Evaluation of Quality Change in the Far-infrared Drying of Komatsuna Leaves Using Cumulative Temperature as an Indicator

Abstract

　In this study, we determined the quality changes (surface color, and l-ascorbic acid, carotenoid, and chlorophyll contents) and drying rate during far-infrared drying and hot-air drying of Komatsuna leaves. Three different drying temperatures ranging from 44℃ to 55℃ were employed in both methods. Cumulative temperature was used as an indicator of quality change. The measured moisture content changes under each set of drying conditions agreed with those calculated using an exponential model. An Arrhenius-type equation was used to establish the relationship between the drying rate constant k and the average surface temperature during drying. Under the ideal drying conditions, the cumulative temperature at the sample surface served as a useful index of the decrease in the l-ascorbic acid content. In addition, the surface color of the Komatsuna leaves changed to a lesser extent as the cumulative surface temperature decreased. Etiolation of the sample during drying was caused by the decrease in the chlorophyll rather than the decrease in the carotenoid content.