| Proso millet(Panicum miliaceum L.)belonged to the gramineous family,which has both food and medicine.It is rich in nutrients and is an important food source.Starch is the main component of proso millet,accounting for more than 70%.According to the difference of amylose content in proso millet,proso millet can be further divided into waxy proso millet(0 ～ 5%)and non-waxy proso millet(17% ～ 32%).Natural fermentation is a traditional method of processing food.Compared with artificial inoculation fermentation,natural fermentation can better preserve the unique texture and flavor of food,so as to meet people’s pursuit of healthy diet.The structure and physicochemical properties of starch will directly affect the edible quality of proso millet Therefore,three kinds of waxy proso millet BS,LS21 and LS23 and two kinds of non-waxy proso millet LM14 and LM8 were selected as the research objects in this study.through the analysis of the basic composition and processing characteristics of the changes,explore the effect of natural fermentation on the quality of proso millet;By measuring the crystal structure,short-range ordered structure,chain length distribution,thermodynamic properties,gelatinization properties,rheological properties and in vitro digestion properties of proso millet starch,the change law of natural fermentation on the structure and physical and chemical properties of proso millet starch was analyzed.The relevant basic research provides a theoretical basis for the further processing of proso millet,in order to develop a new path of comprehensive utilization of proso millet.The main research results are as follows:(1)Analysis on natural fermentation quality and processing characteristics of proso millet.Composition analysis showed that the contents of total starch,fat,protein and ash in waxy and nonwaxy proso millet decreased after fermentation.In addition,the total starch hydrolysis caused by natural fermentation increased the amylopectin content in waxy proso millet BS,LS21 and LS23 from 0.57%,3.21% and 4.17% to 0.74%,4.15% and 5.82%.The amylose content of non-waxy proso millet LM14 and LM8 was significantly increased from 19.65% and 20.42% to 22.97% and 24.35%(P < 0.05).These results confirmed the degradation effect of natural fermentation on components of waxy and non-waxy proso millet,which may be related to the enzymatic hydrolysis of microorganisms in the natural fermentation process.And composition changes have also led to colour change of waxy and non-waxy proso millet,after the natural fermentation,the value of L*for waxy and non-waxy proso millet rised,a*and b* droped,so that the waxy and non-waxy proso millet were showing brighter colour.The inside of the LMR results showed that combined water content of waxy and non-waxy proso millet was significantly increased after through natural fermentation,and the combined water content of waxy proso millet was higher than non-waxy proso millet,which indicated after natural fermented waxy proso millet has better water retention.Cooking quality of the analysis results show that after the natural fermentation the volume expansion rate of waxy and non-waxy proso millet were significantly increased,making them showed better eating quality.Further to determinate flavor substances of waxy and non-waxy proso millet found that after natural fermentation aroma components of waxy and non-waxy proso millet,alkanes substances and aliphatic compounds contributed larger,followed by alcohol and aromatic compounds.(2)Analysis on structural characteristics of starch in naturally fermented proso millet.First of all,analyze the particle structure,natural fermentation waxy and non-waxy proso millet starch granule surface damage and holes,but did not affect the polarization cross structure of particles,and waxy and non-waxy proso millet average particle size of starch granules was reduced,the average grain size of starch granules in waxy proso millet was higher than that in non-waxy proso millet,which indicated the natural fermentation of waxy and non-waxy proso millet starch particles in erosion.Secondly,the results of X-ray diffraction and Fourier infrared spectroscopy(FTIR)showed that the relative crystallinity of starch of waxy proso millet BS,LS23 and LS21 increased from 41.35%,41.07% and 38.39% to 44.17%,44.12% and 43.70%,respectively.Crystallinity of starch of non-waxy proso millet LM8 and LM14 increased to 37.27% from 33.09% and 37.07% and40.10%,and degree of order(DO)of waxy and non-waxy proso millet starch have been improved significantly(P < 0.05).However,no new functional groups were produced,the original basic structure of starch was maintained.The research results of chain length distribution showed that natural fermentation increased the proportion of chain A of waxy and non-waxy proso millet starch,while decreased the proportion of chain B1,B2 and B3,indicating that natural fermentation would lead to the break of the long chain branch of the starch of waxy and non-waxy proso millet,and the long chain distribution of amylopectin tended to be short chain,thus affecting the physical and chemical properties of waxy and non-waxy proso millet starch.(3)Analysis of physicochemical properties of starch in natural fermented proso millet.After the natural fermentation the drainage rate of waxy and non-waxy proso millet starch were drop,thus increasing the acceptable degree of consumers.Transparency increased after the fermentation,and the light transmittance of waxy proso millet starch was higher than non-waxy proso millet.These results indicate that natural fermentation can make waxy proso millet starch show better appearance quality.Natural fermentation significantly increased solubility and swelling power of waxy and nonwaxy proso millet starch under different temperature(P < 0.05).Making it more responsive to the needs of the food industry.The pasting and thermodynamic properties of waxy and non-waxy proso millet starch were improved by natural fermentation.The pasting temperature of waxy proso millet BS,LS21 and LS23 starch increased significantly from 74.19 ℃,75.05 ℃ and 76.76 ℃ to 76.29 ℃,76.51 ℃ and 77.96 ℃(P < 0.05),and the peak viscosity increased by 1.47,1.45 and 1.43 times compared with that before fermentation,respectively.Gelatinization enthalpy increased by 23.03%,56.02% and 47.95%.The pasting temperature of non-waxy proso millet LM14 and LM8 starch increased significantly from 78.32 ℃ and 80.01 ℃ to 80.29 ℃ and 81.94 ℃(P < 0.05),the peak viscosity increased by 1.29 and 1.38 times,and the gelatinization enthalpy increased by 15.57% and10.54% compared with that before fermentation.These results indicated that natural fermentation increased the difficulty of gelatinization but improved the thermal stability of waxy and non-waxy proso millet.The thermal stability of waxy proso millet starch was better than that of non-waxy proso millet.Texture characteristics and rheological properties showed that after the natural fermentation,waxy and non-waxy proso millet starch gel exhibited better viscoelasticity,waxy proso millet showed a good which was formed by the viscous behavior,non-waxy proso millet gel formed by its deformation is recoverable,and its structure present a more rigid structure.The results of in vitro digestion showed that the SDS content of five kinds of proso millet starch increased from23.52 ～ 33.26% to 25.95 ～ 34.05% by natural fermentation.The content of RDS decreased from26.79 ～ 39.07% to 23.83 ～ 30.15%,and the content of RS decreased from 27.78 ～ 41.49% to 36.15～ 42.57%.Natural fermentation delayed the waxy and non-waxy proso millet starch digestion rate,make it easier for people to satiety.(4)Pearson correlation analysis showed that the higher the amylose content and the smaller the particle size,the better the thermal stability.At the same time,the peak viscosity was positively correlated with the content of chain A,negatively correlated with the content of chain B1,and the breakdown value was negatively correlated with the content of amylose,indicating that the change of amylose content and chain length distribution had a great influence on the pasting properties,that is,the proso millet starch with a higher content of short chain generally showed a higher peak viscosity,while the sample with a higher content of amylose might have a weak shear resistance.Moreover,high content of amylopectin in the system will slow down the digestion rate of starch in vitro.In conclusion,the changes in mean grain size,chain length distribution and amylose content caused by natural fermentation can improve the gelatinization properties,texture properties and in vitro digestion properties of proso millet. |
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