| Polyphenol is one of the secondary metabolites widely found in vegetables, fruits and other plants. The unique molecular structure of plant polyphenol exhibits strong antioxidant activity. Epidemiological studies have shown that dietary patterns were significantly associated with the prevention of chronic diseases such as cardiovascular disease, cancer and Alzheimers's disease. Bitter Melon which is the typical vegetable cultivated in southern China, is a edible vegetable with a variety of biological active substances; Sweet corn also called vegetable corn, due to its rich nutrition and good taste, is very popular with all classes of consumers. As grain products, sweet corn can complement the biologically active substances which are not present in significant quantities in some fruits and vegetables. So, sweet corn is a good source of new healthy food. At present, explore and research on the phenolic contend and the antioxidant capacity in fruits and vegetables and their processed products has become a common concern in medicine and food chemistry, but the bitter melon and sweet corn were not included. There are fewer researches on the changes of the phenolics and antioxidant capacity in bitter melon and sweet corn during thermal processing. The purposes of this study is to compare different varieties of the content of phenolic compounds and the antioxidant activity in bitter melon flesh and sweet corn; meanwhile analyzed the changes of phenolics and antioxidants activity in in bitter melon and sweet corn during thermal processing. The major results are as follows:(1)Phenolic content and antioxidant activity of different varieties of bitter melon: The free, bound and total phenolic content of 14 bitter melon cultivars ranged from 11.03 to 26.80,0.45 to 3.78, and 12.36 to 28.45 mg of gallic acid equivalents/100 g fresh weight (FW), respectively. The coefficients of variation (CVs) of free, bound and total phenolic content in bitter melon varieties was 23.50,61.04 and 21.58%, respectively. The percentage contribution of free phenolics to the total was 91.18%. The free flavonoid content of bitter melon samples ranged from 627.62 to 1275.24μg CE/100 g FW, The CV of free flavonoid content in bitter melon varieties was 22.80%. Vanillic acid, Epicatechin and Rutin were detected in bitter melon samples and ranged from 128.34 to 650.37,2461.84 to 8016.55, and 63.60 to 316.90μg/100g FW, respectively. The CVs of Vanillic acid, Epicatechin and Rutin content in bitter melon varieties were 43.85%,26.97% and 33.09%, respectively. The result of FRAP of 14 bitter varieties varied from 19.14 to 49.93 mg trolox equivalents/100 g FW and the CV was 27.67%. The IC50 values of DPPH of 14 bitter samples ranged from 0.80 to 2.37mg GAE/100 g FW and the CV was 35.10%. The IC50 values of ABTS of 14 bitter samples varied from 1.51 to 8.38 GAE/100 g FW and the CV was 63.75%. There was a significantly positive correlation between free phenolic content and FRAP values of 14 bitter samples (P<0.01). While there existed significantly negative correlation between free phenilic content and the IC50 values of DPPH and ABTS of 14 bitter samples, respectively.(2) Phenolic content and antioxidant activity of different varieties of sweet corn: The free, bound and total phenolic contents of 8 sweet corn varieties ranged from 30.35 to 47.76, from 4.45 to 14.47, and from 38.01 to 57.04 mg of gallic acid equivalents GAE/100 g FW, respectively. The percentage contribution of free fraction to the total was from 82.08 to 88.28%. The free, bound and total flavonoid contents varied from 3.03 to 5.08, from 1.56 to 2.60, and from 4.89 to 7.34 mg of catechin equivalents CE/100 g FW, respectively. The percentage contribution of free fraction to the total was from 61.73 to 73.72%. The average values of phenolic compositions including ferulic acid as well as p-counmarin of 8 sweet corn cultivars were 7905.97 and 854.82μg/100 g, respectively. The IC50 value of the free and bound fraction of scavenging DPPH radical of 8 sweet corn varieties ranged from 2.03 to 4.98 mg GAE/100 g FW, and from 0.28 to 0.34 mg GAE/ 100 g FW respectively, while the result of scavenging ABTS radical was 0.96 to 2.5 mg GAE/100 g FW,0.18 to 0.34 mg GAE/100 g FW, respectively. The free, bound and total FRAP values ranged from 19.16 to 32.42, from 6.86 to 20.22, and from 26.02 to 44.04 mg of trolox equivalents (TE)/100g FW, respectively. Total antioxidant activity determined using FRAP assay is significantly correlated to the contents of total phenolics, total flavonoids respectively. These results indicate that there are significant differences in phytochemical content and antioxidant activity of sweet corn among different cultivars.(3) Influence of heat treatment on the polyphenolic compounds and antioxidant activities of sweet corn:Sweet corn was heated at 105-125℃for 25 min using an autoclave. The phenolic compounds and the antioxidant activity in fresh sweet corn, sweet corn syrup and the sweet corn syrup after been processed were analyzed. The result showed that the phenolics content of sweet corn been processed was significant increased. The composition of phenolic compounds has a significant change; The FRAP value was significant increased, and the ability inhibition of DPPH and ABTS radicals was increased. (4) Different drying methods and heating time on the content of phenolic compounds in bitter melon and antioxidant activity:The bitter melon was treated with hot air drying and vacuum drying at 70℃for 5-25 hours. The phenolic compounds and the antioxidant activity in fresh and dried bitter melon were analyzed. The result showed that with the longer drying time, the content of the phenolic compounds was lower; FRAP values was significant decreased; and the ability inhibition of DPPH and ABTS radicals was also decreasd. Compared to hot air dring, the bitter melon with vacuum drying has relative higher content of phenolic compounds and stronger antioxidant capacity. |
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