Modeling fruit internal heating rates for hot air and hot water treatments

Abstract

Hot air and hot water heating methods have been extensively studied as thermal treatments to control insect pests in fruits to replace chemical fumigation. An inherent difficulty in using these methods is that slow heating rates may result in long treatment times and possible damage to fruit quality. Many factors influence heating time. A systematic analysis of those influences is desirable to help in designing effective treatment protocols. A simulation model based on heat transfer theory was developed to study the effect of fruit thermal property, fruit size, heating medium and heating medium speed on heat transfer rates within spherical fruits. The simulation demonstrated that the small variation in thermal diffusivity among fruits had little effect on heating time. Fruit internal heat transfer rate was significantly influenced by fruit size and by heating medium. Water was a more efficient medium than air and increasing air speed increased heating rates. Water circulation speeds had little influence on heat transfer rate. The Biot number showed that internal energy transfer by conduction was a heating rate limiting factor. Combining low frequency electromagnetic energy with hot air or hot water eliminated conduction as a major rate-limiting factor because the energy was directly delivered to the fruit interior.