Various edible algae have been traditionally consumed as healthy food stuffs in Asian countries such as China, Korea and Japan, and roasting treatments have been carried out on some of these edible algae for the improvement of their taste and flavor. In the present paper, we analyzed the effect of roasting treatments on the radical scavenging activity of a typical Japanese edible brown alga, Laminaria japonica (L. japonica, Ma-konbu). The effect was estimated by a stable radical compound, diphenylpicrylhydrazyl (DPPH) and a chemiluminescence assay of superoxide anion generation using hypoxanthine (HPX) and xanthine oxidase (XOD) system. Weak but significant radical scavenging activities against the DPPH radical and superoxide anion were observed in the water extract of L. japonica when it was roasted at 130-150ºC. Very strong radical-scavenging activities were detected under much higher temperature conditions (180-200ºC). The enhancing effect of the roasting treatment on the radical scavenging activity was highly associated with the roasting-induced increase of polyphenol and tannin in the L. japonica extract. The sugar concentrations in the L. japonica extracts under different roasting conditions were significantly, but not strongly associated with their radical scavenging activities. The protein concentrations in the L. japonica extracts, however, were not associated with their radical scavenging activities under different roasting conditions. Furthermore, the extracts of L. japonica roasted under higher temperature conditions (180-200ºC) caused strong radical scavenging effects on the generation of opsonized zymosan (Opz)-induced reactive oxygen species (ROS) generation in human blood neutrophils, which was measured by chemiluminescence assay. These experimental results suggest that the roasting treatment of L. japonica causes an enhancing effect on the radical scavenging activity in the extract of this alga, and is associated with the increase in various radical scavenging substances in the extract. The significance of this finding is discussed from the viewpoint of healthy food science.
Keywords: Laminaria japonica; neutrophils; polyphenol; radical-scavenging activity; roasting treatment.