the increasing hot air temperature.Both RFB+HWB and HWB reduced the heat and mass transfer efficiency of the sample.Under the same hot air temperature,the drying time for the HWB sample>that of the RFB+HWB sample>that of the unblanched sample.The Logarithmic model and Midilli model had a good fitting effect on the drying process of all samples,with R~2 of greater than0.99.(3)As an important pretreatment of sweet potatoes before drying,the blanching treatment could effectively avoid the obvious“surface whitening”phenomenon of unblanched samples after drying while ensuring the ready-to-eat properties of the dried products.After drying,the hardness and frangibility of the two blanched samples decreased while the cohesiveness increased,which was similar to the texture of dried sweet potatoes currently on the market.Compared with the HWB sample,the RFB+HWB sample had better color parameters after drying.The surface of the dried sample after the RFB+HWB was in golden yellow,while the HWB sample was in dark yellow and the color distribution was uneven after drying.After the unblanched sample was dried,obvious starch particles can still be observed,while the starch in both RFB+HWB and HWB samples were completely gelatinized and filled the cell wall.In summary,the RFB+HWB is an efficient and high-quality pretreatment method,which can effectively reduce the POD activity in sweet potatoes,so that dried sweet potato products have better quality parameters.