| Electron beam treatment technology has got much attention because of its good insecticidal and sterilization effect,security,environmental protection and controllability.As a highly toxic,flammable liquefied gas,phosphine has been used as a fumigant in China to control the pest and microbial problems during wheat storage.So electron beam irradiation replacing phosphine fumigation has become a studying trend in the development of science and technology.However,studies on changes in the number of pests and microorganisms and the quality of wheat treated by electron beam during storage were rarely reported.In this study,wheat kernels were treated by electron beam at different doses(1,3,5 k Gy)and then stored at 25°C and 35°C for 8 months.By measuring the number of pests and microorganisms,determining the storage stability of wheat and the pasting property and processing quality of wheat flour during 8 months storage,then characterizing the molecular structure of wheat starch and network structure of gluten protein before and after storage,the reason for changes in quality of electron beam irradiated wheat was revealed.Firstly,the changes in the number of pests and microorganisms in wheat during 8months storage were studied.The result showed that untreated wheat was severely infected by pests after 8 months of storage,but the electron beam-treated wheat was not attacked.After treating at 1,3 and 5 k Gy,the bacteria count in wheat decreased from 2.6×10~5 CFU/g toCFU/g to 100 CFU/g,85 CFU/g and 45 CFU/g,respectively.At 25°C storage,the bacteria count in all untreated wheat were above 1.1×10~4 CFU/g,and the mould count increased to2.5×10~5 CFU/g in the later period of storage.While the bacteria count and mould count in all treated wheat were significantly lower than those in untreated wheat,and the microorganisms count decreased with the increasing of dose.At 35°C storage,the changes of microorganisms count were similar to those at 25°C,but the microorganisms count of wheat was lower due to the decrease of moisture content.In general,electron beam treatment at 1 k Gy dose could effectively kill microorganisms and pests in wheat and inhibit the growth and reproduction of them in the storage process,in order to enhance the storage stability of wheat.Secondly,the storage stability of wheat treated by electron beam was investigated.The results showed that the activity ofα-amylase and the falling number decreased significantly after irradiation,and the range of reduction was positively correlated with the dose of electron beam.But the fatty acid value and the moisture content of wet gluten were slightly increased with the increasing of dose.During the storage at 25°C and 35°C,the lipase activities of the four kinds of wheat samples(0,1,3,5 k Gy)showed a tendency to increase firstly and then decrease.In the later period of storage at 25°C,lipase activity of electron beam treated wheat was significantly lower than that of untreated samples.The fatty acid values of all the four kinds of wheat samples showed an increasing trend,and respectively increased to 42.67,40.86,33.71 and 34.24 mg KOH/100 g after storage at 35°C for 8 months.The activity ofα-amylase in the four kinds of wheat samples showed a slight decrease,but the extent of change in treated wheat was lower than that in untreated wheat.The falling number of the four kinds of wheat samples increased,and the extent of increase after 5 k Gy treatment reduced greatly,but remained above 400 s during the storage.The moisture content of wet gluten of all samples which were all more than 180%showed a slight decrease trend,and the values of treated samples by electron beam were higher than those of untreated wheat.After 8months storage at 25°C and 35°C,not only did the quality of wheat irradiated at three doses meet the wheat storage standards,but their storage stability also enhanced since electron beam treatment reduced the lipase activity and the increase rate of fatty acid value.Thirdly,the pasting properties and the dough processing qualities of wheat flour samples were analyzed.The results showed that higher electron beam irradiation dose had greater influence on the pasting properties and the dough processing qualities of wheat flour.When the dose was up to 5 k Gy,the values of peak viscosity,setback viscosity,pasting temperature decreased by 57.51%,55.23%,1.88%,respectively.What’s more,the water absorption rate of wheat flour increased by 3.42%,stable time decreased by 41.57%and weakening level increased by 73.64%,which indicated that irradiation destroyed the structure of gluten and starch in wheat.Those resulted in the decrease of viscosity of wheat flour,destroyed the gluten network structure,reduced dough stability and gluten strength.During the storage at25°C and 35°C,the peak viscosity and final viscosity of all the samples increased,while the breakdown viscosity increased firstly and then decreased.Meanwhile,pasting properties changed greatly with the rise of the temperature.The changes extent of irradiated wheat was less than that of untreated wheat,which indicated that irradiated wheat could keep the pasting properties that they were prone to gelatinization and was not easy to retrogradation.After 8months storage at 25°C and 35°C,dough weakening level decreased significantly,and the differences between treated wheat and untreated wheat decreased.At 25°C storage,the dough stable time of untreated and treated wheat at the dose of 1,3 k Gy increased,however,the dough stable time of all the untreated and treated wheat samples decreased at 35°C storage,indicating that the dough stability decreased at higher temperature,but the capacity of resistance to stir of all the samples increased.Thus,wheat treated at 1 k Gy irradiation dose could keep the processing properties at low temperature.Finally,the changes of the starch molecular structure and the gluten network structure of the four wheat samples were investigated.The results showed that electron beam treatment significantly affected the starch molecular structure and the gluten network and the influence was greater with the increasing of irradiation dose.When the irradiation dose reached to 5k Gy,the molecular weight of amylose and amylopectin decreased by 32.46%and 23.38%,meanwhile the proportion of long chain(DP≥37)in starch reduced and the proportion of short chain(DP≤12)increased,which indicated that irradiation destroyed the ordered structure of starch and degraded it.Electron beam treatment could make free sulfydryl groups increase as the disulfide bond was destroyed,which resulted in the weakening of gluten network structure.The SEM images showed the same results.After 8 months storage,the starch structure of all the four wheat samples had no remarkable change.However,free sulfydryl groups significantly reduced.The results of SEM images also showed that the gluten network structure was improved after storage and became strengthened. |
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