QUALITY INDICATORS OF Dioscorea rotundata POIRET and Dioscoreaalata LINNAEUS TUBERS FOR UTILISATION IN STARCH, FLOUR AND AMALA

Abstract

Yam is an important tuber crop with numerous species of distinct qualities that dictate its acceptability and utilisation. Inappropriate quality indicator is a risk for yam value chain. There is sparse documented information of reliable indicators for some commercially cultivated yam varieties in Nigeria. This study was undertaken to identify quality indicators that determine utilisation of some varieties of Dioscorea rotundata and Dioscoreaalata tubers as starch, flour and amala. Genetically characterised nineteen varieties of D. rotundata and thirty-six of D.alata were sourced from National Root Crops Research Institute, Umudike. Colour (L*, a*,b*), granule morphology and dry matter contents of tubers were determined by standard methods. Tubers of D. rotundataand D. alatawere processed into flour, starch and elubo (fermented flour) by standard procedures. The samples’proximate composition, amylose, amylopectin, sugar, starch, pH, titratable acidity, phytate, tannin, minerals, Swelling Power (SP), solubility and Water Absorption Capacity (WAC) were determined by standard methods, and pastingcharacteristics (peak, holding strength, breakdown, final and setback viscosities) by Rapid Visco Analyser. Amala was produced from elubo by conventional method and evaluated for stretchability, stickiness, smoothness, hardness, taste, aroma and appearance by panelists. Data were analysed using descriptive statisticsand ANOVA at α0.05. The L*, a* and b* of D. rotundata tubers (66.2-81.2, -3.1-2.3 and 17.2-22.0) were significantly different from those of D.alata(26.9-83.4, 0.2-16.8 and 8.1-22.2), respectively. Granule sizes of D. rotundata (32.4–57.0 x 22.4–35.7 µm)were significantlyhigher than D. alatavarieties (18.3-48.9 x 13.4-33.1 µm). Dry matter of the tubers ranged from 32.6 to 48.2% and 21.6 to 41.6%, respectively. Crude fibre, protein, amylose, amylopectin, sugar, starch, pH and titratable acidity of the yams varied significantly. The D.rotundata varieties had lower phytate (1.0-1.4%) and tannin (0.1-1.26%) contents. Potassium was dominant mineral in the yam species. The SP, solubility and WAC ofD.rotundata andalata were 4.97–8.86 and 2.15–8.45 g/g; 0.55–2.65 and 0.13–5.72% and 56.31–82.59 and 50.18–105.38%, respectively. Pasting properties of D. rotundata and alata starches and elubo were significantly different, with starches showing higher peak (291.08-654.78 RVU and 252.42-467.46RVU), holding strength (140.33-362.58RVU and 106.54-368.63RVU), breakdown (150.75-361.17RVU and 20.83-212.83RVU), final (209.63-493.58RVU and 159.54-533.54RVU) and setback viscosities (69.29-134.88RVU and 53.00-266.58RVU), respectively. Stretchability, smoothness and softness of amala from D. rotundata varieties were significantly rated higher, and lighter in colour with pleasing aroma. Dioscorea rotundata exhibited increasedstretchability with increasing SP and decreasing holding strength; decreased astringent taste with increased sugar and decreased amylose; final viscosity correlated positively with smoothness. For D. alata, there were significant correlations between stickiness, WAC and peak viscosity; taste, WAC and tannin content. Pasting properties had highest contribution (85.64%) in D. rotundata utilization, while functional properties were highest (73.88%) for D. alata. Mineral contents had no significant effects on both species’ utilisation. Pasting and functional properties, amylose, tannin and sugar were the established quality indicators in Dioscorea rotundata and alata utilization for starch, flours and amala.