Phenolic profile and in vitro antioxidant power of different milk thistle [Silybum marianum (L.) Gaertn.] cultivars

• Phenolic classes, antioxidant capacity, and LC–MS/MS targeted phenolics were analyzed in 15 genotypes of milk thistle.
• The flavonolignan silybin, along with chlorogenic acid, comprised major part of the polyphenols in seeds of milk thistle.
• Significant differences were found between genotypes, suggesting different industrial potential.

Milk thistle (Silybum marianum (L.) Gaertn.) is a rich source of phytochemical compounds exhibiting functional roles in plant metabolism and nutraceutical effects on human health. The aim of the present work was to characterize the phytochemical and antioxidant profiles of the achenes from fifteen different genotypes of milk thistle, by measuring the total phenolics, flavonoids and flavonols content as well as antioxidant capacity. In addition, selected individual phenolic compounds were determined by liquid chromatography tandem mass spectrometry. The majority of polyphenols in milk thistle were composed of chlorogenic acid and the flavonolignan silybin. Significant differences were identified between cultivars for total antioxidant capacity and for all tested classes of compounds. Genotypes 1 and 2 had the highest total phenolics and flavonoids content, as well as the strongest antioxidant activity. Genotype 8 contained high levels of caffeic acid, 10–15 times greater than values obtained for the other genotypes. Antioxidant capacity was mainly correlated to total phenolics and flavonoids, rather than to single compounds. The study pointed out the important role of milk thistle genetic background on the actual chemical and antioxidant profiles of milk thistle achenes.