Combined medium- and short-wave infrared and hot air impingement drying of sponge gourd (Luffa cylindrical) slices

Influences of drying air temperature, radiation distance, slice thickness and wavelength on drying time, energy consumption, and product quality (color, shrinkage, rehydration and total saponin) were investigated in combined infrared and hot air impingement drying (IR-HAD). Results illustrated that temperature and slice thickness were the most distinct factors affecting the drying time. Moisture effective diffusivity decreased with decrease in moisture content, and was influenced by drying conditions. Radiation distance significantly affected rehydration capability, while temperature and thickness affected almost all quality attributes. Drying caused an insignificant reduction in total saponin, and similar volume shrinkage results were found due to constant amount of water removed. Thickness of 6 mm maintained better quality than thicker bodies, coincidence with the calculated infrared penetration depth (5.30-7.25 mm). IR-HAD at wavelength of 2-4 mu m increased drying rate slightly than that at 0.25-2 mu m, while insignificant difference in product quality. Particularly, IR-HAD samples showed smaller thickness shrinkage and larger area shrinkage, as opposed to hot air drying. IR-HAD offered many advantages in minimizing the drying time and better preserving the quality over hot air drying under the same conditions, but markedly enhanced the specific energy consumption. This study proves that IR-HAD is a suitable technique that retains quality characteristics of vegetables.