NUTRIENT COMPOSITION, PHYTOCHEMICAL, ANTIOXIDANT AND FUNCTIONAL PROPERTIES OF FLOUR FROM ACHA (Digitaria exilis), TIGERNUT (Cyperus esculentus) AND SORREL SEED (Hibiscus sabdariffa) PROTEIN ISOLATE

Abstract

This study evaluated the nutritional composition, phytochemicals, antioxidants and functional properties of composite flour from blends of acha, tigernut and sorrel seed protein isolate which may serve as potential flour for production of low-glycemic baked foods and dough meal. Flour samples were prepared from acha (ACF) and tigernut (TNF) while protein was isolated from sorrel seed (SPI). The flour blends were formulated using Response Surface Methodology (RSM) design expert 8.3.0.1(trial version) to generate twelve samples. The best two samples were selected (based on their protein, fibre contents and functional properties) alongside the three raw materials (ACF, TNF, SPI) for analysis of their phytochemical, anti-nutrient, pasting and antioxidant properties. Results obtained showed that crude protein content ranged between 3.55% in TNF and 49.17% in SPI, with SPI being significantly higher than other samples (p<0.05). Bulk density, water absorption capacity, oil absorption capacity, emulsion capacity and pH values ranged between 0.54 g/cm 3 in SPI and 0.86 g/cm 3 in ACF; 1.37 g/ml in ACF and 2.47 g/ml in SPI; 1.00 g/ml in DSF and 2.77 g/ml in SPI; 48.89% in DSF and 56.83% in SPI; 5.13 in DSF and 7.00 in SPI respectively. Mineral contents were significantly different (p<0.05) between samples and values obtained ranged from 412.83 mg in SPI to 442.76 mg in ACF for magnesium; 8.02 mg in SPI and 8.49 mg in TNF for iron; 11.95 mg in FCT and 12.13 mg in TNF for zinc; 4.36 mg in ACF and 4.84 mg in FCT for copper; 0.83 mg in SPI and 0.90 mg in TNF for phosphorus; 177.84 mg in TNF and 197.06 mg in FCT for potassium; 43.29 mg in DSF and 44.91 mg in FCT for sodium; 2.16 mg in SPI and 2.19 mg in TNF for calcium/100g samples respectively. Total essential amino acids, were higher in the formulated flour blend samples (FCT- 36.40 and DSF- 34.03) mg/ 100 g of protein, as compared to other samples: acha (32.91 mg/100 g of protein); tiger nut (32.68 mg/100 g protein) and sorrel seed protein isolate (31.63 mg/100 g protein). Total poly-unsaturated fatty acid values of the flour samples ranged between 23.73% in DSF and 41.10% in SPI. Pasting properties of the samples were significantly different (p<0.05) and ranged from 32.0-1383 RUV, 24.0-1174 RUV, 8.0-209, 27.0-2587 cP, 3.0-1413 RUV for peak, trough, breakdown, final and ssetback viscosities, respectively. Furthermore, peak time and pasting temperature ranged from 5.07-5.80 min and 0-93.55 o C, respectively. Sample of SPI had significantly higher (p<0.05) values as compared to other samples for DPPH (88.03-93.81%), FRAP (40.64-120.16 mg/g) and OH (10.89-15.16 mg/g) (p<0.05), while ABTS and Fe 2+ chelation ranged from 33.21% in SPI to 34.21% in DSF and 2.65% in TNF to 5.68% in FCT, respectively. Significant difference was observed in the insoluble fibre content of the samples with values ranging from 56.61% in DSF to 83.98% in ACF (p≤0.05), while soluble fibre ranged between 16.03% in ACF and 33.39% in DSF. In-vitro protein digestibility and starch digestibility of the flour samples were significantly different from each other and ranged between 0.19% in ACF and 4.62% in SPI and 2.44% in TNF and 8.54% in SPI, respectively (p≤0.05). The formulated composite flour from acha, tigernut and sorrel seed protein isolate contained appreciable amounts of protein, essential amino acids, fatty acids, and minerals. The composite also exhibited good free radical scavenging abilities and better functional properties compared with the control samples. However, sample FCT (50% acha, 45% tigernut and 5% sorrel seeds protein isolate) showed better in-vitro starch digestibility, when compared with 100% acha, tiger nut and sorrel seed protein isolate flours (control); hence, FCT may serve as a potential flour blends for production of low-glycemic baked foods and dough meal.