Studies on tuber starches III. Impact of annealing on the molecular structure, composition and physicochemical properties of yam (Dioscorea sp.) starches grown in Sri Lanka

This study reports the impact of one step annealing on the composition, molecular structure, granule morphology and physicochemical properties of starches isolated from cultivars of Dioscorea esculenta (kukulala, java-ala-nattala) and Dioscorea alata (hingurala, raja-ala), yam tubers grown in Sri Lanka. In all starches, granule morphology (shape, size, surface appearance), birefringence patterns, acid hydrolysis profile and X-ray patterns remained unchanged on annealing. Crystallinity remained unchanged on annealing in hingurala, kukulala and java-ala. However, crystallinity of raja-ala and nattala increased and decreased, respectively, on annealing. In all starches, annealing decreased the gelatinization temperature range (kukulala ∼ hingurala > nattala ∼ raja-ala > java-ala), amylose leaching (raja-ala > nattala > hingurala > kukulala > java-ala), granular swelling (raja-ala ∼ hingurala > kukulala > java-ala > nattala), peak viscosity (raja-ala > hingurala > kukulala > java-ala > nattala), enthalpy of retrogradation (kukulala ∼ java-ala ∼ nattala ∼ hingurala ∼ raja-ala) and susceptibility towards acid hydrolysis (java-ala > raja-ala > hingurala ∼ nattala > kukulala). However, annealing increased gelatinization temperatures (kukulala ∼ java-ala ∼ nattala ∼ raja-ala ∼ hingurala) and the enthalpy of gelatinization (kukulala > hingurala > java-ala > nattala > raja-ala). Set-back viscosity increased in nattala, but decreased in the other starches (raja-ala > hingurala > kukulala > java-ala) on annealing. The study showed that the different responses shown by the cultivars of the Dioscorea starches towards annealing were to a large extent influenced by their composition and molecular structure.