Effect of fixed bed drying on the retention of phenolic compounds, anthocyanins and antioxidant activity of roselle (Hibiscus sabdariffa L.)

In order to evaluate the use of lixiviated roselle calyxes as an alternative source of bioactive compounds, they were subjected to dehydration in a fixed bed dryer, followed by the evaluation of some chemical parameters such as the retention of monomeric anthocyanins, phenolic compounds, polymeric color and antioxidant activity. The experimental variables were temperature (40, 60 and 80°C), bed ratio (1, 2 and 3), and superficial air velocity (0.75, 1.5 and 2.25m/s). Drying curves showed both a constant drying period and a decreasing falling rate period. To determine kinetic parameters, the drying data were fitted to a first order chemical model, based on the moisture content ratio versus the drying time. The k value ranged from 9.0×10 ^-4 to 2.7×10 ^-3s ^-1. The different drying conditions showed significant differences in the monomeric anthocyanin content (188.13±6.78-230.23±5.30mg D3S/100g residue d.b.), the total phenol content (435.03±16.8-637.65±12.8mg GAE/100g residue d.b.) and the antioxidant activity (181.13±2.27-288.44± 18.22μmol TE/g of dry extract) (p<0.05). Our results confirmed that lixiviated roselle calyxes can compete with other traditional products, such as grapes or bilberries, in terms of their antioxidant activity and their low cost associated with waste materials.