| Oat is the fifth largest cereal crop with full nutrition. China is one of cradlelands of Avena nude L (naked oat). In recent years, oat consumption progressively increases with insight for oat nutrition value, and it is becoming a scarce resource. Making full use and maximizing value of oat are common goals of researchers. Germination of seed, as one part of plant life cycle, possesses very complex mechanism and process. Using this natural non-harmful digestion could improve solubility of insoluble nutrients and promote digestibility of them, at the same time, some bioactivities products could be desired. Depending on these characteristic of germination, people have produced nutritive foods for a long time, and malts of soybean and barley are the best examples. This thesis focused on improvement of oat digestibility and nutrition value by germination, and investigated structure and importantly properties of oat starch which is the main storage substance during this period. The aim of this study was to supply some references to utilization of oat and oat starch, and SEM, laser partical size distribution analyzer, RVA, DSC, TGA, DMA, photoelectron spectroscopy, low field NMR and high performance ion exchange chromatography were employed to analysis or characterize oat starches. The important results are:1 Main nutrients and biochemical properties of germinated oatThe growth curve and oat malt was described, and color of oat malt after drying at 50℃was investigated by colorimeter. It was found that there was a significant correction between them. The free amino acid and reducing sugars in oat increased above 10 times during germination, and reached 0.37% and 14.47%, respectively. The free sugars and total polyphenols also increased from 5.23% to 28.11% and from 0.2% to 0.91%. The content of phytic acid at the end of germination remarkable decreased to 0.11% compared with 0.35% of raw oat. Total starch enzymes activity in oat seed germinated for 144 h was 5 fold higher than that of raw oat. The activities of catalase and peroxidase increased one and tow folds during germination, and MDA (Malondialdehyde) content presented one "L" type curve with lowest point at soaked process. At the end of germination, the MDA return to 1/2 content of raw oat which was 4.0×10-3μmol/g.2 Granules and aggregating structure of starches from germinated oatThe X-ray powder diffraction was used to characteristic granule crystal property of starches isolated from raw, soaked and germinated oat, all sample presented typical A-type patterns, only little difference in diffraction peaks was found at 2θ= 17°or 18°between samples, the crystallinity varied from 8.8% to 16.0%. Starch of soaked oat present the lowest crystallinity, while the 144 h one was the highest. All starch granules were polygonal or oval, and surface of granules was smooth and regular, and no size difference could be perceived under scanning electron microscopy (SEM). The size of most granules was smaller than 10μm, but a few aggregated granules were found in each sample. Using laser particle size analyzer evaluated particle size and distribution of all starch samples, and found that the size was bigger than that observed by SEM and the granules would shrink with germination time. The particle size of starch was distinguished at different concentration ethanol for the same sample, but the change order of D50 was same as follows:70% ethanol＞50% ethanol＞30% ethanol; 70% ethanol＞90% ethanol. The difference of special surface area was the biggest for the same starch at 50% ethanol, the special surface area of 144 h sample increased by 100 m2/Kg compared with raw sample, and reached 236.28 m2/Kg.3 Thermal properties of starches from germinated oatPasting property of oat starches was studied by DSC, no obvious difference was found between samples. The onset temperature, peak temperature and end-point temperature were 56.7-58℃,60.8-61.7℃and 71.2-75.6℃, respectively. During the second-half of germination, the pasting temperature band broaden, and the range of temperature of 144 h sample was the biggest one and up to 18℃. The pasting enthalpy of oat starch increased during this period, and the maximum value was 11.32 J/g. The thermogravimetric analysis (TGA) was employed to investigate thermal degradation of oat starches, and temperature range of degradation varied from 275℃to 363℃. As heating rate increased, degradation zone moved to higher temperature. According to degradation reaction analysis by Coast-Redfern method, all samples’thermal degradation was the first order reaction. The free energy of reaction would decrease with germination time from 219.44 KJ/mol to 157.75 KJ/mol.4 Rheology properties of starches from germinated oatThe 24 h sample presented the highest peak viscosity as oat starches was characterized by rapid viscosity analyzer (RVA), beside that, peak viscosity increased with germination time. The lowest viscosity of oat starch in medium germination stage was higher than that of samples isolated from the beginning and end of germination. However, the breakdown value of oat starch in medium germination stage was lower than that of samples isolated from the beginning and end of germination. The viscosity of oat starch pastes would increase with concentration and present property of pseudoplastic flow, and power low function can describe the property of oat starches very well. Thixotropy was obvious at high concentration, and thixotropy index would be steady as shear cycle increased. Oat starch pastes changed into soft gel as concentration decreased, the germination time longer, vary trend of soft gel was more obvious. Oat starch slurry was not appropriate to be characterized by dynamic rheology analyzer at 5% concentration. Starches from oat germinated for 120 h and 144 h presented the lowest G’, and their G’s only increased at higher temperature. Probably, those starches pack amylose very tough. The strength of gels formed by them was worse than that formed by other samples. Visco-elastic of starch from raw oat depended on temperature very tough, elastic of 96 h sample depended on concentration of itself, and viscosity of soaked sample depended on very weak.5 Structure characterizations of starches from germinated oatThe amylopectin and amylose content of oat starches were determined by size exclusive chromatograph (SEC), amylose in other samples gradually increased during germination and up to 25.8%, while amylose of starch in oat soaked reached 37.6% compared with 13.3% of raw oat. The intrinsic viscosity of soaked and 144 h samples were highest and reached 191 g·g-1 and 196.1 g·g-1, respectively, this value in raw oat was only 130.8 g·g-1. There were positive corrections between amylose content and intrinsic viscosity (correlation coefficient 0.86, P＜0.01), and between medium molecular weight content and hydrolysis index (GI) in vitro digestion (coefficient 0.82, P＜0.01). The ratio of short chains of starch decreased, and the same time the ratio of long chain of starch increased during oat germination. This is maybe because that the former was degraded for life activities of oat malt in preference to the later. The debranching of starch happened in fractions of 33＜DP＜45, and debranched part with low DP degraded into signal sugar and oligsaccarides. The results of in vitro digestion showed that the starch of raw oat had the maximum equilibrium concentration and GI of 102.21, and that GI and glycemia index would decrease in some content during different germination time. The photoelectron spectroscopy was used to check change of carbon and oxygen content in starch pastes surface of raw oat and oat germinated for 144 h, the result indicated that the later was not easy retrograde in comparison with the former. |
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