| As one of the traditional foods consumed in China, soymilk is not only rich in high-qualityproteins and essential fatty acids, but also rich in bioactive compounds, such as soybeanisoflavones, gallic acid and catechinic acid. However, the content and antioxidant activity ofbioactive compounds are easily affected depending on soybean varieties and processing,especially heating. Besides bioactive compounds, soymilk also contains antinutrition factorssuch as trypsin inhibitors. Heat treatment is one of the most efficient ways to inactivate theiractivities. However, phenols and isoflavones are very sensible to thermal, and prone toconversion and degradation during heat treatment. To date, the antioxidant activity ofbioactive compounds (such as isoflavones) in soymilk is widely reported. Nevertheless, asystematic study about the influence of soybean variety, heat treatment and gastrointestinaldigestion on the profiles and activity of bioactive compounds in soymilk is still needed. Forthis purpose, soybeans of10widely cultivated varieties were used as materials. The effect ofsoybean variety, heat treatment, heat treatment combined with in vitro gastrointestinaldigestion on the profiles and activity of bioactive compounds in the processed soymilks werestudied. Moreover, the protective effect of digested soymilks on oxidative stress was alsoinvestigated. The results of the present study can provide meaningful information for theunderstanding of changes and potential health-promoting effect of bioactive compounds in theprocessed soymilks during the processing of heat treatment and gastrointestinal digestion.Meanwhile, it is useful for the production of high quality soymilks with higher antioxidantactivity and bioaccessibility. The main results are as followings:The total phenolics, total flavonoids and isoflavones contents as well as the antioxidantactivities [1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity, ferric reducingantioxidant power (FRAP) and oxygen radical absorbing capacity (ORAC)] of soymilks weresignificantly influenced by soybean varieties. The influence of processing procedure on theprofiles and antioxidant activity changes of soymilk varied with the soybean variety applied.Compared with grain soybean, the content of total phenolics and total isoflavones in rawsoymilk was declined. For most soymilks, the DPPH radical scavenging activity and ORACwere decreased whereas FRAP was increased. In raw soymilk, significant correlations wereestablished among total phenolics content, total flavonoids content, DPPH radical scavengingactivity and FRAP.In order to investigate the influence of temperature and time on the content and profile of isoflavones,95℃,121℃and143℃were applied to simulate pasteurization, in-containersterilization and ultra-high-temperature (UHT) which are widely used in industry. Thedegradation kinetics of predominant isoflavones in soymilk was also studied. Results showedthat heat treatment enhanced the conversation and degradation of individual isoflavones.Significant decreases of malonyldaidzin and malonylgenistin contents were induced by heattreatment. It was found that decrease of malonylglucosides caused increase of glucosideisoflavones at95℃, and aglycone and glucoside isoflavones at121℃. In addition,95℃and121℃treatment enhanced the conversation of individual isoflavones whereas143℃treatment enhanced the degradation of isoflavones. The conversation and degradation ofmalonyldaidzin and malonylgenistin followed the first-order reaction kinetics at heatingtemperature95℃,121℃and143℃. Moreover, the degradation rate constants increased withthe increase of temperature.In vitro gastrointestinal digestion was applied to investigate the content and antioxidantactivity changes of bioactive compounds in the heat-treated soymilks during gastrointestinaldigestion. Results revealed that the acidic pH of in vitro gastric digestion enhanced the releaseof phenolics and flavonoids from soymilk, improved the DPPH radical scavenging activity,FRAP and ORAC. In addition, gastric digestion increased the content of glucoside isoflavoneswhereas decreased the content of aglycone isoflavones. Compared with gastric digestionfraction and duodenal digestion fraction, the absorption fraction (represents the fraction canbe absorbed and utilized by human) had much lower total phenolics, total flavonoids andisoflavones contents and antioxidant activity. The bioaccessibilities of total phenolics, totalflavonoids and total isoflavones were detected as97.27-120.01%,34.63-67.19%and34.40-41.22%respectively. Heat treatments inceased the bioaccessibility of total phenolicswhereas most of the heat treatments increased the bioaccessibility of total flavonoids (exceptfor143℃-40s). Daidzein and its derivates were proven as the most bioaccessible isofalvones.Glucoside isoflavones showed the highest bioaccessibilities followed by malonylglucosides,acetylglucosides and aglygcones. The DPPH radical scavenging activity and FRAP ofabsorption fraction were decreased by73.85-78.08%and30.93-50.93%as compared withthose of undigested soymilk. Heating at95℃increased the FRAP and ORAC of absorptionfraction. Overall, the soymilks treated at95℃60min and121℃9min had higher bioactivecompounds contents, antioxidant activities and bioaccessibilities in the absorption fractions.The present study also evaluated the potential protective effect of raw and heat treateddigested soymilk against the chemical oxidative stress induced by 2,2′-azobis(2-amidinopropane) dihydrochloride (AAPH) on human erythrocytes. Resultsshowed that the digested soymilks effectively inhibited the hemolysis of erythrocytes inducedby AAPH. Heat treatment prior to in vitro gastrointestinal digestion improved the protectiveeffect of digested soymilk on erythrocytes hemolysis, since the hemolysis inhibition rate wasincreased from50.48%to54.28-88.02%. However, the hemolysis inhibition rate wasdecreased with the further increase of temperature. Results revealed the digested soymilksprotected erythrocytes from oxidative damage by enhancing the enzymatic and non-enzymaticdefence system. In other words, digested soymilks alleviated the hemolysis of erythrocytesand lipid peroxidation resulted from oxidative stress by suppressing the accumulation of ROS,reducing the increase of SOD activity and decrease of non-enzymatic antioxidant GSH andenzymatic antioxidant GPx activity. Compared with raw soymilk, heat treatment improved theprotective effect of digested soymilk on erythrocytes against oxidative stress via improving itsfree radicals scavenging activity. |
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