| Proso millet is one of important crops in arid and semi-arid areas in China, and rich in protein, starch, dietary fiber and elements, which has been attracting more and more attention from the nutritionists and consumers as a medical food native material with higher nutritional and medical values. While proso millet has not been applied extensively due to its bad palatability and processing characters. Starch is the main component of proso millet grains, its physicochemical properties are the important factors affecting the processing and functional features of proso millet. The reports on the research of the physicochemical properties of proso millet starch are still rare at present. In this study, 12 non-waxy proso millet varieties and 7 waxy proso millet varieties were selected as materials, the differences of starch physicochemical properties, including starch granule shape and size, crystal structure,amylose content, transparency, freeze-thaw stability, retrogradation, solubility and swelling power, pasting properties and thermal properties, from different proso millet varieties and three different geographic regions were investigated. The main conclusions as follows:(1) There were significant differences between the amylose content, transparency,freeze-thaw stability, retrogradation, solubility and swelling power, and pasting properties from the starches of various non-waxy proso millet varieties. The starch granules morphology of different non-waxy proso millet varieties were similar, presenting more large polygonal shapes with smooth edges and few small spherical shapes, and there were honeycomb structures on the surfaces of some starch granules. The range of particles size was 3.76 ~12.02 μm, and the crystal structure was type A. The range of amylose content, light transmittance, freeze-thaw syneresis rate and supernatant volume percentage of different non-waxy proso millet varieties were 8.01%~28.02%, 9.93%~18.03%, 32.43%~47.06%,15.5%~18.03%, respectively. When the temperature was 90℃, the range of solubility and swelling power of non-waxy proso millet starches were 11.87%~22.67%, 15.37%~21.46%,respectively. The peak viscosity(PV), breakdown value(BD), setback value(SB) and pasting temperature(PT) ranged within 1.979~2.887 Pa·s, 0.491~0.827 Pa·s, 0.739~1.136 Pa·sand 62.80 ~ 64.45℃ separately. Among the tested varieties, Gu05-223 has highest transparency, best freeze-thaw stability and cold paste stability, stronger anti-aging ability,which is suitable for the processing and utilization of beverage products and frozen foods.(2) There were also significant differences between the amylose content, transparency,freeze-thaw stability, retrogradation, pasting properties and thermal properties from the starches of various waxy proso millet varieties. The starch granules morphology of different waxy proso millet varieties also presented more large polygonal shapes with smooth edges and few small spherical shapes, depression structures existed on the surfaces of some starch granules. The range of particles size was 2.86~10.08 μm, and the crystal structure was also type A. The range of amylose content, light transmittance, freeze-thaw syneresis rate and supernatant volume percentage of different waxy proso millet varieties were 0.124% ~1.391%, 26.47%~32.05%, 18.42%~26.94%, 9.78%~10.27%, respectively. The PV, BD,SB and PT ranged within 3.144~3.515 Pa·s, 0.779~0.906 Pa·s, 0.197~0.566 Pa·s and62.80 ~ 63.75℃ separately. The onset temperature of pasting(To), peak temperature of pasting(Tp), conclusion temperature of pasting(Tc) and heat enthalpy(ΔH) of different waxy proso millet varieties ranged within 67.87~70.79℃, 74.57~75.94℃, 80.38~81.06℃ and10.367 ~ 10.670 J/g separately. Among the tested varieties, Fu07-405 has the lowest retrogradation and best thermal stability, which is more suitable for used as a food stabilizer and processing material for tinned food with high temperature.(3) The agronomic traits yield performance and physicochemical properties of starches in the three geographic regions all exited significant differences. The average yield of 10 proso millet varieties for Shaanxi Yulin region(6488.0 kg/hm2) significantly higher than that of Shaanxi Yan’an(2764.0 kg/hm2) and Shanxi Datong(3172.3 kg/hm2) region, this may be related to the higher precipitation and sunshine hours is Shaanxi Yulin during the growth period of proso millet.The starch granules morphology of proso millet from three geographic regions also presented more large polygonal shapes with smooth edges and few small spherical shapes, depression structures existed on the surfaces of some starch granules. The order of the average size of three geographic regions was: Yulin(7.54 μm) >Yan’an(7.42μm) > Datong(7.18 μm). The starch crystal structures of proso millet from different geographic regions were all type A, but the diffraction intensity at the same 2θ existed differences. The orders of the diffraction intensity of different proso millet varieties, with 2θat about 15°, were consistent: Yan’an > Yulin > Datong, while there was no consistent regularity at other 2θ. The order of the amylose content, light transmittance and freeze-thaw syneresis rate was: Yulin(22.77%) > Yan’an(20.52%) > Datong(19.61%), Yulin(12.73%) >Yan’an(11.93%) > Datong(11.82%), and Yan’an(46.89%) > Datong(41.86%) > Yulin(39.02%), respectively. The orders of PV, BD and SB of proso millet starches from three geographic regions were consistent: Datong > Yan’an > Yulin. Similarly, in Yulin, Yan’an and Datong three geographic regions, Gu05-223 had the lowest amylose content(8.01%, 7.65%,8.81%), freeze-thaw syneresis rate(32.43%, 39.44%, 35.52%) and SB(0.739, 0.693, 1.046Pa·s), the highest PV(2.887, 3.065, 3.333 Pa·s) and BD(0.827, 0.943, 1.111 Pa·s); oppositely,Mengjingmi7 the highest amylose content(28.02%, 25.70%, 22.38%), freeze-thaw syneresis rate(47.07%, 55.50%, 45.83%) and SB(1.136, 1.349, 1.459 Pa·s), the lowest PV(1.979,2.721, 2.928 Pa·s) and BD(0.491, 0.580, 0.691 Pa·s). Variance analysis showed that the amylose content of starch, freeze-thaw stability and pasting properties of proso millet starches were all highly significantly affected by the varieties, environment, the interaction between varieties and environment at the same time, while the transparency was only significantly affected by varieties and environment, had no significant relation with the interaction between varieties and environment.(4) The amylose contents, transparency, freeze-thaw stability, solubility and swelling power and pasting properties of starches between the proso millet and the buckwheat, maize and potato all had significant differences. The starch granules morphology of proso millet presented more large polygonal shapes with smooth edges and few small spherical shapes,depression structures existed on the surfaces of some starch granules, which were similar with the buckwheat starch and maize starch, but different from potato starch. The order of the average size of tested starch granules was: waxy proso millet(6.04 μm) < buckwheat(7.02μm) < non-waxy proso millet(7.18 μm) < maize(12.15 μm) < potato s(33.67 μm). The starch crystal structures of proso millet were type A, same with the buckwheat starch and maize starch, but different from potato starch(type B). There were some differences of diffraction intensity between proso millet starches, buckwheat starch and maize starch, with 2θ at about15°, 17°, 18°, and 23°, the order of the average diffraction intensity was: non-waxy proso millet(377.5 cps, 395.5 cps, 422.5 cps, 338.5 cps) < waxy proso millet(391.5 cps, 461.0 cps,471.0 cps, 384.0 cps) < maize(435.0 cps, 538.0 cps, 554.0 cps, 485.0 cps) < buckwheat(461.0, 570.0, 576.0, 514.5 cps). The order of the average crystallinity of tested starches was:buckwheat(35.77%) < potato(38.81%) < maize(40.65%) < waxy proso millet(45.21%) <non-waxy proso millet(45.36%). The order of the amylose content, light transmittance and freeze-thaw syneresis rate was: waxy proso millet(1.36%) < potato(22.21%) < maize(23.95%) < non-waxy proso millet(27.15%) < buckwheat(39.01%), buckwheat(7.37%) <maize(14.60%) < non-waxy proso millet(15.31%) < potato(25.10%) < waxy proso millet(31.50%), and potato(8.39%) < waxy proso millet(20.21%) < maize(23.31%) < buckwheat(47.46%) < non-waxy proso millet(47.49%), respectively, and all the tested starches appeareddelamination after one freeze-thaw cycle. When the temperature was 90 ℃, the orders of the solubility and swelling power tested starches were consistent: buckwheat(11.00%, 10.88%) <maize(11.53%, 11.67%) < non-waxy proso millet(25.54%, 12.22%) < waxy proso millet(34.42%, 18.46%) < potato(47.04%, 59.98%). The order of the BD and SB of the tested starches were: common buckwheat(0.412 Pa·s) < maize(0.415 Pa·s) < non-waxy proso millet(0.498 Pa·s) < tartary buckwheat(0.825 Pa·s) < waxy proso millet(0.975 Pa·s) <potato(2.123 Pa·s), waxy proso millet(0.542 Pa·s) < common buckwheat(0.737 Pa·s) <tartary buckwheat(0.803 Pa·s) < non-waxy proso millet(0.999 Pa·s) < maize(1.176 Pa·s) <potato(2.435 Pa·s), respectively. In general, non-waxy proso millet starches have better hot paste stability, were suitable for the food needs a high temperature during processing; while the waxy proso millet starches have higher transparency, better cold paste stability and freeze-thaw stability, stronger anti-aging ability, were suitable for the processing industries that need better transparency or frozen stability. |
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