| Rice is one of the hardest preserved food grain varieties prone to aging deterioration during storage, causing huge economic losses. In order to control the rice ageing during storage effectively, the mechanism of ageing needs to be further elucidated. Rice ageing is a multi-component complex issue. This thesis focus on the improvement effects of protein changes on rice aging characteristics and its impact mechanisms, trying to reveal the mechanism of aging through the performing roles of protein modifiers and the effect of rice protein on the pasting properties.1. Changes of rice quality during storageThrough studying the variation of the quality indicators of rice during storage, it is important to find an effective indicator of the rice aging to evaluate the degree of aging and safety of rice. Milled rice was sealed in airtight bottles stored at37℃for accelerated aging, periodically sampling was carried out to determine the changes in selected physical and chemical indicators. The following results were obtained:(1) Fatty acid value, gel consistency and alkali spreading value of rice in storage changed irregularly, while the total amylose, hot water soluble amylose and hot water insoluble amylose content did not change significantly.(2) Rice odor evaluation by electronic nose showed that the bad smell is much larger than the good smell in contribution to the total odor of the stored rice.(3) The parameter changes in three color space mode showed conformably that the internal color of rice grain than its surface color is more white during storage, which suggests storage yellowing in rice originate from the surface of contact with air.(4) The changes of RVA pasting characteristic parameters showed that the peak viscosity and breakdown increased from0to2nd month, decreased from2to8th month, setback declined from0to2nd month, rose from2to8th month, while pasting temperature increased from0to8th month. In addition, the original two-stage swelling of stored rice merged into a one-time swelling after4months, showing that the ageing causes a higher swelling temperature of the amorphous region, which should swell prior to occurrence of the crystallization region. Because that amorphous region composes mainly of amylase, protein and lipid, it is speculated that ageing led to amylose, protein and lipid changes in the nature. In summary, the RVA characteristic parameters appear to be the most sensitive, regular and provide the richest information to characterize the degree of rice aging during storage. So, RVA characteristic parameters on behalf of pasting properties were considered as indicators in the subsequent chapters of this thesis to study the changes of rice quality during storage.2. Effect of pretreatment before storage on pasting properties of rice duration storageThe milled rice which was subjected to treatments before storage were accelerated to ageing at37℃to study the effects of changes in enzyme activity of Alpha-amylase and lipoxygenase, and in protein oxidation. The results showed that:(1) Alpha-amylase was not the main inducement of rice ageing, while the stabilization role of lipoxygenase by EDTA·2Na slowed the ageing of stored rice.(2) Among three reducing agents, Vc treatment before storage has the most important impact on rice aging. When the rice stored at37℃after4months, Vc as prooxidant accelerated rice aging, while tea polyphenols and DTT delayed rice aging during the storage period of4and8months, showing that rice aging was closely related to the oxidation process. Vc accelerated rice ageing duration storage of rice, and this finding has not been reported in the literature yet.3. The improvement role and its mechanism of post treatment after storage on pasting properties of aged riceIn order to find a similar or even better additives compaer to DTT which is food additives and can improve the quality of aged rice, and at the same time, to infer rice ageing mechanism through the nature and function of quality improvement of aged rice, the effects of treatments on aged rice were investigated. The results followed that:(1) Sodium sulfate treatment showed that aging decreased the normal water absorption speed of rice.(2) DTT treatment before storage (Chapter3,2.2.1DTT treatment before storage at0month) with the fresh rice treated (treated in pasting process) has the same effect, suggesting that DTT play a role in the heated gelatinization process rather than in before pasting which react directly with compound in the rice. The increasing of starch granule swelling degree of aged rice by DTT treatment was very limited, yet Vc showed bigger enhancing role than DTT on the swelling of aged rice, even more swelling than the fresh rice as the control. In contrast, tea polyphenols only retards the cooling retrogradation of amylase. The facts indicate that the disulfide cross-linking is not the only major change when the rice aging occurs, there may be other force to prevent the starch granule swelling except for disulfide bonds, and Vc could destroy the main force of inhibiting aged rice from swelling, and showed a dose-effect relationship, confirming other aging mechanisms exist except the sulphydryl is oxidized to the disulfide bonds. The effect of SDS on aged rice showed that a large number of hydrogen bonds and hydrophobic interactions might be formed in rice ageing, and these forces inhibited gelatinization swelling of the starch granules.(3) the results of water absorption index, soluble solid and microscopic images confirmed that Vc simultaneously promotes water absorption and soluble solids, while DTT only promoted water absorption but inhibited soluble solid, and the role of Vc in promoting water absorption was much higher than that of DTT. This was different in mechanism of improving pasting properties between Vc and DTT, and also showed that water absorption of starch granule was more important than losses of the soluble solids on the pasting properties.(4) the effect of SH-blocking agent NEMI on fresh rice showed that the disulfide bond exists only in the swelling starch granules. The results of NEMI dealing with aged rice showed that aged rice in the gelatinization process could still form disulfide cross-linking which make the pasting viscosity increased, but the contribution of sulphydryl cross-linking on the viscosity of aged rice paste was very small, and that of swelling degree (Vc promoting swelling) was bigger. The comparison of treatment results between Vc separately and in combination with NEMI indicated that Vc prevented the formation of disulfide cross-linking of aged rice in gelatinization process.(5) the effect of Vc treatment on aged rice of accelerated ageing by Vc before storage showed that the Vc does have a dual function on rice ageing, which was that the treatment before storage on fresh rice accelerated aging and the treatment after storage on aged rice improved pasting properties. This once again showed the nature of the rice aging was associated with oxidative changes.4. Effects of rice storage protein on pasting properties of aged riceRemoving protein from aged rice after storage were carried out to explore the effect of rice protein changes in ageing on pasting properties, and removing protein from fresh rice before storage were carried out to explore the combined effect of rice protein changes and interactions with other components in ageing process on pasting properties. The results were:(1) the changes of albumin and globulin in aging process promoted rice ageing, the changes of glutelin and prolamin in ageing process delayed rice ageing, while the changes of fat in ageing process have a dual role on rice ageing.(2) the combined effects of each component changes and the interactions with other components in ageing process were as that:fat and albumin promoted rice ageing, globulin and prolamin delayed rice ageing, while glutelin played dual role on rice ageing. Comparison of above mentioned imformation suggest that rice ageing is not caused by separated change of one or several components, but is more related to the interactions between the various components, especially the interactions between the globulin and other components, followed by the interactions between glutelin and fat and other ingredients.5. Structural changes of rice storage protein in ageing processThe structural changes of the four rice storage protein and interactions between them and other components during ageing process were characterized using infrared and Raman spectroscopic techniques in this chapter. The results were following:(1) Infrared spectrum showed albumin may be glycoprotein, and albumin, no matter it was in fresh or aged rice, both presents sulfoxide peak. Raman spectrum showed that after ageing, the random coil in albumin reduced, the protein hydrophobic group buried, and the intermolecular hydrogen bonding decreased, probably suggesting that the associations between albumin and starch molecules become looser.(2) Infrared spectrum showed that sulphydryl oxidation of globulin from aged rice was more obvious than fresh rice with main products of sulfone and sulfoxide, and glycosylation trend of age rice globulin significantly increased duration storage. Raman spectrum indicated that random coil of the globulin increased and sulfoxide occurred after rice ageing, and the degree of intermolecular hydrogen bonding strengthened, suggesting that ageing promotes binding of globulin and starch molecules.(3) Infrared spectrum showed the glycosylation trend of age rice glutelin significantly decreased than that of fresh rice. Raman spectra displayed that the random coil in gluten reduced after aging and free sulfhydryl peaks appeared.(4) Infrared spectrum of prolamin after ageing have no obvious changes, Raman spectrum reveal anti-parallel β-sheet structure in aged rice prolamin increased, the conformation of disulfide bond changed, tyrosine residues changed embedded state into the exposed state. In summary, after aging, the interactions between globulin and starch presented the greatest change, followed by glutelin, while the interactions between albumin and prolamin and starch changed insignificantly. The interactions of globulin with starch molecules were strengthened, and the interactions between glutelin and starch molecules were weakened. |
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