Study On The Change Of Internal Temperature Field Of Cold Sensitive Fruit During Heat Shock Treatment And Its Effect On Cold Resistance

People’s daily lives are inseparable from the vitamins,minerals and other nutrients provided by fruits and vegetables.Some of the fruits and vegetables are extremely sensitive to changes in temperature,which we call cold-sensitive fruit and vegetables.Cold-sensitive fruit is generally sent to a low-temperature cold storage for storage,thereby avoiding decay and insect damage and prolonging the commodity period.However,when cold-sensitive fruits and vegetables are stored at a temperature lower than a certain temperature,cold damage will occur,and depressions,brown spots and the like will occur.As a non-toxic,harmless and non-polluting post-harvest pretreatment method,heat shock treatment can enhance the stress resistance of cold sensitive fruits and vegetables,improve the toughness of cell membrane and the activity of antioxidant enzymes,thereby increasing the self-repairing ability during low temperature storage.Most of the current research on the selection of heat shock conditions is often obtained through previous experience,and there is no correlation between the heat transfer properties of fruits and vegetables during heat shock treatment.During the heat shock process,the internal temperature field of fruits and vegetables is a dynamic process.In this paper,the treatment of cucumber fruit with hot water was carried out.The variation of the internal temperature field of the fruit during heat shock was simulated by establishing a numerical model of heat transfer.The relationship between the temperature of the hot water treated fruit and its cold resistance at different temperatures was studied and verified.The main research contents and results are as follows:(1)A numerical model for heat treatment of cylindrical and spherical fruit hot water was established.The maximum relative error MD of the cylindrical model was 1.35 %,the root mean square error RMSE was 1.62 %,and the maximum temperature difference was 1.1 °C.The average of the spherical model The relative error MD is 1.47%,the root mean square error RMSE is 2.42%,and the maximum temperature difference is 2.3 °C.The model is reasonably usable and has the effect of predicting changes in the temperature field of the fruit.(2)By studying the changes of internal temperature field and heating rate of cucumber fruit and apple fruit under different heat shock conditions,the variation of temperature field inside the fruit during heat treatment was discovered.The results show that when the heat shock temperature is increased,the heat transfer rate of the fruit is greatly increased,but the time to achieve temperature uniformization is basically the same;when the medium flow rate is increased,the overall heating rate is increased,and the peak temperature rise rate is advanced,which is beneficial to The heat shock efficiency is improved;the adaptability of the fruits of different radii to the heat shock temperature varies,and the appropriate heat shock temperature and heat shock time should be selected according to different radii to achieve the optimal heat shock effect.(3)The comprehensive cold resistance of cucumber fruit at different center temperatures was studied for three heat shock temperatures of 40 °C,45 °C and 50 °C.The physiological and biochemical status of fruits at different central temperatures were analyzed by measuring malondialdehyde content,peroxidase activity,superoxide dismutase activity,electrolyte permeability,weight loss rate and fruit firmness under different storage days.Through the membership function method and dimensionality reduction analysis method,the six indicators were combined into 2-3 comprehensive indicators with a contribution rate greater than 80% to evaluate the comprehensive cold resistance of cucumber fruits under different storage days.The results showed that: compared with the non-heat treated control group,hot water treatment can reduce cell membrane lipidation,increase enzyme activity and fruit firmness,reduce electrolyte permeability,weight loss rate and chilling damage during storage,thereby improving cucumber fruit.Cold resistance at low temperature storage.Among them,the content of malondialdehyde decreased in the early stage of storage,which may be due to the fact that the fruit in the early stage of storage did not have serious cold damage.The cells themselves had strong self-repairing ability,and the peroxidase activity was discovered by experiments.The activity is greatly increased,and sometimes malondialdehyde content can be converted into a normal fatty acid.The peak of antioxidant enzyme activity appeared within 6-9d,indicating that the chilling injury occurred at this time,and the antioxidant enzyme activity was not weakened,which could generate a large amount of neutralizing reactive oxygen free radicals and maintain the normal operation of the cells.It has been found that the optimum center temperature is 35 °C when the heat shock temperature is 40 °C,the optimum center temperature is 30 °C when the heat shock temperature is 45 °C,and the optimum center temperature is 25 when the heat shock temperature is 50 °C.So preliminary conclusion: the optimum heat shock interval of cucumber heat treatment is 25-35 °C,and the temperature of the optimal heat shock center shifts to the low temperature zone with the increase of heat shock temperature.By studying the changes of various indexes at the same heat temperature at different heat shock temperatures,it was found that 45 °C hot water treatment can induce the cold resistance of fruits and improve the stress resistance of fruits in the high incidence of cold damage compared with 40 °C and 50 °C.And short-term high temperature stress treatment can stimulate the cold resistance and antioxidant enzyme activity of fruit cell membrane.(4)By studying the changes of weight loss rate and chilling injury index of cucumber fruits under different heat shock temperatures,it was found that the trend of chilling injury index and fruit weight loss rate were basically the same.In the low temperature region,the weight loss rate and the chilling injury index of the fruit were lower in the central temperature range of 30-35 °C.In the high temperature region,the weight loss rate and chilling injury index of the fruit in the central temperature range of 25-30 °C were lower.It can be verified that as the heat shock temperature increases,the optimum heat shock center temperature shifts to the low temperature region.By simulating the changes of average temperature and average heating rate of cucumber fruits under different heat shock temperatures,it was found that the optimum average temperature range during heat shock was 37-39 °C,and shifted to the high temperature range as the heat shock temperature increased.The optimum average heating rate range is 0.5-2.0 °C/min,and shifts to the high heating rate region as the heat shock temperature rises.