| In recent years,with the improvement of consumers'health awareness,there is a increasing demand for new type of whole-grain foods which are both convenient and nutritious.Therefore,the research object in this study was whole-grain brown rice.Firstly,the phenolic composition and the thermal stability of phenolic extracts of brown rice was studied.Afterwards,the effects of extrusion processing on phenolics and antioxidant activity of brown rice were studied,and the results were compared with those of wheat and oat.Then the effects of extrusion with thermostable and mesophilic?-amylase on the phenolics and antioxidant activity of brown rice were investigated respectively.Finally,the phenolic content and antioxidant activity of extruded brown rice were furtherly improved by Choerospondias axillaris fruit peels addition,and meanwhile the in vitro starch digestibility and expected glycemic index were significantly decreased.The main conclusions of this study are as follows:There were 27 phenolics and phenolic derivatives were identified in phenolic extracts of brown rice,and most of them were phenolic glycosides.The content of phenolics and flavonoids of phenolic extracts of brown rice were decreased by5.7%9.2%?3.5%5.8%respectively after 30 min of hydrothermal treatment at 60,80 and 100°C,and the antioxidant activity determined by DPPH,FRAP,and ABTS assay were decreased by 4.9%5.5%,0.5%1.3%,6.5%7.4%,respectively.After120 min of hydrothermal treatment,the content of phenolics and flavonoids were decreased by 4.5%7.3%,1.1%4.6%respectively,the antioxidant activity was decreased less than 10%,and there was no significant difference in the effect of different temperatures on phenolic content,and the retention rate of flavonoid content was the highest after 100°C treatment.In addition,the content of ferulic acid and p-coumaric acid in phenolic extracts of brown rice were obviously increased,while the content of corresponding phenolic glycosides were obviously decreased,and the content of glucose was obviously increased with the increase of treatment temperature.The above results indicated that phenolics of brown rice might undergo deglycosylation during hydrothermal treatment,but the majority of the phenolics and antioxidant activity were still remained after hydrothermal treatment.After extrusion processing,the content of free form of phenolics,flavonoids,phenolic acids,and antioxidant activity in brown rice,wheat,and oat were decreased by 24.3%69.0%,23.7%52.6%,4.9%45.7%,14.4%63.1%respectively,while the content of bound form of phenolics,flavonoids,phenolic acids,and antioxidant activity were increased by 3.6%11.9%,17.4%57.6%,6.4%49.1%,1.6%8.2%respectively,and the bioaccessible phenolics and antioxidant activity of the three whole grains were decreased by 13.5%30.2%,4.6%21.3%,respectively.The in vitro bioavailability of phenolic acids were extremely low?<1%?before and after extrusion,and most of the bioaccessible phenolic acids were from the free phenolics.Furthermore,the water absorption index?WAI?,water solubility index?WSI?,and pasting properties of the three whole grains were changed significantly after extrusion,indicating that the structure of brown rice starch was obviously changed during extrusion,thus significantly influenced the the interaction of phenolics with starch and the extraction efficiency of phenolics.After enzymatic extrusion with 0.1%thermostable?-amylase?ETA?,the content of free phenolics,antioxidant activity and total phenolic acids of brown rice were increased by 23.2%,5.8%18.9%,53.7%respectively compared with those of normal extruded brown rice,and they were significantly increased with increasing enzyme addition.However,the content of free flavonoids in extruded brown rice was significantly decreased with increasing enzyme addition.Additionly,the brown rice starch was significantly hydrolyzed during ETA processing,the reducing sugar content was significantly increased,the WAI,WSI,and pasting properties of extruded brown rice was significantly changed.The reducing sugar content in extruded brown rice was significantly positively correlated with the content of free phenolics,total phenolic acids and antioxidant activity.Therefore,the degradation of brown rice starch and the protective effect of reducing sugars during ETA processing played an important role in the retention of free phenolics and antioxidant activity.After enzymatic extrusion with 0.1%mesophilic?-amylase?EMA?,the content of free phenolics,antioxidant activity and total phenolic acids of brown rice were increased by 69.6%,49.3%89.4%,10.8%respectively,and the content of free flavonoids was increased by 85.1%,compared with those of normal extruded brown rice,and they were obviously increased with the increasing enzyme addition.Additionly,the brown rice starch was significantly hydrolyzed during EMA processing,the reducing sugar content was significantly increased,the WAI,WSI,and pasting properties of extruded brown rice was significantly changed.The reducing sugar content in extruded brown rice was significantly positively correlated with the content of free phenolics,flavonoids,total phenolic acids and antioxidant activity.Therefore,the degradation of brown rice starch and the protective effect of reducing sugars during EMA processing played an important role in the improvement of the retention rate of free phenolics and antioxidant activity.The comparison results of EMA and ETA showed that although the EMA processed brown rice had lower reducing sugars,it had higher retention rate of free phenolics,flavonoids and antioxidant activity.Therefore,the effect of extrusion temperature on the free phenolics,flavonoids and antioxidant activity was more critical than the enzymatic hydrolysis of?-amylase and the protective effect of reducing sugars.Choerospondias axillaris fruit peels?CFP?was rich in free phenolics?5.2×104?g GAE/g DW?and free flavonoids?4.0×104?g CE/g DW?.After CFP addition,the content of phenolics,flavonoids and antioxidant activity of extruded brown rice were obviously increased,and they were significantly positively correlated with the addition amount of CFP?r=0.9960.999?.Besides,the WAI,WSI,and pasting properties of extruded brown rice was significantly changed after CFP addition.When the addition amount of CFP was 1%,the“equilibrium hydrolysis rate”?74.2%?,kinetic constant(14.7×10-3)and expected glycemic index?79.8?of extruded brown rice during in vitro starch digestion were significantly lower than those of extruded brown rice without CFP addition.,which may be due to the inhibition effect on digestive enzymes by the free phenolics and flavonoids in CFP.However,the physicochemical properties of extruded brown rice was significantly changed after a large amount of CFP addition?2%10%?,and the interaction of phenolics with starch might be enhanced,thus the in vitro starch digestibility showed no significant difference compared with that of normal extruded brown rice. |
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