| Thinned young kiwifruit is a kind of agriculture by-products produced in the process of kiwifruit thining.Its yield can reach 30% of the total young kiwifruit output every year.Normally,thinned young kiwifruit is directly discarded,resulting in enounous resource waste as well as environmental pressure.Based on this sense,it is meaningful for reasonal development and utilization of thinned young kiwifruit,which can not only turn waste into treasure with improved economic added value of kiwifruit industry,but is also able to eliminate pollution from the source to ensure the healthy industrial development.From the perspective of resource,environment and economy,recycling and utilization of thinned young kiwifruit are of great significance and imperative.In this study,the phenolic content and antioxidant activity in thinned young and mature kiwifruits were analyzed by both spectrophotometry and electrochemical methods.Furthermore,monophenolic compounds in thinned young and mature kiwifruits were characterized and quantified using our new established UPLC-QQQ-MS/MS method.On this basis,the realistic application value of thinned young kiwifruit polyphenol was further evaluated in terms of its preservative function during beef storage.Our results revealed the enormous phenolics contained in thinned young kiwifruit,with epicatechion as its prevailing individual phenolic compound.Also,the antioxidant preservation effect of thinned young kiwifruit polyphenol was confirmed in beef.These finding laid a theoretical foundation for the application of thinned young kiwifruit in food,agriculture and medicine industry,and added a new source of natural antioxidants.The main results are as follows:(1)Spectrophotometric method was used to determine phenolic contents and antioxidant activities of kiwifruit polyphenol extracts.Growth stage specificity,variety specificity and tissue distribution specificity were ascertained for the phenolic contents and antioxidant activities in the detected kiwifruits.For growth stage specificity,the phenolic contents and antioxidant activities in kiwifruits showed the trend of the first batch of thinned kiwifruits > the second batch of thinned kiwifruits > mature kiwifruits.The contents of total polyphenols,total flavonoids,total flavanols and the antioxidant activities of ABTS,DPPH and FRAP in the first batch of thinned kiwifruits were 7.9,17.5,10.1,10.4,8.7 and 8.3 times of those in the whole mature kiwifruits,respectively,revealing the abundant phenolic compounds and antioxidants contained in thinned young kiwifruits,especially the first batch of thinned young kiwifruits.For variety specificity,the contents of total polyphenols,total flavonoids,total flavanols,and antioxidant activities of ABTS,DPPH and FRAP in skin and flesh of mature ‘Zesy002’ and ‘Zesh004’ were significantly higher than those in mature ‘Hayward’,which highlighted the advantages of ‘Zesy002’ and ‘Zesh004’ in term of phenolic antioxidants.For tissue distribution specificity,the average contents of total polyphenols,total flavonoids,total flavanols and antioxidant activities of ABTS,DPPH and FRAP in the skin of the three kiwifruit cultivars were 3.3,28.4,42.1,3.3,2.5 and 1.6 times higher than those in flesh,respectively.Correlation analysis showed that the antioxidant activities of phenolic extracts in kiwifruits were significantly correlated with the contents of total polyphenols,total flavonoids and total flavanols(p < 0.01),demonstrating that phenolic compounds were not only the main component of antioxidants in mature kiwifruits,but also the important source and contributor for the strong antioxidant activities detected in thinned young kiwifruit.(2)A fast,efficient and sensitive quantitative method for phenoloc compound in the MRM mode of UPLC-QQQ-MS/MS system was optimized and established,and effective separation and precise quantification of 15 phenolic compounds were successfully achieved in 22 minutes of running time.The detection limit of this method is between 0.010-0.240 mg/L and the quantitative limit is between 0.041-0.480 mg/L.The relative standard deviation(RSD)of both retention time and peak area of mass spectrometry response is less than 6.79%,and the recovery rate is between 93.60-102.22%.These data confirmed the good repeatability and accuracy of this method.Then the new UPLC-QQQ-MS/MS method was used to characterize and quantify 15 monophenolic compounds in the extracts of thinned young and mature kiwifruits.Results showed that epicatechin was the most abundant phenolic compound in all kiwifruit extracts with the highest content of 2989.23 g/g FD present in the first batch of thinned young ‘Zesy002’.Besides,the epicatechin contents in kiwifruits were significantly correlated(p < 0.01)with the contents of total polyphenols,total flavonoids,total flavanols and antioxidant activities of ABTS,DPPH and FRAP,proving the decisive role of epicatechin in the antioxidant activities of thinned young and mature kiwifruits.Quercetin appeared to be the second largest phenolic compound in kiwifruit extracts with the highest content of 587.61 g/g FD detected in the second batch of thinned young ‘Hayward’.However,the contents of quercetin in kiwifruit extracts are much lower compared to those of epicatechin.The average contents of flavanols,flavonols,hydroxybenzoic acids and hydroxycinnamic acids in thinned young kiwifruits of the three cultivars were 3.8,27.6,1.2 and 7.7 times higher than those in mature kiwifruits,which further clarified the abundant phenolic resources in thinned young kiwifruit.(3)Cyclic voltammetry was used to elucidate the redox behaviors of 16 phenolic compounds as well as ascorbic acid,and to analyze the antioxidants present in thinned young and mature kiwifruits.For standards of phenolic compounds and ascorbic acid,the electrochemical behaviors were specific,showing significant structural-activity relationship.Compounds with catechol structure,such as catechins,quercetin,gallic acid and caffeic acid,had strong electron donor capacity and showed relatively low anode peak potentials in the cyclic voltammogram.Compounds with isolated phenol groups,such as vanillic acid,syringic acid,ferulic acid and p-coumaric acid,were oxidized by a small number of electrons with weak reducing ability and higher oxidation potential.Quercitrin,hyperoside and rutin,which are derivatives of quercetin,exhibited higher oxidation potential than quercetin due to their substituents on c-3 position.The phenolic contents and antioxidant activities determined in thinned young and mature kiwifruits using cyclic voltammetry also showed growth stage specificity,variety specificity and organization distribution specificity.Furthermore,the electrochemical quantitative results correlated well with the spectrophotometric results(total polyphenols,flavonoids,and flavanol contents and ABTS,DPPH and FRAP antioxidant activities)as well as the UPLC-QQQ-MS/MS results of 8 monophenol compound contents(p < 0.01).In addition,cyclic voltammetry analysis showed epicatechin as the dominant antioxidant in kiwifruit extracts,which was consistent with the quantitative result of UPLC-QQQ-MS/MS method.Therefore,our finding verified the effectiveness of cyclic voltammetry in predicting phenols and reducing powers of kiwifruit samples,thus providing a new efficient and convenient way for the analysis of antioxidant in kiwifruit.(4)Polyphenols were obtained from the first batch of thinned young ‘Hayward’ through ethanol extraction and macroporous resin purification.The total phenolic content in the obtained thinned young kiwifruit polyphenols was 0.53 g EE/g,and the antioxidant activities of ABTS,DPPH and FRAP were 10.60 mmol TE/g,6.76 mmol TE/g and 2.89 mmol TE/g,respectively.Epicatechin,quercitrin and catechin were identified as the main phenolic compounds in thinned young kiwifruit polyphenols with their contents being 139.57 mg/g,36.00 mg/g and 5.86 mg/g,respectively.The purified thinned kiwifruit polyphenols were applied in fresh beef for 7 days of preservative test(4°C),and epicatechin,potassium sorbate and blank was set as comparison.Results indicated that thinned kiwifruit polyphenols can effectively reduce beef TBARS values during cold storage,inhibit the formation of TVB-N,prevent the oxidation of polyunsaturated fatty acids,stabilize brightness and maintain structures of beef.The results of sensory assay showed that beef treated with thinned kiwifruit polyphenols had comparable sensory scores with the control group at the early stage of storage,but obtained better scores than control at the later stage of storage in terms of odour,taste and texture.Hence,thinned young kiwifruit polyphenols could significantly inhibit beef lipid and protein oxidation,stabilize beef physicochemical properties,and effectively improve beef quality and shelf life during cold storage without affecting beef sensory characteristics.Combined with its natural,abundant and economic properties,thinned young kiwifruit polyphenols could thus be used as a kind of natural food preservative with more advantages than synthetic antioxidants. |
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