Development of porous structure during air drying of agricultural plant products

Abstract

The porous structure of dehydrated plant materials is a key parameter that affects the transport properties and a number of quality characteristics of these materials. The porous structure may be determined by the bulk porosity, pore sizes and pore size distribution parameters. Helium pycnometry and mercury porosimetry were used for porosity and pore 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 they were dried partially by air and partially by freeze drying, therefore a wide variety of porosities and pore size distributions were measured. It was found that the freeze drying resulted in a very high bulk porosity, while a gradual involvement of air drying resulted in a decreasing porosity. Two discrete peaks of the pore size distribution function were found for potatoes, cabbage and apples and three peaks for carrots. The three peaks for the above materials were found at around 20 μm, 1 μm and 0-2-0-04 μn, depending on the material. The 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 during dehydration. The surface areas of freeze dried samples were of the order of 1 m 2/g, which is typical value for dehydrated foodstuffs.