Thermophysics Research on Postharvest Spherical Fruit during Heat Treatment
|School||Shanghai Ocean University,|
|Course||Refrigeration and Cryogenic Engineering|
|Keywords||spherical fruit heat treatment heat transfer rate biological parameters environmental parameters|
As a large agricultural country, China has the largest of fruit and vegetable cultivation area and the most of production. However, the losses caused by corruption are very serious. Recently, heat treatment before storage, as one of the preservation methods, using hot water or hot air, has been investigated extensively for fruit disinfestation but without chemical residues.Varying degrees of efficacy have been reported both from home and abroad. After themal treatment, fruit could maintain a high hardness and bright colour, improve the flavor, inhibit ethylene release rate and respiratory rate during fruit storage, reduce some enzymes activity, remove the reactive oxygen from the cells effectively, delay ripening and senescence, improve resistance, inhibit some physiological diseases, prevent or reduce the chilling injury, reduce the decay index significantly, prevent rot, extend the storage life effectively.However, a common difficulty with hot air or water heating treatment is the slow rate of heat transfer resulting in hours of treatment time.External and internal damage caused by heat over long exposure times including peel browning, poor color development, pitting and abnormal softening. There is a need to fully understand the influence of various factors on heat transfer in fruit to minimize adverse effects on fruit quality.In recent years, with the further development of temperature measurement and the improvement of the accuracy of measuring instruments, the thermal properties and the internal temperature of fruit can be tested more accurately and faster, which can increase the accuracy both of experimental research and theoretical model for spherical fruit thermal treatment. The fluent software, which has been successfully applicated in the food industry, makes a model of higher accuracy and stability, but save time.To improve the thermal treatment efficiency, it should be investigated deeply that what to effect convection between the environment and spherical fruit and conduction of internal spherical fruit and how they work. This paper tested biological and environmental parameters of thermal treating spherical fruit, and made experiments and modelled, to research fundamentally how the parameters effects the heat transfer rate, in order to provide reliable theory and basic data.Fifteen kinds of fruits and vegetables had been measured for their thermal conductivity, density and specific heat capacity, in order to calculate thermal diffusivity. The experimental results demonstrated that there is small variation in thermal diffusivity among fruits, distributed in about 4×10-8 m2/s. Using apple fruit and tomato as experimental subjects, spherical fruit surface heat transfer coefficients were determinated experimentally. The results demonstrated that: spherical fruit surface heat transfer coefficient increased as the air velocity increased, decreased as radial dimension of spherical fruit increased. As biology, spherical fruit transpiration enhanced its heat transfer effect significantly. Tomato’s moisture content is more than apple’s, and its transpiration and heat transfer effect are both more significant. Using the experimental correlation, surface heat transfer coefficients for apple fuits of from 0.040m to 0.080m in hot air under air circulating speeds of from 1m/s to 4m/s had been figured out. Using an empirical formula, surface heat transfer coefficients for apple fuits of from 0.040m to 0.080m in hot water under air circulating speeds of from 1m/s to 4m/s had been figured out as well. Figured out Reynolds numbers, fluid state can been ascertained as laminar flow.Based on heat transfer theory, experiments were taken to study the effect of fruit thermal property, fruit size, heating medium and heating medium speed on heat transfer rates within spherical spherical fruit. The experimental results demonstrated that the small variation in thermal diffusivity among fruits and vegetables had little effect on heating time. In general, the variations in thermal diffusivity among spherical fruit varieties and types may not influence heating time to cause any practical concern in real treatment systems. It is possible to use a substitute fruit of desired size for heating tests when the fruit is out of season and not available. Spherical fruit internal heat transfer rate was significantly influenced by heating medium and by spherical fruit size. Water was a more efficient medium than air and increasing air speed increased heating rates. Attention should be paid to uniform air distribution to ensure uniform heating among spherical fruit in bins and chambers. Water circulation speeds had little influence on heat transfer rate. When using water as the heating medium, increasing water circulation speed will not practically increase the heating rate. In this case the only benefit of water circulation is to ensure temperature uniformity among the individual spherical fruit.According to the experimental results, spherical fruit internal heat transfer rate was significantly influenced by heating medium and by hot air speed rate and by spherical fruit size. Using Fluent soft, a simulation model was developed to further and refinedly study the effect of hot air speed rate and spherical fruit size. The simulation demonstrated that spherical fruit internal heat transfer rate was significantly influenced by air speed rate. With the increasing of air speed, the variation in thermal treatment time gradients are smaller and smaller. Spherical fruit internal heat transfer rate was significantly influenced by spherical fruit size. With the increasing of spherical fruit size, thermal treatment time gradients gradients are more and more significant.