Physico-chemical, functional and pasting properties of native and chemically modified water yam (Dioscorea alata) starch and production of water yam starch-based yoghurt

Water yam starch was isolated and subjected to chemical modification by acetylation, oxidation and acid thinning. Chemical composition, functional properties and pasting characteristics of native and modified starches were determined using standard methods. Chemical modification reduced the fat, protein, reducing sugar and amylose content. Amylose content ranged between 9.42 and 12.53% for native and modified starches. Acetylation and oxidation reduced the ash content while acid thinning increased the ash content. There were increases in resistant starch content and oil absorption capacity while the water absorption capacity and bulk density were reduced due to modification. Acetylation and oxidation improved gelation while acid thinning reduced gelation. Swelling power and solubility were pH and temperature dependent. Oxidation improved clarity but acid thinning reduced it. The result of pasting characteristics showed that overall, commercial cassava flour was the best; while for the native and modified water yam starches, acid thinned was the best. Sensory evaluation revealed that yoghurt produced from acetylated starch was the best. While water yam generally had good starch qualities, acid-thinned water yam had the best overall result which could be adopted for industrial uses.