| Citrus flavonoids,a kind of important plant dietary flavonoids,are mainly distributed in the pericarp of citrus.They possess a variety of physiological functions related to human health,such as anti-oxidant,anti-cancer,anti-inflammatory,anti-bacterial and anti-allergy activities and so on.They can be divided into flavanones,flavones,isoflavones,flavonols and anthocyanidins based on their chemical structures among which flavanones are the most abundant and the common ones including hesperidin and its aglycone hespertin,naringin and its aglycone narinenin.However,citrus flavonoids generally have the problem of low bioavailability due to their poor water solubility,which greatly limit their direct application in food,medicine and cosmetics industries.The research on the solubilization behaviors and their mechanisms of citrus flavonoids is currently in initial exploration stage,so it is necessary to select appropriate solubilization supplements and methods for encapsulating the industrially extracted citrus flavonoids and it is also an important problem to be solved urgently in citrus by-product industry in China.Octenyl succinic anhydride modified starches(OSAS),also called pure gum,are the modified starches produced by esterification of starches with octenyl succinic anhydride(OSA)under weak alkaline conditions.They are presently the only alkenyl starch esters allowed in food and there are no limited amounts to the application but their degree of substitution(DS)should be less than 0.02 according to the regulations of the US Food and Drug Administration(FDA).OSAS,as a new amphiphilic surfactant,owns dual hydrophilic and hydrophilic properties and good emulsification and stability.OSAS has become a hot spot in the research and development of modified starches at home and abroad in recent years.Four kinds of starches(corn starch,wheat starch,potato starch and sweet potato starch)were selected to prepare OSAS with different DS and their physicochemical and structural properties were characterized.The effects of OSAS prepared from different starch origins,different mass concentrations and different degrees of substitution on the solubilization behaviors of hesperidin and hesperetin were studied and the difference of solubilization behavior between them was also analyzed.The contents and results are as follows:1.The four most common starches(corn starch,wheat starch,potato starch and sweet potato starch)on the market were selected for OSA modification.Different DS of OSAS were prepared by controlling the reaction time.The results showed that DS and reaction efficiency(RE)significantly increased with an increase in reaction time(p<0.05),its order was octenyl succinic anhydride modified corn starches(OSA-CS)>octenyl succinic anhydride modified sweet potato starches(OSA-SPS)>octenyl succinic anhydride modified wheat starches(OSA-WS)>octenyl succinic anhydride modified potato starches(OSA-PS).The DS of OSA-CS reached 0.0163 at 2 hours.In addition,some physicochemical properties of native and modified starches were analyzed,including transparency,retrogradation,viscosity and gelatinization properties.The transparency of OSA-CS,OSA-WS,OSA-SPS significantly increased with the increase in DS,however,OSA-PS significantly decreased from 73%to 58%(p<0.05).The stability of OSA-CS,OSA-WS,OSA-SPS and OSA-PS significantly improved with an increase in DS(p<0.05).Compared with native starches,the viscosity of OSA-CS,OSA-WS,OSA-SPS and OSA-PS significantly increased with elevated DS.But its gelatinization temperature and time significantly decreased with an increased in DS(p<0.05),indicating that esterified starches were more easily gelatinized.2.The structural characteristics of OSAS with different starch origins and DS were analyzed using modern analytical techniques.The results of scanning electron microscope(SEM)showed that native and modified starches of corn and sweet potato had polygonal structures,while those of wheat and potato were elliptical ones.Compared with the smooth surface of native starches,the surface of their OSAS became rough and the damage degree became more obvious with elevated DS.Fourier transform infrared spectroscopy(FT-IR)results showed that OSA-CS,OSA-WS,OSA-SPS and OSA-PS all had a new absorption peak at 1724 cm-1,corresponding the stretching vibration of ester carbonyl RCOO-.Its absorption intensity gradually increased with an increase in DS,indicating that the esterification reaction had indeed occurred and OSA groups had been successfully introduced into the starch skeleton.X-ray diffractometer(XRD)results showed that native and modified starches of corn,wheat and sweet potato had strong diffraction peaks at 15°,17°,18° and 23° which specificly exhibited starch of A-type diffraction,whereas native and modified starches of potato appeared sharp diffraction peaks at 5.6°,14.8°,17°,22° and 24°which belonged to B-type diffraction one.The crystalline form of starches did not change after esterification,but its crystallinity significantly decreased with an increase in DS(p<0.05).In conclusion,the esterification did exist and primarily happened in the amorphous regions and the surface of starch granules rather than in the crystalline one.3.The solubility of hesperidin generally decreased with an increase in mass concentrations of OS AS in the region of 1-4%.OSA-SPS on the solubility of hesperidin was better than that of OSA-CS,OSA-WS and OSA-PS.The solubility of hesperidin increased with elevated DS firstly and then decreased after a certain value,indicating that the excessive amount of anhydride would inhibit the solubility of hesperidin.In addition,OSA-SPS had the best solubility on hesperidin in a concentration of 0.8%and DS=0.0051 through further optimization.The optimum results of external conditions on the solubility of hesperidin were as follows:stirring power 2400 r/min,stirring time 6 h,stirring temperature 50℃ and unadjusted pH.Under above these conditions,the solubility of hesperidin was improved by 6.52 times compared with that of the control(without addition of OSAS).Different from their physical mixture,hesperidin has been encapsulated into OSA-SPS by means of FT-1R,XRD and DSC and the inclusion complex model was speculated by means of NOESY and 1H NMR,indicating that hesperidin did enter into the inner cavity of OSA-SPS from the wide port side of H-3 protons.4.The solubilization efficiency of hesperetin gradually increased with an increase in mass concentrations of OSAS in the range of 1-4%(the solution would be supersaturated above 5%).OSA-CS on the solubilization efficiency of hesperetin was better than that of OSA-WS,OSA-SPS and OSA-PS and it would reach the highest at a mass concentration of 4.4%and DS=0.0158 of OSA-CS.The effects of DS on the solubilization efficiency of hesperetin was the same as that of hesperidin and showed a trend of rising first and then falling.The solubilization efficiency of hesperetin would reach the best at stirring power 1600 r/min,stirring time 4 h,stirring temperature 40℃ and unadjusted pH and it was 9.87 times higher than that without OSAS under the same conditions.The inclusion between hesperetin and OSA-CS has been confirmed by FT-IR,XRD and DSC and the inclusion model has also been deduced by 1H NMR and 2D NMR.Similar to the inclusion model of hesperidin and OSA-SPS,hesperetin also entered into the inner cavity of OSA-CS from the wide port side of H-3 protons.5.Although hesperetin is the hydrolysate of hesperidin,their solubilization was obvious difference.The absorbance of hesperidin gradually decreased with the stepwise increase in mass concentrations of OSAS,however,hesperetin’s absorbance generally increased.OSA-SPS was the optimum to solublize hesperidin,while OSA-CS was most suitable for hesperetin.The optimal DS was 0.0051 for OSA-SPS to solublize hesperidin,while that for OSA-CS to solublize hesperetin was 0.0158.It could be concluded that OSAS with polyhedral structure and crystalline type A had better solubilization on hesperidin/hesperetin based on the analysis of the structural characteristics of OSAS.The effects of external factors on the sobulity of hesperidin/hesperetin were also some differences.The solubility of hesperidin was the best when stirring power was 2400 r/min,while that for hesperetin was the optimum at 1600 r/min.The optimal stirring temperature for hesperidin was 50℃,but 40℃ was the best for hesperetin.The stirring time for hesperidin was the best at 6 h,whereas hesperetin only took 4 h.The main reasons for these above phenomena was that the molecular weight of hesperidin is larger than that of hesperetin.The results of FT-IR,XRD and DSC further confirmed that the inclusion of hesperidin with OSA-SPS was less than that of hesperetin with OSA-CS,but their action sites were the same,all from the wide port side of H-3 protons into the inner cavity of OSAS by means of 1H NMR and 2D NMR. |
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