Vacuum drying of roselle ( Hibiscus subdariffa Linn.) calyces for retention of bioactive compounds and antioxidant activity of functional beverage blended with grapes and watermelon

Functional beverages cater to specific group of consumers who demand health-protective food items developed from natural ingredients without synthetic food additives. Roselle, coloured grapes and watermelon are rich sources of bioactive compounds. Roselle calyces were vacuum dried at 40 °C and − 30 mm Hg pressure for 22 hours to a dry matter content of 16%. Roselle-watermelon and roselle-grapes nectar was formulated by blending 10% aqueous extract of roselle with 10% juice of watermelon and grapes separately, in combination with 10% ginger extract. The formulated beverage after pasteurization, was stored at 5 ± 2 °C for 3 months. The product maintained its sensory properties even after 3 months of storage. The blend of roselle: grapes retained significantly higher ascorbic acid (14.11 mg100g−1), total phenolics (22.50 mg100g−1) and anthocyanins (9.16 mg100g−1) while lycopene (4.479 mg100g−1) was detected only in roselle: watermelon beverage. Antioxidant activity in terms of DPPH assay (IC 50 value) was also greater in roselle: grapes (6.48 μgmL−1) as compared to roselle: watermelon blend (14.81 μgmL−1). Correlation analysis of bioactive compounds and antioxidant activity revealed strong positive correlation between total phenolics and anthocyanins throughout the storage with an ‘r’ value ranging from 0.999 to 1.0, significant at 0.001 level.