The variable nature of Biot numbers in food drying

Biot numbers (Bi) relate internal and external resistances to mass or heat fluxes and are useful to identify controlling mechanisms. Their variation during drying received little attention despite its importance in model selection. Therefore, variable Bi were analysed in drying of wheat, a low-moisture product, and in the formation of a low-calorie, dehydrated apple-leather (LCAL), a high moisture product. Wheat drying was predicted with a numerical solution from a m.c. of 0.35 kg water/kg dry matter, to find a variable mass transfer Bi that shifted the controlling mechanism from internal–external to internal. From a lower m.c. of 0.205 kg water/kg dry matter, the entire process was internally controlled. Drying of shrinking LCAL was solved with the energy balance. The mass transfer Bi varied from 0.5 to 1600, while the heat transfer Bi, only from 0.25 to 0.5, and a modified version, from 0.25 to 0.