Studies on karkade (Hibiscus sabdariffa): protease inhibitors, phytate, in vitro protein digestibility and gossypol content

Karkade defatted flour extract had chymotrypsin inhibitor activity almost half that of trypsin (21.8 and 40.5 enzyme inhibitor activity units/mg protein, respectively). The protein isolate had a weak chymotrypsin inhibitor activity compared to that of tryprin (4.9–28.3 enzyme inhibitor activity units/mg protein, respectively). The trypsin inhibitor activity of the protein isolate was 70% of that of the defatted flour extracts. Extraction of trypsin inhibitor by citrate buffer (pH 4.6) gave a higher specific activity of trypsin than by phosphate buffer (pH 7.6), Tris buffer (pH 9.0) or distilled water. Heating the extract in boiling water for 10 min destroyed 66.1% of the trypsin inhibitor activity. Karkade protein isolate had lower free and total gossypol than did the whole seed and defatted flour. The in vitro protein digestibility of karkade defatted flour and protein isolate was lower than that of casein. Phytic acid was higher in karkade defatted flour than in soybean defatted flour.