EEFECTS OF PROCESSING ON CHEMICAL COMPOSITION, FUNCTIONAL AND ANTI-NUTRITIONAL PROPERTIES OF RED SORREL (Hibiscus sabdariffa) SEED FLOUR

Abstract

The study investigated the effects of processing on chemical composition, functional and anti-nutritional properties of red sorrel (Hibiscus sabdariffa L.) seed with the view to see if sorrel seed flour could be useful as a cheap source of protein substitute in dietary supplements and in the prevention of PEM.Mature sorrel seeds were processed to obtain raw sorrel seed flour (control), dehulled sorrel seed flour, boiled sorrel seed flour, roasted sorrel seed flour and sprouted sorrel seed flour. The flours were subjected to physicochemical, anti-nutritional and antioxidant analyses to assess the impact of processing. Results of proximate analysis showed that protein content and total ash increased from 24.93% to a range of 27.65 – 29.71% and from 4.36% to a range of 5.16 - 5.41% respectively in processed seed flours. Boiling significantly (p≤0.05) increased crude fat than other processing methods. Processing resulted in significant (p≤0.05) increase in sodium, calcium, potassium, magnesium and manganese in processed flours; however no significant (p≤0.05) increase was observed in zinc while roasting and sprouting resulted in reduction of iron. Processing significantly(p≤0.05) increased the level of some essential amino acids including: leucine, lysine, threonine, phenylalanine, histidine and arginine; sulphur-containing amino acids (methionine and cystine) were however found to be limiting in both raw and processed flours. Processing did not have any significant (p≤0.05) effects on myristic acid while palmitic acid was increased by roasting from 18.29% to 18.66%. The flours were found to contain abundant unsaturated fatty acids with oleic acid ranging from 22.02 - 23.26% and linoleic acid ranging from 49.86 - 50.24%. All processing methods reduced anti-nutrients; phytate was significantly (p≤0.05) reduced from 9.12mg/g to a range of 3.93 – 7.89 mg/g in the processed sorrel seed flours and tannin was reduced from 6.04 mg/g to a range of 3.28 – 4.82 mg/g. The processed flours have significantly (p≤0.05) higher total phenolic content (7.70 – 10.24mg/g) than the control flour (7.36mg/g). However, there was reduction in total flavonoid content in the processed flours ranging from 0.33 – 0.57mg/g as compared to the control flour which had a total flavonoid content of 0.96mg/g. DPPH radical scavenging abilities of the dehulled flour was significantly higher (p≤0.05) than other samples but there was no significant (p≤0.05) difference between DPPH level in the control and boiled seed flours. Ferric Reducing Anti-oxidant Power (FRAP) was significantly (p≤0.05) reduced by all processing methods. There was reduction in Vitamin C content in all processed flours. It significantly (p≤0.05) decreased from 2.69mg/100g in raw flour to a range of 1.66 – 2.37 mg/100g in the processed flours. Water absorption and oil absorption capacities were significantly (p≤0.05) increased by all processing methods, however there was reduction in foaming and emulsifying capacities in processed flours. Swelling index was significantly (p≤0.05) reduced by all processing methods, except dehulling which increased it from 0.74 – 0.93. Results of protein solubility show that the trend for protein solubility of sorrel seed flours irrespective of the processing methods was inconsistent. In vitro protein digestibility decreased from 7.52 mg/g to a range of 1.06 – 5.87 mg/g in processed flours. In vitro starch digestibility was however increased from 276.44 mg/g in the control to a range of 343.65 – 565.89 mg/g in processed flours. The study concluded that the processing methods improved the nutritional qualities and reduced the anti-nutrients of sorrel seed flours.