The impact of annealing on the molecular structure and properties of Dioscorea Starches

Dioscorea starches are presently not used in the food industry due to their poor functionality. Annealing has been shown to improve starch functionality. Therefore, the objective of this study was to determine the structural changes within the amorphous and crystalline domains of starches isolated from varieties of D. esculenta and D. alata tubers on annealing, and the impact of these changes on functional properties. Starches from varieties of Dioscorea esculenta (kukulala, java-ala, nattala) and Dioscorea alata (hingurala, raja-ala) tubers grown in Sri Lanka were isolated and their yield, morphology, composition (phosphorus, bound lipid, total amylose and lipid complexed amylose), molecular structure and physicochemical properties were studied in their native and annealed states. Among the D. esculenta starches, nattala exhibited the highest levels of phosphorus (0.1 0%), bound lipid (0.44%) and lipid complexed amylose chains (22.01 %). However, between the D. alata starches, the highest levels of phosphorus (0.05%), bound lipid content (0.25%) and lipid complexed amylose chains (8.34%) was in hingurala. All isolated starches were pure, undamaged and with granule sizes ranging from 3 to 40 μm (D. esculenta) and 30 to 45 μm (D. alata). The granules had smooth surfaces with shapes ranging from polygonal (D. esculenta) to truncated oval or truncated spade in D. alata starches. The amylopectin unit chain length distribution and the average chain length of native D. esculenta ranged from 24.57-25.85% (Degree of polymerization [DP] 6-12), 56.55-59.64% (DP 13-24), 12.61-13.58% (DP25-36), 3.17-4.41 (DP 37-50) and 17.93-18.33%. However, the corresponding values for D. alata starches were 17.89-20.68, 57.40-59.76, 17.46-17.47, 4.46-4.87 and 19.29-19.61%. Among the D. esculenta starches, the highest proportion of DP 37-50 and average chain length was in nattala starch. D. esculenta starches displayed a B-type X-ray pattern. However, a B-type (raja-ala) and Ca-type (hingurala) X-ray patterns were displayed by the D. alata starches. Crystallinities ranged from 49-53%, in the D. esculenta (nattala>java-ala> kukulala) starches, but were similar (43.0%) in the D. alata starches. A V-lipid amylose complex peak was also visible in the X-ray pattern of all starches. -- The gelatinization transition temperatures (To [onset], Tp [mid point], Tc [conclusion]) and enthalpy (ΔH) of gelatinization of native D. esculenta starches ranged from 72.55 to 85.40°C and 17.32-18.07 J/g, respectively. The corresponding values for the D. alata starches being 75.45-92.70°C and 18.60-18.98 J/g, respectively. The extent of amylose leaching (AML) at 90°C, ranged from 5.58 to 6.19% and 13.20 to 13.60% in the D. esculenta and D. alata starches, respectively. Variations in AML among varieties of each species was negligible. Swelling factor at 90°C, ranged from 53.61 to 64.97% in the native D. esculenta (nattala>java-ala>kukulala) and from 36.60 to 38.64% in the D. alata (hingurala>raja-ala) starches. D. esculenta starches exhibited a higher peak viscosity (nattala>kukulala> java-ala), lower pasting temperatures, a greater degree of viscosity breakdown (nattala> java-ala> kukulala) and lower set-back (kukulala>nattala>java-ala) values than D. alata starches. A similar trend was also observed for peak viscosity (raja-ala>hingurala), viscosity breakdown (hingurala> raja-ala) and set-back (raja-ala>hingurala) among the D. alata starches. The rate and extent of acid hydrolysis of D. esculenta starches (nattala>java-ala>kukulala) were higher than those of the D. alata (raja-ala>hingurala) starches. D. esculenta java-ala>kukulala>nattala) starches were more susceptible than D. alata (hingurala>java-ala) towards α-amylolysis. The extent of retrogradation of D. esculenta java-ala>kukulala>nattala) was higher than that of the D. alata (hingurala>raja-ala) starches. -- The second phase involved a study of the impact of annealing on the structure and properties of Dioscorea starches. The granule surface, birefringence patterns, concentric growth rings and the amylopectin unit chain length distribution of all starches remained unchanged on annealing. The crystallinity of some varieties of D. esculenta (kukulala, java-ala) and D. alata (hingurala) starches remained unchanged on annealing. However, crystallinity decreased in nattala (D. esculenta) but increased in raja-ala (D. alata) starches. In addition crystalline polymorphism remained unchanged on annealing. Annealing increased the gelatinization temperatures to nearly the same extent for all starches. However, the gelatinization temperature range (T c-T 0 ) decreased on annealing (hingurala> kukulala>raja-ala>nattala>java-ala). The enthalpy of gelatinization increased marginally (kukulala > hingurala > java-ala > nattala >raja-ala) on annealing. Amylose leaching (AML) and swelling factor (SF) decreased (D. alata> D. esculenta) on annealing. At 90°C, the decrease in AML in the D. esculenta and D. alata starches followed the order: nattala-kukulala>java-ala and raja-ala>hingurala, respectively. However, at the same temperature, the decrease in SF for the D. esculenta and D. alata starches followed the order: kukulala>nattala>java-ala and raja-ala-hingurala, respectively. In the D. esculenta starches, annealing decreased the peak viscosity (kukulala>java-ala>nattala). However, annealing increased peak time (kukulala > nattala > java-ala) pasting temperature (kukulala > nattala-java-ala) and thermal stability (kukulala>nattala>java-ala). The set-back decreased on annealing in kukulala, but increased in both nattala and java-ala (nattala>java-ala) starches. In the D. alata starches, annealing decreased peak viscosity (raja-ala>hingurala), but increased peak time (raja-ala>hingurala), pasting temperature (raja-ala>hingurala) and thermal stability (hingurala>raja-ala). However, set-back decreased in raja-ala but increased in hingurala on annealing. In all starches, the acid hydrolysis pattern remained unchanged on annealing. However, the extent of acid hydrolysis decreased (D. esculenta [java-ala>nattala>kukulala] - D. alata [raja-ala~ hingurala]) on annealing. α-amylolysis decreased on annealing. The extent of this decrease was more pronounced in D. alata (raja-ala>hingurala) than in the D. esculenta java-ala>kukulala) starches. However, annealing increased the susceptibility of nattala starch towards α-amylolysis. The enthalpy of retrogradation decreased in both D. esculenta (kukulala-nattala>java-ala) and D. alata (raja-ala>hingurala) starches. The major findings of this research are: (1) the different responses shown by the Dioscorea cultivars towards annealing were mainly influenced by differences in their composition and molecular structure, (2) differences in physicochemical properties between native and annealed starches are influenced to a large extent by structural changes within the amorphous (interaction between AM-AM and/or AM-AMP chains) and crystalline (AM-AMP and/or AMP-AMP interactions, crystallite reorientation, crystallite perfection, reorientation of phosphate groups on amylopectin) domains of the starch granule during annealing, (3) annealing has a major impact on the thermal stability and extent of retrogradation of Dioscorea starches. Consequently, annealed Dioscorea starches can be used for applications in foods that are subjected to thermal processing and frozen
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