| China is the world’s largest potato producer and its planting area and production rank first in the world.At present,more than 95%of potatoes are mainly sold as vegetable,processing volume is low,and there are few fast-food and semi-finished products.Blanching is an important operation before processing fruits and vegetables.Blanching can passivate enzymes that cause deterioration of fruits and vegetables,such as peroxidase（POD）and polyphenol oxidase（PPO）;prevent color change and reduce odor;reduce microbial to extend shelf life;eliminate intercellular air to increase heat and mass transfer rate;soften tissue to obtain products with a specific texture.There exist short backs of blanching fruits and vegetables with hot water or steam such as slow heat conduction,large loss of soluble nutrients,huge water wastage and difficulty of sewerage management.Similar to microwave,radio frequency（RF）is a no ionizing radiation with larger penetration depth.Heating uniformity,cellular morphology,texture,color change and kinetics of enzyme inactivation are the key issues facing fruits and vegetables blanching using RF.There appeared little research on the discipline of RF blanching,which is of urgent need to be revealed.In this paper,potato was selected as the research object.The effects of radio frequency on blanching potato and its mechanism were researched.Firstly,the temperature-dependent and frequency-dependent dielectric properties of potato were studied,then to demonstrate the effect of electrode gap,sample height and electrical conductivity of NaCl solution on the heating profile and temperature distribution of potato cuboids subjected to RF treatment,to analyze the effect of RF heating on inhibiting of PPO of potato and cell morphology,finally to evaluate the effect of different blanching methods on the physical and chemical properties of potato cuboid.The main conclusions are as follows:（1）Both dielectric constant（ε’）and loss factor（ε’）values decreased with an increasing frequency.At a frequency of 27.12 and 40.68 MHz,ε’increased with an increasing temperature from 25 to 60°C,and then decreased.ε’decreased with an increasing temperature at 915 and 2450 MHz,respectively.However,ε’’increased obviously with an increasing temperature at 27.12 and 40.68 MHz,respectively.ε’’did not change much at 915and 2450 MHz,respectively.（2）The results revealed that the electrode gap,sample height and NaCl solution had significant effects（P<0.05）on the temperature distribution and heating uniformity of the sample after RF heating.The optimum RF heating uniformity was obtained at an electrode gap of 120 mm,a sample height of 60 mm and when immersed in NaCl solution of 0.5 s m–1.The central heating pattern was presented in a sample.Cold spots were located at the edge of the top surface of the sample.（3）The results showed that PPO relative activity decreased rapidly as temperature increasing at all electrode gaps.Under the same final temperature,potato cuboids showed the lowest relative activity of PPO at the electrode gap of 120 mm compared to 110 mm and 130mm.Different potato matrices had significant effects（P<0.05）on the PPO relative activity.The enzyme extract showed the lowest PPO relative activity,followed by the cuboids and puree.RF heating destroy the structure of potato tissue leading to the cell wall ruptured,the contents flowed out.（4）Both electrode gap and temperature had a significant effect on potato weight loss,color,texture,and electrolyte leakage（P<0.05）.Compared with RF heating,the weight loss of the potato cuboid was lower after hot water blanching,but the electrolyte leakage was higher subjected to hot water blanching.After hot water blanching,the change of color and texture of the sample were consistent with that blanched under RF heating.Both RF heating and hot water blanching had some influence on the starch granules in potato cuboids. |
