EFFECT OF PROCESSING ON NUTRITIVE CHARACTERISTICS OF SOME EDIBLE TUBERS, ENZYME DIGESTIBILITY AND MORPHOLOGY OF THEIR EXTRACTED STARCHES

Abstract

Three tuber samples namely cocoyam (Colocasia esculenta), sweet potato (Ipomoea batatas) and white yam (Dioscorea rotundata) were subjected to processing (boiling and roasting) while the unprocessed served as the control. Each sample was divided into two portions. One portion was analysed with respect to their proximate compositions, functional properties, phytochemical screening, anti-nutrient contents, minerals, functional groups as well as digesting the starch using alpha-amylase enzyme. The second portion was subjected to starch extraction and the extracted starch was characterised to different sugars after digestion with the enzyme; functional groups were also determined using Fourier transform infrared spectroscopy (FTIR) and the morphology of the starch granules were analysed using scanning electron microscopy (SEM). The result on proximate revealed that the moisture content ranged from 9.59 g/100g in fresh white yam to 11.94 g/100g in fresh cocoyam. Crude fibre (CF) was highest (3.35 g/100g) in fresh sweet potato and the lowest value of 1.78 g/100g was found in roasted white yam. Crude protein (CP) was significantly different (P≤0.05) with the highest value of 12.78 g/100g recorded for roasted white yam while the least value (2.28 g/100g) was found in boiled cocoyam. Ethyl extract (EE) was highest in the fresh white yam and the ash content was generally highest in cocoyam fresh and processed samples. There was no significant difference (P≤0.05) in the nitrogen free extract (NFE) in all the samples analysed. Among the functional attributes, water absorption capacity (WAC) ranged from 200% in fresh sweet potato to 550% in boiled of the same sample. Oil absorption capacity (OAC) ranged 45% in roasted white yam to 225% in fresh of the same sample. Foaming capacity (FC) varied from 1.95% in boiled sweet potato to 28.54% in fresh white yam. There was no significant difference (≤0.05) in values obtained in emulsion capacity (EC) and bulk density (BD) while least gelation capacity (LGC) had least value (4%) in fresh sweet potato and highest value of 12% in the boiled of the same sample. Results on phytochemical screening showed the presence of saponin, tannin and terpenoid in all the fresh and processed samples, phlobatannin was present only in the fresh and processed cocoyam and white yam, while alkaloid and anthraquinone were absent in all the samples. Results on the anti-nutrients determined showed tannin, saponin, terpenoid, oxalate, phytate were in the range 0.52 - 1.70 mg/g, 17.36-169.27 mg/g,1.31-5.22 mg/g,0.18-1.16 mg/g and 6.59-22.25 mg/g, respectively. Cardiac glycoside ranged 6.48 mg/g- 9.26 mg/g was present only in the white yam, flavonoid was absent in the cocoyam, while phlobatannin and alkaloid were below the detection limit in sweet potato. Ca, K, P and Mg were the most abundant minerals in all the tuber samples, but processing especially boiling significantly (P≤0.05) decreased the mineral contents in the samples. FT-IR analysis of the tubers and their extracted starches showed the presence of OH, C2H2n+1, C2H2, CnH2n-2, R-X and ROR while the presence of aromatic nitro compound was detected in the extracted starch of boiled sweet potato. Results on the SEM showed increased surface area in the boiled samples when compared to the roasted and fresh samples. The result on the complex carbohydrates digestion before and after alpha-amylase enzyme digestion revealed that amylase digested the starches to monosaccharide (glucose) and disaccharide (sucrose and maltose) and the rate of digestion were highest in roasted sweet potato, roasted white yam and roasted cocoyam. Processing generally had effects on the nutritive characteristics of all the tubers analysed.