Effects of Roasting on Antibacterial and Antioxidant Properties of <i>Sophora japonica</i> Buds—The Involvements of Rutin and Quercetin Constituents

Roasting is the most common thermal processing method established for <i>Sophora japonica</i> (SJ) buds applied as traditional medicines, and it has also been reported to alter several of their therapeutic functions. However, there have been no studies investigating the influences of roasting on the effects of these materials against bacteria. Therefore our study was performed to examine the alterations that this process would induce in SJ buds’ antibacterial properties. Fresh buds were subjected to hot air drying or different roasting methods, as described in Materia Medica, including yellow-, dark yellow-, scorched-, and charred-roasting conditions. Antibacterial effects, total polyphenol and flavonoid contents, antioxidant activities, as well as rutin and quercetin concentrations in methanol extracts obtained from those materials, were then measured and compared. The results showed that dark yellow-roasted SJ buds exerted the strongest antibacterial and antioxidant activities and were also the richest in polyphenol contents. Analysis of rutin and quercetin revealed that, following the increment in heating temperatures up to 240 °C, the reduction in rutin content occurred in a parallel manner to the increment in quercetin content. However, overheating at 300 °C reduced both concentrations. Among the five tested samples, dark yellow-roasted SJ had the highest amounts of quercetin. Furthermore, the comparison of rutin and quercetin in antibacterial effects and antioxidant activities showed that the latter was significantly stronger in both of these functions, suggesting that the increment in quercetin content as a result of heat treatment was responsible, at least in part, for the potentiation of the two therapeutic effects.