| Apple is the highest yield fruit in China.Apple contains Vc,polyphenols,soluble sugar,pectin,minerals and trace elements,which has the function of preventing cardiovascular disease and lowering blood pressure.In recent years,fruit and vegetable processing industry develops rapidly.Blanching is one of the main technologies in fruit and vegetable processing.The purpose of blanching is to make enzyme inactivation that produces adverse changes in catalytic reaction,prevent color deterioration,reduce the bad taste of some raw materials,accelerate the dehydration and infiltration of sugar and improve the quality of can products.However,traditional blanching methods such as hot water blanching and steam blanching have slow heating transfer rate,which are easy to cause the loss of nutrients and the changes of color and texture,and even consume a large amount of water resources,resulting in a large number of sewage.Therefore,it is very necessary and urgent to discover and try a new blanching method.Radio frequency(RF)blanching is a new way of being applied in fruits and vegetables,which has the advantages of higher heating speed,larger penetration depth and better uniformity than traditional methods that make better quality of products.In this paper,apple was selected as the research object,and RF heating was used for blanching treatment.The specific research contents were as follows:(1)Using the open-ended coaxial probe method studied how the dielectric properties of Fuji and Qinguan pulp changed with the frequency and temperature.The electromagnetic wave to the penetration depth of apple pulp was calculated.(2)Under the condition of RF treatment,the effects of different factors on the heating rate and uniformity were studied.The heating rates of apple pieces in the process of hot water,RF and hot water assisted RF treatment were compared.(3)The changes of the activity of polyphenol oxidase(PPO),the color and texture,and the soluble solids content(SSC)and polyphenol content of apple pieces were studied.The results were as follows:(1)Under the same temperature,the dielectric constant and loss factor of apple decreased with the frequency increasing.Under the same frequency,the dielectric constant of apple pulp was decreasing as the temperature was increasing,but the loss factor increased with the temperature rising.The variation of the dielectric constant and loss factor with the frequency and temperature of Fuji pulp was similar to that of Qinguan pulp.(2)Under the condition of RF treatment,the apple pieces of 2×2×5 cm and 5×0.8×5 cm(length × width × height)showed a central heating phenomenon.The heating rate at the center position of the height to the bottom surface of 2.5 cm was the fastest,the height to the bottom surface of 4 cm was the slowest and the height to the bottom surface of 1 cm was between the two.The heating rate of a single apple piece was much faster than that of more than one apple piece,which placed vertically in parallel showed a phenomenon that the apple piece near the edge had faster heating rate compared to the apple piece near the middle position.(3)Compared with hot water heating,the heating rate of the RF heating and the hot water assisted RF heating was much faster,while the hot water assisted RF heating was much faster than that of RF heating.(4)Under electrode gap of 120 mm,one apple piece and several apple pieces placed vertically in parallel were respectively heated to 85?,with the time being 90 s and 357 s.The relative enzyme activity of polyphenol oxidase was below 20%.The hardness was decreasing,while the temperature was increasing in the process of RF heating.Soluble solids and polyphenol contents decreased with the temperature increasing. |
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