Grape seeds are the major component of mushroom culture media. This study aimed to examine the effects of different grape seeds on the antioxidant activities of Pleurotus ostreatus polyphenols. Four different assays were employed to measure antioxidant activity, including the Folin–Ciocalteu assay, flavonoid content determination assay, 2,2′-azino-bis-3-ethylbenzthiazoline-6-sulphonic acid(ABTS) scavenging activity assay, and the ferric reducing ability of plasma assay. Polyphenol content was assessed using...