Quantitative investigation and systematic studies of quercetin, total phenolics, flavonoids, antioxidants, antibacterial and antibiofilm or antibiofouling properties of methanolic extracts of onions obtained from six different varieties have been carried out to explore their relative merits in terms of biological activities of fresh and aging onions. Total phenolic content in the extracts was examined spectrophotometrically using Folin-Ciocalteau's phenol reagent and total antioxidant activity was studied by FRAP and DPPH methods. In vitro antibacterial activity of the extracts was investigated on Gram-negative (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus and Bacillus cereus) respectively, by using a modified Kirby-Bauer disc diffusion method. Antibiofilm activity was tested by crystal violet assay. The best results against biofilm formation were observed for the extracts obtained from onions stored for three months. The total phenolic and antioxidant content found to be increased upon aging in all the six varieties; red skinned onion (Happyhong) showed the highest level of total phenolics (5110.07 ± 196.56 μg GAEg-1 FW) and total flavonoids (2254.00 ± 154.82 μg QEg-1 FW) after three months. The results showed that in all the varieties, quercetin content as well as biological activity increases with aging in the stored onions compared with the fresh ones.

中文翻译：

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活性化合物作为抗衰老洋葱抗菌和抗生物膜剂的系统研究

对来自六个不同品种洋葱的甲醇提取物的槲皮素、总酚类、黄酮类、抗氧化剂、抗菌性和抗生物膜或抗生物污染特性进行了定量调查和系统研究，以探索它们在新鲜和老化洋葱的生物活性方面的相对优势. 使用 Folin-Ciocalteau 苯酚试剂分光光度法检测提取物中的总酚含量，并通过 FRAP 和 DPPH 方法研究总抗氧化活性。使用改进的 Kirby-Bauer 圆盘扩散法，分别研究了提取物对革兰氏阴性菌（大肠杆菌和铜绿假单胞菌）和革兰氏阳性菌（金黄色葡萄球菌和蜡状芽孢杆菌）的体外抗菌活性。通过结晶紫测定测试抗生物膜活性。对于从储存三个月的洋葱中获得的提取物，观察到对生物膜形成的最佳结果。6个品种的总酚类和抗氧化剂含量随着陈酿而增加；三个月后，红皮洋葱 (Happyhong) 的总酚类 (5110.07 ± 196.56 μg GAEg-1 FW) 和总黄酮 (2254.00 ± 154.82 μg QEg-1 FW) 水平最高。结果表明，与新鲜洋葱相比，贮藏洋葱中的槲皮素含量和生物活性均随着老化而增加。三个月后，红皮洋葱 (Happyhong) 的总酚类 (5110.07 ± 196.56 μg GAEg-1 FW) 和总黄酮 (2254.00 ± 154.82 μg QEg-1 FW) 水平最高。结果表明，与新鲜洋葱相比，贮藏洋葱中的槲皮素含量和生物活性均随着老化而增加。三个月后，红皮洋葱 (Happyhong) 的总酚类 (5110.07 ± 196.56 μg GAEg-1 FW) 和总黄酮 (2254.00 ± 154.82 μg QEg-1 FW) 水平最高。结果表明，与新鲜洋葱相比，贮藏洋葱中的槲皮素含量和生物活性均随着老化而增加。