Studies On Physicochemical Properties Of Starches Isolated From Proso Millets (Panicum Miliaceum L.)

Proso millet, belonging to annual family Gramineae (panicum miliaceum L), is an important cereal for the diets of human in the North of China, which can grow well under the condition of aridity, semi-aridity and low soil fertility. It is a rich resource with many advantages such as strong stress resistance, short growth period and wide variety. Proso millet is rich in starch, but there are few attentions paid on proso millet starch as ordinary maize and potato starch about its detailed studies and applications. Therefore, it has important significance to research the proso millet starch and probe its potential applications in food industry.In this study, proso millets from four different production areas in China (Lvliang city, Shanxi province, Baotou city, Inner Mongolia, Gulang County, Gansu province and Shuangliao) were used as experimental materials. On the basis of the nutrient constitutes analysis of proso millets, the basic constitutes, granules, crystallinity, particle size distribution, pasting, thermal and in vitro digestibility properties of starches isolated from proso millets with alkali extraction method were studied by means of scanning electron microscope (SEM), X-ray diffraction (XRD), laser light scattering particle analyzer (LLSPA), Brabender viscoamylograph, differential scanning calorimetry (DSC) and other physicochemical analyses. The results are as follows:(1) The studies of nutrient components of four proso millet samples showed that the content of starch, crude protein and moisture were 67.05-73.12%, 9.50-12.14%, 10.59-12.07%, respectively, and the crude protein content is higher than that in rice and wheat. The inorganic element contents of Mg,K,Fe and Ca were high, and followed by Cu. In addition, proso millets almost contained all the amino acids, the maximum content was found in glutamic acid (Glu) (1.24-1.50%), and leucine (Leu) took up the highest percentage (14.41 - 15.39%) of all essential amino acids (EAA). Unsaturated fatty acid (73.6-79.42%), especially linoleic (51.69-61.06%) which was necessary to humans were enriched in proso millet oil.(2) The analysis of basic constitutes of proso millet starches showed that the content of starch, crude fat and crude protein were 82.69-86.30%, 0.12-0.22% and 0.24-0.58%, respectively, and the content of amylose was 18.52-20.98%, which was slightly lower than that in maize and potato starches. SEM analysis found that proso millet starch granules showed a high percentage of presence of large polygonal to small spherical shape, and the surfaces of the granules appeared to be smooth and showed no evidence of fissures. The average diameter of proso millet starch granules were 6.5-7.0μm with a diameter range of 0.3 - 12μm. XRD showed type A. Moreover, the sharp and light micrographs of proso millet starch granules in polarized mode were clear and presented the shape of "+", with the umbilical point close to the center of starch granules. The full particle size distribution indicated that proso millet starches exhibited a smooth bimodal size distribution and consisted of a major population of large granules of 2.82- 14.16μm in diameter. The volume mean size of starches was 6.47-6.95μm.(3) Analyses of pasting properties showed: the solubility, swelling power, freeze & thaw stability and euphotic ratio of proso millet starches were small. The Brabender viscoamylograph indicated that pasting onset temperatures of proso millet starches were at 73.00-74.65℃and Baotou sample gave the highest peak viscosity (457BU), followed by Shuangliao and Gulang sample, while Lvliang the lowest (219BU), it suggested that there were some distinctions between varieties of proso millet starches. As to the stability of pastes, the heat stability of Lvliang and Gulang samples were much better than others, while the cold stability of Baotou sample was best. In addition, with the increases of temperature, NaCl, NaHCO₃ concentration and rotation speed, the apparent viscosity of pastes decreased, while sucrose had an opposite effect. Anyhow, the apparent viscosity changing scales were small affected by above factors. When pH value was at 8, the apparent viscosities were highest, but when pH value exceeded 10, the apparent viscosities tended to be flat.The transition temperatures and enthalpy of gelationization (â–³H) of proso millet starches, investigated using DSC, were 62.81-82.44℃and 10.4-14.46J/g, respectively.Texture analyzer was used to indicate the texture properties of starch gels. The results showed : in the concentration range of 8-16%, with the increase of starch concentration, the hardness and chewiness of Lvliang and Gulang proso millet starches rised, but the concentration had no significant effect on Baotou and Shuangliao samples. Moreover, at the same concentration, the hardness of gel resulted in the follow order: Gulang≈Lvliang > Shuangliao > Baotou. The spingness and resilience of gels had not been significantly affected by the starch concentration.Effects of various processing on in vitro digestibility of proso millet starches indicated that in vitro digestibilities of starches were rised by boiling, microwave cooking, high-pressure cooking and baking on the contrast of unprocessing. And the high-pressured starch digestion in vitro best, moderate in microwave cooking and the baked starch digestion in vitro relatively poor.