Effects of annealing, acid hydrolysis and citric acid modifications on physical and functional properties of starches from four yam (Dioscorea spp.) cultivars

The effects of annealing, acid hydrolysis and citric acid modification on the physical and functional properties of starches of four Nigerian yam cultivars were studied. Loose (0.3636–0.7658 g/cm3) and packed (0.5606–1.0165 g/cm3) bulk densities and CIE L* of native, and modified starches decreased significantly (p < 0.05) with annealing, but increased significantly with acid hydrolysis and citric acid modification. The CIE a* value and %W (whiteness degree) increased, while b* value reduced with all modifications. The CIE L*, a* and b* values of extracted starches ranged between 74.8–98.3, −4.21–4.80 and 4.40–12.9, respectively. Among the native starches, L* ranked: bitter yam > white yam > water yam > yellow yam. Starch granule shapes varied with cultivar but not with the modifications. Starch granule sizes reduced significantly with the modifications, with increased incidence of small and medium sized granules, and decrease in large sized granules. Short and long length of native white, yellow, water and bitter yam starches ranged from 19.5–54.9 and 15.6–42.9 μm, 27.30–58.50 μm and 15.60–42.90 μm, 23.4–58.5 μm and 15.6–35.1 μm, and 2.50–7.50 μm and 2.50–5.0 μm, respectively. Chemical modifications caused etching, fragmentation and roughening of some granules' edge. The WAC (0.63 g/g–1.047 g/g) increased significantly with acid hydrolysis and citric modification, but reduced significantly on annealing while OAC (0.437 g/g–1.053 g/g) was not significantly affected by the modifications. Generally, alkaline water retention (1.017 g/g–0.670 g/g) and foam capacity of the starches decreased with all the modifications. Annealed yam starches showed better foam stability than citric modified and acid hydrolysed yam starches. Generally, emulsion index (0.337–0.621) increased insignificantly with citric acid modification, while annealing and acid hydrolysis had variable effect.