| As the most common problem in rice storage,aging often leads to the deterioration of rice edible quality.At present,the research on rice aging mechanism needs to be further deepened.In order to avoid the interference of the outer layer of rice grains,this paper takes15% of the inner layer of rice grains removed by grinding as the research material,and grinds them into four kinds of rice grains(endosperm cell aggregates S1,starch complex granules aggregates S2,starch single granule aggregates S3,and starch single granule S4).The effects of soaking temperature,soaking time and ultrasonic treatment on the disaggregation between starch particles after rice aging were studied,as well as the changes in the properties of aged rice flours after ultrasonic disaggregation.The corresponding fresh rice sample was taken as a reference,so that the changes in the disaggregation between starch granules after aged were revealed through pretreatment,and the basis was provided for elucidating the aging mechanism of rice and improving the edible quality.(1)Effects of soaking conditions and particle size of rice flour on the disaggregation of starch granules after rice aged.The dyeing results showed that the starch granules in fresh rice had strong water absorption and swelling ability.After soaking at room temperature(25℃)for 10 min,the whole rice granules swelled,the granules were larger,the cell boundaries became blurred,and the protein bodies were less.In aged rice,the swelling of starch granules is small,the complex granules and cell structure are obvious,and a large number of protein bodies can be seen between complex granules and cell walls.To some extent,aging inhibited the swelling of starch granules,so it did not cause obvious extrusion or coating of protein bodies.Particle size distribution analysis shows that when the particle size of raw rice flour is100-125 μm(S1),prolonging the soaking time at room temperature can obviously promote the disaggregation of starch granules in fresh rice,and soaking for 16 h has the most obvious effect,but soaking time has no obvious effect on aged rice.Increasing the soaking temperature and prolonging the soaking time in the range of 25-55℃ can promote the disaggregation of starch granules in fresh rice and aged rice.Compared with the soaking time,the soaking temperature has a greater influence on the disaggregation of starch granules in aged rice.When the soaking time is 10 min,and the particle size of raw rice flour is 20-40μm(S2),increasing the soaking temperature can only obviously promote the disaggregation of starch granules in fresh rice,but has little effect on aged rice.When the particle size of raw rice flour is 8-20 μm(S3)and 3-8 μm(S4),the soaking temperature has no significant effect on the disaggregation of starch granules in fresh rice and aged rice,which may be due to the small particle size of raw rice flour itself which is difficult to disaggregate.According to the peak particle size of S1 and S2 particle size distribution,both fresh rice and aged rice can disaggregate into endosperm cells under the same soaking condition.The difference is that fresh rice is easier to disaggregate from endosperm cells into starch granules(single and multiple starch granules),while aged rice is difficult to disaggregate into starch granules.Therefore,as the particle size of raw rice flour decreases,it is more difficult for starch granules to disaggregate.After aging,when the particle size of raw rice flour is reduced to S2,increasing soaking temperature has no obvious effect on the disaggregation of starch granules in aged rice.These results show that it is more difficult to disaggregate starch granules after rice aged,and the results of optical microscopy well support the results of particle size analysis.The scanning electron microscope of cooked rice section shows that the starch in fresh rice is easier to gelatinize,and starch granules are easier to disaggregate,while starch granules in aged rice are more closely arranged.Moreover,the scanning electron microscope shows that the amount of water added has a great influence on the gelatinization degree of starch granules in rice grains.The limited water steaming method can’t completely gelatinize the starch granules in rice grains,and the shape of the granules is still intact.In fresh rice,less protein bodies can be observed and swollen,and most of the protein bodies can fall off with the starch granules,while the protein bodies in aged rice are tightly adhered to the gaps between the starch granules.These results show that the swelling degree of protein body decreases after rice aging,and the binding degree between protein body and starch particles increases,and it is difficult to separate starch particles,which may be an important reason for the change of physical and chemical properties of rice after aging.(2)Effect of ultrasonic treatment on the disaggregation and crystal structure of starch granules after aging.Fresh rice and aged rice S1、S2 were selected as ultrasonic treatment objects.The results of particle size distribution showed that the peak volume fraction of starch particles of fresh rice was larger than that of endosperm cells without ultrasonic treatment,while that of aged rice was the opposite,which indicated that the endosperm cells of fresh rice were easy to disaggregate and starch particles were easy to disaggregate.However,in aged rice,only endosperm cells were easy to disaggregate but starch granules were difficult to disaggregate.Therefore,the combination of starch granules was strengthened due to aging.After S1 ultrasonic treatment for 180 min and S2 ultrasonic treatment for 120 min,the starch granules in aged rice can also be completely disaggregated,which indicates that the close bond between starch granules caused by aging can be destroyed by ultrasonic treatment.The results of light and scanning electron microscope showed that the starch granules in aged rice gradually disaggregated with the extension of ultrasonic time,and the number of starch granules increased obviously.After ultrasonic treatment in aged rice S1 and S2 for 180 min and 120 min respectively,they were completely disaggregated into starch granules,which was consistent with the results of particle size distribution.Infrared analysis showed that the short-range ordering degree of starch molecules was enhanced to some extent after rice aged,while ultrasonic treatment decreased the ordering degree of starch molecules in fresh rice and aged rice.X-ray diffraction shows that the diffraction peaks of aged rice at 17 and 18 are stronger than those of fresh rice,which indicates that the microcrystalline bundles of starch after rice aged are strengthened,the crystallization degree is increased,and the microstructure becomes more compact.However,after ultrasonic treatment,the diffraction peak intensity of fresh and aged rice decreased,the crystalline structure of starch was destroyed to some extent,the amorphous structure increased and the crystallinity of starch decreased,but ultrasonic treatment did not change the crystalline form of starch in fresh or aged rice.The above results show that the starch granules are difficult to disaggregate after rice aging,and the short-range ordering degree and crystallization degree of starch are enhanced.However,ultrasonic treatment can obviously promote the disaggregation of starch particles in aged rice.Although it destroys the order and crystallinity of starch granules to some extent,it is not enough to change the molecular structure and crystallization type of starch.(3)Effect of ultrasonic treatment on pasting properties of aged rice.Compared with fresh rice,aged rice disaggregated starch granules after ultrasonic treatment,and then studied the changes of pasting properties after ultrasonic treatment,in order to reveal the influence of starch granules disaggregation on pasting properties in aged rice.The results showed that ultrasonic treatment improved the water binding capacity of aged rice by promoting the disaggregation of starch granules,and obviously improved the texture of aged rice gel.SEM results showed that the internal cells of aged rice flour particles were more complete before ultrasonic treatment,and the internal structure of rice gel was supported by cell walls and cell spacers,so the overall hardness was higher.After ultrasonic treatment,starch particles in rice flour particles gradually disaggregate,endosperm cells are destroyed,and the whole structure of rice gel is mainly supported by starch granules,thus the hardness decreases,which is consistent with the results of rice gel texture.The pasting properties of RVA of rice flour S1 showed that before ultrasonic treatment,the peak viscosity(PV)and breakdown(BD)of aged rice were significantly lower than those of fresh rice,while the hageding strength(HS),final viscosity(FV),setback(SB)and pasting temperature(Pa T)were significantly higher than those of fresh rice,which indicated that the edible quality of rice deteriorated due to aging.After ultrasonic treatment for 180 min,PV,HS,BD,FV,SB of fresh rice decreased,while Pa T increased.For aged rice,PV and BD increased first and then decreased,but the PV and BD values after ultrasonic treatment for 180 min were still significantly higher than those without ultrasonic treatment.HS value first increased and then decreased,but after ultrasonic treatment for 180 min,HS value was lower than that without ultrasonic treatment.FV,SB,Pa T decreased with the extension of ultrasonic time.For aged rice S2,compared with ultrasonic treatment for 0 min,after ultrasonic treatment for 120 min,the PV and BD values obviously increased,FV also increased,HS and Pa T obviously decreased,and the change trend was basically consistent with S1.The results showed that ultrasonic treatment of rice flour not only promoted the depolymerization of starch granules,but also improved the water binding capacity of rice flour,which obviously promoted the improvement of pasting properties of aged rice.Therefore,ultrasonic treatment can promote the disaggregatation of starch granules in aged rice,which can obviously improve the edible quality of aged rice.Therefore,ultrasonic treatment can promote the disaggregatation of starch granules in aged rice,which can obviously improve the edible quality of aged rice.To sum up,the main reasons for the deterioration of rice quality after aged are as follows:aging enhances the combination degree between protein bodies and starch granules,the ordering and crystallization degrees of starch granules are enhanced,and it is difficult to separate starch granules from each other.Soaking and ultrasonic pretreatment can obviously promote the disaggregation of starch granules in aged rice,and the effect of ultrasonic treatment is more obvious.The analysis of physiochemical properties showed that the water binding capacity,texture and pasting properties of aged rice were obviously improved after the starch granules were effectively separated by ultrasonic treatment.The research results of this topic provide fresh ideas for clarifying the aging mechanism of rice,regulating the quality of rice,and providing more basis for improving the storage and eating quality of rice and its products,delaying the aging deterioration speed and reducing the post-harvest loss of grain caused by aging deterioration. |
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