Studies on tuber starches. II. Molecular structure, composition and physicochemical properties of yam (Dioscorea sp.) starches grown in Sri Lanka

Starch from varieties of Dioscorea esculenta (Lour.) Burkill (kukulala, java-ala, nattala) and Dioscorea alata L. (hingurala, raja-ala) tubers grown in Sri Lanka was isolated and its morphology, composition, structure and physicochemical properties were studied. The yield of starch was in the range 12.2–18.0% on a whole tuber basis. The shape and size of D. esculenta and D. alata starch granules were round to polygonal (3–10 μm) and spade shaped (30–45 μm), respectively. There was considerable variation in total amylose, phosphorous, bound lipid, amylose leaching, swelling factor, gelatinization parameters, extent of retrogradation, pasting characteristics, susceptibility towards enzyme and acid hydrolysis crystallinity and lipid complexed amylose chains between and among the starches belonging to the two species. The chain length distribution of debranched amylopectins of the starches showed that the proportion of short branched chains (dp 6–12), medium chains (dp 25–36) and average chain length of amylopectin were significantly (P < 0.05) higher in the D. esculenta starches. However, variations in amylopectin structure among varieties of both species were not significant (P < 0.05). D. esculenta starches exhibited a ‘B’-type powder X-ray diffraction pattern. However, in the D. alata starches, hingurala and raja-ala displayed ‘C’- and ‘B’-type patterns, respectively. The results showed that some of the starches from the above species could be used in foods subjected to thermal processing and frozen storage, respectively, with minimal physical or chemical modification.