DEVELOPMENT OF POROUS STRUCTURE DURING AIR-DRYING OF AGRICULTURAL PLANT-PRODUCTS

The porous structure of dehydrated plant materials is a key parameter that affects the transport properties and a number of quality characte ristics of these materials. The porous structure may be determined by the bulk porosity, pore sizes and pore size distribution parameters. H elium pycnometry and mercury porosimetry were used for porosity and po re size distribution measurements, respectively. The porous structure was measured in four key plant materials: apple; potato, cabbage; and carrot The dehydrated materials were either air- or freeze-dried or th ey were dried partially by air and partially by freeze drying, therefo re a wide variety of porosities and pore size distributions were measu red. It was found that the freeze drying resulted in a very high bulk porosity, while a gradual involvement of air drying resulted in a decr easing porosity. Two discrete peaks of the pore size distribution func tion were found for potatoes, cabbage and apples and three peaks for c arrots. The three peaks for the above materials were found at around 2 0 mu m, 1 mu m and 0.2-0.04 mu m, depending on the material. Tile pore size of air dried materials was much smaller than the size of freeze dried samples, due to the collapse of structure in air dried samples d uring dehydration. The surface areas of freeze dried samples were of r ite order of I m(2)/g, which is typical value for dehydrated foodstuff s. Copyright (C) 1996 Elsevier Science Limited.