Effects of different time periods in intermittent heat treatment on the temperature field and storage quality of cherries

Abstract: Temperature can trigger the internal physiological changes of fruit during heat treatment. It is of great significance to explore the fruit tissue temperature and the gradient at different time for better heat treatment preservation. Taking the cherry as the samples, this study aims to determine the effects of different time in the intermittent heat treatment on the temperature field and storage quality in the fruits. The alternately spraying was adopted by the 50℃ and 20℃ water during treatment. At the same time, three types of time of 20, 100, and 180 s were selected as the treatment, named I1, I2, and I3 processing groups, respectively. The storage quality of the fruit was taken as the index. A non-stationary heat transfer model was developed for the individual fruits, and then to numerically analyze the heat transfer and dynamic response of cherry tissue during intermittent heat treatment. The heat transfer model was better agreed with the experimental data, with a root-mean-square error of 0.316 ℃. The results showed that the heat treatment with intermittent spraying improved the storage quality of fruits. There were significant differences in the effect of different spraying times on the temperature field and quality of cherry fruit. The optimized parameters were achieved, where each and total treatment time were 180 and 540 s, respectively, with the spraying treatment at 50 ℃ for 3 times. A better heat transfer between the cherries was obtained in the treatment medium under the optimal condition, with a mean temperature of 27.71 ℃ at the individual fruit r/2, and 30.85 ℃ at the center. The decay rate of fruits in the control, 20 s, 100 s, and the optimal group were 12.99%, 10.49%, 6.51%, and 5.66%, respectively. Therefore, the intermittent heat treatment maintained the fruit firmness to reduce the firmness of stored fruits. Specifically, the firmness of fruits was the lowest, when the treatment time was 100 s. The weight loss rate of cherry fruit was reduced during storage, but there was no outstandingly different variation in the soluble solid content between the heat treatment and the control group. The heat transfer analysis showed that the temperature curve of heat treatment group I1 increased stepwise. The temperature curves of groups I2 and I3 showed a wave shape, where the temperature dropped in the cooling stage, leading to relatively less influence by the temperature lag. The distributions of fruit internal temperature were all the same under the treatment of I1 and I2, where the temperature at r/2-r/4 was the highest, and the surface temperature of the fruit was the lowest. Therefore, too short a heat treatment resulted in heat accumulation in the middle of the fruit, where the heat failed to transfer to the inside of the fruit. There was a great impact on the biological time delay in this case. Relatively longer treatment time was allowed for the deeper heat transfer and much more uniform temperature field within the cherry fruit, particularly for the better maintenance of fruit and vegetable quality. The heat transfer analysis and storage quality indicators were combined to determine the extended treatment time for the better thermal properties of fruits. In addition, the peak value of the temperature change rate decreased with the increase in treatment time, and the final temperature at r/2 gradually increased. After the third heat treatment, the final temperature of different treatment groups increased by 7.5%, 1.8%, and 0.2% respectively. Furthermore, there was a gradually decreased influence of intermittent time on the final temperature inside the fruit, as the treatment time increased. Heat transfer models can be used to predict the change rate of temperature for the different fruits and vegetables, in order to determine the intermittent heat treatment time. The findings can provide a strong reference for the intermittent heat treatment of agricultural products.