Effect Of Combined Radio Frequency With Oven Baking Process And Its Drying Characteristics Of Sweet Potatoes

Sweet potatoes are rich in carbohydrates,provide about one-third of the energy of rice or wheat of the same weight,and are also rich in nutrients,and China is the first producer of sweet potatoes in the world.For sweet potatoes,which are food crops with extremely high moisture content,strong pest attraction,and relatively fragile,storage is more troublesome,and processing can effectively reduce the loss of sweet potatoes.Due to the change in consumption concept,the proportion of sweet potatoes in China has gradually increased in recent years,about 65% of sweet potatoes are ripened and then processed for consumption,or deeply processed,dried sweet potatoes that are cooked and dried without additives are popular with consumers.And different cooking methods have a greater impact on the quality of sweet potatoes.The new heating method has gradually become an alternative to the traditional method,and radio frequency(RF)heating stands out in the new heating technology with its ideal penetration depth and heating uniformity.Traditional oven baking is a common method of processing sweet potatoes.Since single oven baking has a certain adverse effect on product quality,the combination of RF heating and oven baking(RF-O)was explored as a new sweet potato baking method,and the feasibility of RF-O baking as a cooking method before drying preserved sweet potato was explored.The main contents and achievements of the thesis are as follows:(1)Comparing the effects of individual oven baking and RF-O on the baking time and product quality of cylindrical sweet potatoes.For cylindrical samples,the effects of sweet potato diameter and electrode gap on the RF heating rate and heating uniformity were also explored before comparing the baking characteristics of individual oven baking and RF-O methods.The results showed that the RF heating uniformity was good and the heating time was acceptable when the electrode gap was 90 mm and the sample diameter was 55 mm.The total baking time for baking in the individual oven was 35.9 min,while the total baking time for the combined baking method was significantly reduced to 25.1 min.At the same time,sweet potatoes baked in RF-O had better color or texture values,lower weight loss,and lower acrylamide content than samples baked in the oven alone.Therefore,RF-O can be recommended as an effective,uniform,high-quality baking method for sweet potatoes.(2)Based on the specific RF-O process parameters explored in the first part,the influence of RF-O as a cooking pretreatment method for preserved sweet potatoes on the traditional hot air drying of sweet potatoes was also explored.Samples after combined baking were tested with hot air drying at 55,65,and 75 °C.Moreover,the model fitting of hot air drying was carried out on the dried samples as a whole,and it was found that the samples fitted well with each model,and all of them followed the slow drying law of traditional hot air.When the quality study of dried sweet potatoes at different hot air temperatures was conducted,the experimental phenomenon that the higher the temperature,the shorter the drying time was found.After combined baking,sweet potatoes dried at different hot air temperatures had similar quality within the same layer.This shows that preserved fruit can attract consumers through the color change brought by the Maillard reaction.To ensure a relatively ideal temperature in the RF cavity when hot air-assisted RF drying and RF heating combined with hot air drying(two-stage drying),65°C was used as the temperature of the hot air in the RF cavity in this study.(3)Based on the combined baking process,its effects on the hot air-assisted RF drying of preserved sweet potato and two-stage drying of the RF heating combined with the hot air process were studied,and the color and texture of the samples were studied after drying.The changes in characteristics and quality by hot air-assisted RF drying of sweet potatoes under the three electrode gaps were explored.The target moisture content of 0.5 kg/kg d.b.for drying the samples at electrode gaps of 90,95,and 100 mm required drying times of 160,180,and200 min,respectively.During the whole drying process,the bottom sample was dried faster because the hot air was transported from the bottom to the top and the electromagnetic field accumulates at the bottom.In the same layer,the color and texture value of dried sweet potato after drying under different RF electrode gaps were similar,so the electrode gap of 95 mm was selected for two-stage drying.The color and texture of the samples were similar after hot air drying,hot air assisted RF drying,and RF combined with hot air drying,but the texture values were significantly higher compared to the RF-O baked samples,which were acceptable to consumers.In summary,RF-O is a better sweet potato baking method than oven baking,and can also be used in the processing of preserved sweet potato fruit as a ripening treatment method before drying.