Physicochemical characterization and functional properties of starch and flour fraction from five yam (Dioscorea alata) cultivars

Abstract

Yam (Dioscorea alata), as a bulbous plant, was rich in starch content. Yam starch pastes naturally had a character that was resistant to high temperatures, and this character was needed in food processing. A fairly high diversity, quitely high plant productivity and easily in cultivation of yam plant indicated the potential utilisation of yam tuber as raw material in food industry. Starch in food processing applications would result in the final product that was affected by refined starch physicochemical properties. As for food processing, information about the starch physicochemical characteristics was necessary, which was typical characteristic to the type (source) of starch. Physicochemical character was influenced by several factors such as the type (source) of starch, granule size, amylose content and starch crystallization. Starch with different on its granules sizes had different chemical composition that was contribute to the different pasting character and paste viscoelastisity. Pasting characters and paste viscoelastisity themselve were important functional characters of the starch in food processing applications as thickeners. In relation to that and for the development usage of yam tuber, this study was aimed to observe and to study the physicochemical characteristics and functional properties of starch granules and flour particle size fractionation from several yam cultivars, especially in the paste formation and paste viscoelastisity profile. The chemical composition of starch was specific for each yam cultivar which influenced the nature of the starch physical and functional properties. It was found that yam starch had unimoda asymmetrical distribution of its granule size with a higher portion on the large size, and the size ranged 7-100 μm. The morphology of yam starch granules was round disc-shaped, elliptical, oval and round with smooth wavy surfaces. The chemical composition of yam starch had lipid content ranged 16-30 % db, amylose ranged 23.44-26.99 % db and phosphorus ranged 268.15- 365.36 ppm db. Yam starch crystallinity structure had type-B pattern with low crystallinity which was ranged 20.6-30.4 %. Based on the viscoelastisity profile of its cold paste, yam starch had strong resistance to high friction up to =107 rad/s. of the sweep test frequency Fractionation of yam starch was achieved by dry sieving Retsch Siever with 400 and 500 meshes, and was resulted in unperfectly separated of the size fractions. The fractionation was provided a significantly different on average value among the fractions for all cultivars with the range of average of 20.89- 27.93 μm as smaller fraction, 22.41-28.82 μm as medium size fraction and 25.00- 34.80 μm as large size fraction. Based on the average value of the fraction, the larger the granule size contributed to the increase of amylose and phosphorus content and reduced starch crystallinity. The difference in the granule size fraction was not enough to produce any significant effect of size on gelatinization character. In the paste formation, the starches showed type-B profile and higher paste stability could be found in the bigger granule size. The viscoelastisity character showed that the value of storage modulus (G') was increase as the granule size increase which was lead to the stronger gel formation of the paste from small granule size against heat and stirring. Study on yam starch size, granule size wether as a whole or as a fractionated, consistently was effect on the level of amylose and phosphorus content as well as crystallinity, and altogether produced a distinctive paste character which was related to the starch granule size fraction. Yam starch granule size was also consistently contributed to the character of the greater starch gelatinization temperature and lower energy needed as more uniformity of the granule size. It was also indicated that the levels of amylose, phosphorus and starch crystallinity were closely related to the size and size distribution of granule which were together contribute to the gelatinization, pattern of paste formation, and cold paste viscoelastisity characters. Study on yam flour and the starch character contribution showed that in increasingly refine flour particle size performed paste profile more identical to its starch paste profile (type-B), while the large particle size of flour the paste profiles changed to type-C. Other component than starch and the wider size range in the flour particles causing much lower paste viscosity of yam flour than its pure starch. Uniformity of size was higher in small particle size, could increase the paste viscosity, lowered the temperature and shortered time of the paste formation than larger sized particle. Overall, observation on yam starch, granule fractions and flour particle fractions of five yam cultivars, was found that the size of the starch granules was influenced on its physical (size distribution, crystallinity and gelatinization), and chemical characters (phosphorus and amylose content). The physicochemical characters then was effect on the starch paste profile where the bigger the size the more stable the paste against heating, cooling and stirring. Moreover, viscosity was increases as the granules size smaller, but the strength and stability of the pastes increased as the granules size bigger. These starch character were contribute to the flour paste profile better with increasingly fine particles. Uniformity of granule size might caused to the lower energy requirements and the narrower temperature range in starch gelatinization