| Pomegranate peel polyphenols are a class of phenolic compounds with more hydroxyl groups in pomegranate peel,which account for 23.83% of the total pomegranate fruit.Pomegranate peel polyphenols have various biological functions,such as anti-oxidation,anti-aging,antibacterial,anti-inflammatory,and so on.China is rich in pomegranate resources,but the utilization value of pomegranate peels as main by-products is very low.In order to improve the comprehensive utilization of pomegranate resources,this paper studied the effect of sweep frequency and pulsed ultrasound on the stability of pomegranate peel polyphenols and revealed the mechanism of strcture change caused by ultrasound.The AB-8 resin and Sephadex LH-20 gel resin were used to prepare high purified ellagic acid from pomegranate peel.The antioxidant properties of six pomegranate polyphenols were analyzed,which determined the key ingredients that played an important role on antioxidant activity of pomegranate peel polyphenols.Finally this paper explored the interaction mechanism of gallic acid,ellagic acid,and punicalagin with ovalbumin,as well as the effects of this interaction on the ovalbumin structure and digestive characteristics,and antioxidant properties of pomegranate peel polyphenols.The main researches and conclusions are as follows:(1)The sweep frequency and pulsed ultrasound significantly changed the total content,·DPPH scavenging rate,and the contents of gallic acid,ellagic acid,punicalagin A,and punicalagin B in pomegranate peel extracts.The biggest impact of ultrasonic treatment was determined at conditions as follows: ultrasonic frequency of 28 kHz,power density of 50W/L,sweep cycle of 100 ms,amplitude of ±2 kHz,duty ratio of 77%,and ultrasonic time of120 min.Under this condition,the total content of polyphenols was decreased by 5.08%,and ·DPPH scavenging rate was reduced by 14.57%.At the same time,the gallic acid content in pomegranate peel extracts was reduced by 4.16%,ellagic acid content was increased by49.85%,and punicalagin A and B contents were decreased by 17.65% and 17.65%,respectively.The sweep frequency and pulsed ultrasound significantly decreased the contents of four polyphenol sandards.The contents of gallic acid,ellagic acid,punicalagin A,andpunicalagin B were reduced by 3.49%,4.8%,11.77%,and 3.52%,respectively.Mass spectrometry analysis showed that ultrasound could cause polymerization reaction of gallic acid and ellagic acid,which generated the corresponding five polymer and dimer,respectively.Punicalagin was decomposed into C24H14O16,C25H18O15,C24H14O15 by ultrasonic treatment.(2)The adsorption isotherm of AB-8 resin for pomegranate peel polyphenols was consistent with the Langmuir equation,which was the monolayer adsorption.Under the condition of sample injection concentration 5 mg/mL,sample injection speed 1.0 mL/min,gradient elution with 10%,30%,50%,and 70% ethanol,elution speed 1.0 mL/min,two fragments(P1 and P2)were collected,respectively.The purity of ellagic acid in P1 fraction was 40.57%,and the purity of punicalagin was 60.43% in P2 fraction.Compared with the pomegranate peel extracts,the purities of ellagic acid and punicalagin were increased 54.82 and 24.20 times,respectively.The P1 fraction was further purified by Sephadex LH-20,the purity of ellagic acid reached 94.66%,and the yield of purified ellagic acid was 3.9 mg/100 g(dry pomegranate peel).(3)The antioxidant experiment analysis found that pomegranate peel polyphenols with strong antioxidant ability mainly depended on the key ingredients of punicalagin,ellagic acid,gallic acid,and tannin.Reducing power of ellagic acid,gallic acid,tannic acid,and punicalagin was higher than Vc,respectively.·DPPH scavenging rate of gallic acid was higher than Vc.·OH scavenging rate of punicalagin,ellagic acid,gallic acid,and tannic acid was higher than Vc,respectively.ABTS +·scavenging rates of all the samples were higher than Vc.Reducing power,·DPPH,·OH,O2-·,and ABTS+· scavenging rates of pomegranate peel polyphenols were significantly better than that of pomegranate juice,and antioxidant capacity was increased with the increase in concentration of polyphenols.(4)Spectroscopy technology results confirmed that three kinds of pomegranate peel polyphenols could interact with ovalbumin.The fluorescence quenching type between gallic acid with ovalbumin was static quenching,and the interaction force was electrostatic attraction.There was non-radiation energy transfer between them.Ellagic acid and punicalagin can cause ovalbumin fluorescence intensity enhancement and the red shift of maximum fluorescencepeak.The circular dichroism results showed that gallic acid can reduce the ?-helix content of ovalbumin,and increase the contents of ?-fold and random curl,which increased the polarity of microenvironment,declined the hydrophobic property,and caused structure uncoiling.While ellagic acid and punicalagin can increase the ?-helix content of ovalbumin and reduce the content of ?-fold,which could increase protein hydrophobicity,and therefore structure was more stable.In addition,the interaction between polyphenols and ovalbumin can significantly reduce the antioxidant activity of gallic acid,ellagic acid,and punicalagin.Pomegranate peel polyphenols can lower hydrolysis rate of ovalbumin in stomach digestion phase,and reduce its availability. |
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