Identification And Antioxidant Activity Of Phenolic Compounds In Brown Rice

There are high incidences of intestinal diseases and a variety of non-communicable chronic diseases when people are in the pursuit of fine grain processing.Regular consumption of whole grains and whole grain products has been correlated with reduced risk of developing various chronic diseases,including cardiovascular disease,type II diabetes,obesity,and cancer.The obsjective of the present study was to investigate the phenolic compounds and their health benefits,included the effect of variety,color,processing methods and digestion on the phytochemicals and antioxidant activity of brown rice.It will reveal the health benefits of whole grain rice to promote the development of whole grain foods and health grain consumption.The main results obtained in the present study were shown as follow:Ferulic acid is the predominant phenolics found in cereals,and its dehydrodimers（DFA）are also found in significant amounts.However,given the inherent differences of mass spectrometers and experimental conditions used in different laboratories,the MS spectra of the same DFA among different studies show significant variability.Thus,subsequent studies were unable to confirm the identity of individual DFAs rapidly through MS spectra.In the present work,a UPLC-DAD-ESI-Q-TOF-MS technique in CID mode was successfully developed to identify the dehydrodimers of phenolics in cereals,it solved the problem of rapid identification of DFAs.Six DFAs were detected in rice for the first time,i.e.syn-8-8′aryltetralin DFA,trans-cis-8-8′DFA,cis-cis-8-8′DFA,trans-trans-8-5′DFA isomer,trans-cis-8-5′DFA isomer and trans-cis-8-O-4′DFA.In addition,the determination method of phenolics by RP-HPLC in cereals was optimized.All the phenolics were separated in RP-HPLC.The cis-isomers of monomeric p-hydroxycinnamic acids（ferulic acid,p-coumaric acid,sinapic acid and caffeic acid）were all separated and detected.It solved the problem of detection of cis-isomers in RP-HPLC.The phytochemical content and antioxidant activity of eight varieties of brown rice were reported.The total phenolic contents of brown rice ranged from 72.45 to120.13 mg of gallic acid equiv./100 g.The phenolics from bound fraction contributed40.6%–50.2%of the total phenolic content.The total flavonoid content of brown rice were determined by the SBC method,it ranged from 75.90 to 112.03 mg catechin equiv./100 g,which were 43%–112%higher than the values of rice determined by the traditional AlCl₃ colorimetric method.The flavonoids from the bound fraction contributed 26.9%–48.2%of total flavonoids.Trans-ferulic acid was the predominant phenolic acid in brown rice.Total trans-ferulic acid content ranged from 161.42 to374.81μg/100 g.The percentage of trans-ferulic acid in bound fraction ranged from96.4%to 99.2%.Onlyα-andγ-tocopherols and-tocotrienols were detected in brown rice withα-tocopherol andγ-tocotrienol being the predominant.The total peroxyl radical scavenging capacity（PSC）of brown rice ranged from 18.29 to 40.33 mg vitamin C equiv./100 g.The bound fraction contributed 67.2%–77.2%of total PSC.Boosted regression trees analysis（BRT）was used to analyze the contribion of phenolic compounds to the antioxidant activity of brown rice.Quercetin and trans-p-coumaric acid were the most significant compounds,contributing 25%and 25.7%,respectively,to the antioxidant activity in the free fraction.On the other hand,syringic acid and trans-ferulic acid were the most significant variables in the bound fraction（with contributions of 20.4%and 15.6%）.Additionally,other predictor variables（cis-p-coumaric acid,catechin and vanillic acid）were also important in both extracts.The phytochemical contents and antioxidant activities of free and bound fractions of rice with different colors were reported.Black rice had highest total phenolic content and total flavonoid content in free fraction,followed by red,brown,and white rice.Black rice had the highest percentage contribution of free flavonoids to free phenolics（81.0%）,followed by red rice（55.8%）,brown rice（50.9%）and white rice（29.8%）.Black rice had highest total phenolic content in bound fraction,followed by red,brown,and white rice.However,Red rice had highest total flavonoid content in bound fraction,followed by black,brown,and white rice.Contribution of bound flavonoids to bound phenolics was highest for brown rice（31.1%）,followed by red rice（28.5%）,white rice（22.1%）and black rice（21.7%）.Trans-ferulic acid was the predominant phenolic compound in rice.Black rice had much more free phenolic compounds than other rice,such as protocatechuic acid,vanillic acid,catechin,and quercetin.Total in vitro antioxidant activity was highest for black rice,followed by red,brown and white rice,with the free fractions contributing 96.6%,88.3%,39.0%and 38.6%,respectively.Total cellular antioxidant activity was highest for black rice,followed by red rice,brown and white rice,with free fractions contributing 97.1%,63.2%,33.6%and 33.3%in the no PBS wash protocol,and contributing 97.5%,82.6%,15.8%and 18.9%in the PBS wash protocol,respectively.Tocopherols and tocotrienols contents were highest in red rice,follow by black,brown,and white rice.PCA allowed differentiation among the two fractions of rice with different colors.It was shown that seven phenolics（PRA,CA,CHA,CFA,SYA,QU,and QUG）,TPC,TFC,PSC and CAA were most likely concentrated in the free fraction of black rice,while nine phenolics（p-HA,VA,p-COA,trans-FA,cis-FA,8-5′DFA,5-5′DFA,8-O-4′DFA and 8-5′benzofuran DFA）were most likely concentrated in the bound fraction of black,red,and brown rice.The effect of three staple food processing methods（cooking,long and short screw extrusion）on the phytochemical profiles and antioxidant activity of free,soluble-conjugated,and bound fractions of brown rice was studied.The total phenolic contents and antioxidant activities of free and soluble-conjugated fractions were decreased after processing,whereas those of bound fraction were increased by cooking and long screw extrusion,but not affected by short screw extrusion.RP-HPLC was used to analyze the changes of the free,soluble-conjugated,and bound phenolics of brown rice.Processing increased the content of free phenolic acids,but decreased the content of soluble-conjugated phenolic acids.The content of bound phenolic acids was increased by cooking and long screw extrusion,but not affected by short screw extrusion.PCA allowed differentiation among the three fractions（free,soluble conjugated,and bound）,the TPC,PA composition and AOA（TEAC,FRAP,and PSC）were most likely concentrated in the bound fractions.The effect of in vitro simulated gastrointestinal digestion on the phytochemical profiles and antioxidant activity of brown rice and other three grains（wheat,corn and oat）was studied.It was found that the digestion process of residues after extraction of free phenolics of grains could release the phenolic compounds which were bound to the proteins.After determination using HPLC,the main phenolic compounds were phenolic acids and flavonoid,and DFAs were found in the digestive fraction for the first time.After digestion the total phenolic contents of grains were significantly increased,the digestive fraction contributed 57.7%–79.6%to the total phenolic contents of grains.The digestive fraction also had significant in vitro antioxidant activity and contributed 6.3%–24.9%of the total antioxidant activity.However,no cellular antioxidant activity was observed in the digestive fractions of the grains.The present research expanded the free and bound fractions in the antioxidant research of grains into three fractions:free,digestive and bound,which would provid reference for the release of different forms of phenolics in different digestive parts of the human body.