| Green pea is one of grain legumes cultivated and consumed worldwide for its rich nutritional value.Green peas are highly seasonal crops with a preference of mild humid climate and no tolerance to hot and drought.After being harvested,fresh peas are still strong in respiration and are easily contaminated by mold.For long term preservation,green peas are often frozen or dried after blanching.Blanching was an important pretreatment in fruit and vegetable processing to inactivate the endogenous enzymes that result in the deterioration of the quality of plants.Radio frequency(RF)heating is a penetrative electromagnetic heating technology with advantages of rapid heating,large penetration depth,and high heating efficiency which has been used in heating,drying and sterilization.Compared with traditional water blanching,the application of RF in blanching could save water and energy consumption,and improve production efficiency.However,few researches have been done on RF blanching,and the discipline of RF blanching need to be further revealed.In this paper,effects of RF blanching of fresh green peas were studied.The changes of dielectric properties of peas with frequency and temperature were determined.To improve heating uniformity,a rotation system was introduced and the effects of rotation rate,electrode gap and loading weight on heating efficiency and uniformity were discussed.The effects of RF heating on lipoxygenase and peroxidase,physiochemical properties and microstructure of peas were studied.The main conclusions are as follows:(1)The dielectric properties of pea were measured by an impedance analyzer system with an open-ended coaxial probe.Both dielectric constant(?')and the dielectric loss factor(?')of peas decreased with increasing frequency.At 13.56,27.12,40.68,915,2450 MHz,the pea dielectric constant increased with increasing temperature between 25 and 55 °C,and decreased beyond 55°C.Dielectric loss factor increased with increasing temperature between 25 and 75 °C,while decreased after 75 °C.(2)For rotary assisted RF heating,improvement of rotation rate within 0-60 rpm could accelerate heating rate of green peas treated by RF.From 0 to 15 rpm,the heating uniformity of peas was significantly improved(P < 0.05),while no significant improvement(P > 0.05)existed in heating uniformity as rotation rate was higher than 15 rpm.Narrowing the distance of electrode gap(100-120 mm)increased the heating rate.However,electrode gap exerted few effects on heating uniformity.Within 40-100 g,increasing the loading weight could increase the heating rate of peas.But excessive loading weight(120 g)had a significant deterioration effect(P < 0.05)on RF heating uniformity of peas which caused a decrease in heating rate.(3)When pea was processed at electrode gaps of 105,110,115 mm respectively,the activities of lipoxygenase and peroxidase decreased with increasing temperature while these two enzymes showed different temperature sensitivity.During RF heating,the weight loss,color,texture,electrolyte leakage,and ascorbic acid content of peas significantly changed(P < 0.05).Chlorophyll and mineral contents of peas lost with different levels and cell structure of pea cotyledon was destroyed with disintegrated cell wall and gelatinized starch granules.(4)Edible qualities of quick freezing pea samples by RF blanching or hot water blanching in frozen storage for 120-240 d were well maintained.There was no significant change in the color,texture and sensory score of two blanched pea samples. |
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