Drying Kinetics, Quality Attributes and Moisture Sorption Isotherms During Storage of Roselle (Hibiscus sabdariffa) Dried under Solar Drying Conditions

D rying kinetics, quality attributes and moisture sorption isotherms of roselle were investigated using a mixed-mode forced convection solar dryer. The results showed that, the moisture content decreased continuously with the drying time. The drying rates were higher at the beginning, then decreased later with decreasing the moisture content and increasing the drying time. The quality attributes of solar dried roselle (at 40°C) were as good as that of the fresh one, all the dried samples showed optimum color values. The drying process caused a sharp decrease in the microbial load for all the dried roselle samples as compared to the fresh one. The values of pH, acidity, anthocyanins, total phenols and flavonoids were also decreased as a result of the drying process. The water vapor transmission rate (WVTR) of the packaging materials was also determined. The data showed an increase in the WVTR values along with increasing the relative humidity (RH) levels. The values were slightly high in the low density polyethylene (LDPE), whereas, there was no marked difference in the WVTR values of both high density polyethylene (HDPE) and laminated polyethylene/nylon (LPE/N) bags. The results dealing with the moisture sorption isotherms showed that, the equilibrium moisture content (EMC) values for all samples were slightly high in the whole calyces, followed by roselle powder. The EMC values for the samples increased with increasing the storage RH. However, an inverse relationship was observed between the storage RH and the time required to reach equilibration for all samples. The appropriate RH for storage of solar dried roselle should be below 43% using suitable packaging materials with low WVTR such as HDPE or LPE/N bags.