Finite element analysis on fruit temperature fields based on postharvest disinfestations with hot air and water treatments

Abstract: Hot air and hot water heating have been extensively studied as effective physical treatment methods to replace chemical fumigation for controlling insect pests in fruits and vegetables because of environmental benefit and easy control. However, it is lack of systematic research on heating rate, heating time and temperature distribution, the insect mortality requirement is not met or fruit quality is negatively affected. To study the heat transfer mechanism of postharvest fruit heat treatment process, a computer simulation model was developed to analyze the heating rate and the transient temperature distribution in thermally treated fruits using finite element-based commercial software, COMSOL, based on the unsteady heat transfer. a set of differential equations that govern heat transfer in fruits was reduced into a group of algebra equations in the simulation model. The measured surface and center temperatures of fruits during hot air and water heating were compared against to the results obtained from the finite element simulation. The root mean square error between simulated and measured temperatures was all below 8%. The comparison analysis showed that the simulation results were in good agreement with the measured values, which indicates the reliability of the simulation model. With the validated simulation model, the impacts of various parameters on heating rates were systematically studied using forced hot air and water treatments. To reach the same thermal effect, hot water (55°C) heating time was about 30% in hot air (55°C) treatments. Water was more efficient medium than air. Increasing air speed increased heating rates, but water circulation speeds had little impact on heat transfer rate. The most important parameters in the model included the fruit size, fruit shape and the heating medium followed by the heating medium speed and thermal diffusivity. This study demonstrated that the computer simulation model can be used to evaluate the impacts of various heating parameters on the temperature-time history in fruits. These parameters include fruit size, fruit shape, heating medium speed, and thermal diffusivity. Combining the hot air or hot water treatment with the fast heating method, e.g. radio frequency or microwave energy, further improves the fruit heating efficiency since electromagnetic energy may eliminate conduction as a major rate-limiting factor directly delivered to the fruit interior. Furthermore, when combined with insect mortality and quality kinetic information, the model can be used to support for the heat treatment process and optimize the process parameters in postharvest thermal disinfestations.