Effect Of Ohmic Heating On Apple Tissue Structure And Activity Of Polyphenol Oxidase

Apple is very popular because of its beautiful flavor and high nutritional value.The output of apples in Shaanxi is very large.One part is used for direct food,the other is used for deep processing to increase the added value of agricultural products.Fruit juice,canned,dried apples,apple cider vinegar,apple wine are the main products of apple deep processing.However,the quality of products has been seriously affected by the browning caused by polyphenol oxidase.In order to solve the browning problem using pretreatment method of ohmic heating,Shaanxi red apple Fuji as the research object,to study the effects of ohmic heating conditions on the residual activity of polyphenol oxidase,and analyze their interactions with the response surface method,determined the optimum conditions of ohmic heating;according to the ohmic heating after Apple the size of the conductivity changes of Apple block internal organizational structure are described,and using scanning electron microscopy to observe the microstructure changes of Apple block ohmic heating,ohmic heating electric heating to explore through the coordination mechanism;finally,effect of ohmic heating on the quality of apple juice.The main conclusions are as follows:(1)In order to investigate the effect of ohmic heating conditions on the residual activity of polyphenol oxidase,polyphenol oxidase activity was detected under different heating conditions,It was found that the residual activity of polyphenol oxidase decreased with the increase of heating temperature and electric field strength.Holding time had no obvious effect on it.The higher the heating frequency,the higher the residual activity of polyphenol oxidase,the worse the effect of the enzyme.Using response surface methodology(RSM)design of four factor and two level orthogonal regression rotation center combination test of five levels,established two multinomial mathematical model for the relationship between the heating temperature,holding time and electric field intensity,frequency and the frequency and the residual activity of the polyphenol oxidase after ohmic heating of the apple,determine the factors of residual effect of apple polyphenol oxidase activity in the order of temperature,intensity of electric field intensity,frequency and time.Activity of residual interaction on apple polyphenol oxidase influence order for temperature and frequency,electric field intensity and frequency.It is concluded that the best ohmic heating condition is 70?,the electric field intensity is 62V/cm,the holding time is 92 s,the heating frequency is 210 Hz.(2)According to the size of electrical conductivity and the microstructure by scanning electron microscope(SEM)of Apple block after ohmic heating,the change of the internal structure of Apple block was described.The results show that the ohmic heating rate of the apple block is positively correlated with the electric field strength,and is negatively correlated with the frequency of the electric field,which is much higher than that of the water bath heating;With the change of electrical conductivity with temperature,the changes of the structure of Apple tissue with temperature can be divided into two stages:the temperature is less than 60?,apple structure changes slowly,cell structure appeared slight damage;when the temperature is higher than 60? to 65?,apple structure is destroyed sharply,cell rupture,the conductivity increases rapidly.The electrical conductivity of Apple block increases linearly with the increase of electric field intensity above 45V/cm.That is,under the double action of electricity and heat,the apple tissue cells were subjected to continuous irreversible destruction,and the temperature increased to make the electrical damage more significant.The damage degree of Apple sample was positively correlated with the electric field intensity,which was negatively correlated with the electrical frequency,which showed that the electrical conductivity increased linearly with the electric field intensity,and decreased with the increase of the power frequency.When the electric field intensity was greater than 75V/cm,the cell wall and cell membrane were damaged severely,and the cells were torn.It is speculated that the cells of Apple cells have been found to accelerate the cell rupture.The damage degree of Apple tissue is expressed by the Z of electrical conductivity structure,and the Z value of Apple tissue is in accord with the first order reaction model;The relative Zsat of damage rate is positively correlated with temperature,and the initial failure time t0 is negatively correlated with temperature;The relationship between the reaction rate K and temperature is in accordance with the Arrhenius formula,and the activation energy is 122.88 kJ/mol.The activation energy of the Arrhenius curve of characteristic damage time and temperature was 157.01 kJ/mol,which was found to be more sensitive to temperature than the tissue of the peach tissue and the sugarcane tissue.(3)The mass loss rate,hardness,color,sugar content and pH were detected in different conditions,in order to investigate the effect of ohmic heating on the physical and chemical properties.The results show that when the temperature reaches 65?,the cell structure is destroyed seriously,the expansion pressure is lost,and the liquid flow out of the cell,the mass loss rate increases and the hardness decreases greatly;Ohmic heating decreased the activity of polyphenol oxidase in Apple block,inhibited browning,and changed its color;The sugar content of fruit juice,pH almost no effect;Ohmic heating is more conducive to maintaining the nutritional content of apple,contribute to the processing of Apple products,such as processing,so great development prospects.