Compositional Characterization of Milk Thistle (Silybum marianum) Seeds and Press Cake with Emphasis on Press Cake Valorization

Milk thistle (Silybum marianum) oil production generates substantial quantities of seed cake, an underutilized by-product with potential as a source of nutrients and bioactive compounds. This study aimed to characterize milk thistle cakes from two industrial sources (MTC1 and MTC2) and their corresponding seeds (MTS1 and MTS2), focusing on compositional properties, fatty acid profile, and antioxidant activity assessed using the DPPH scavenging assay. Proximate analysis showed that the cakes retained significant residual oil (9.26–14.51 g 100 g−1) and protein (16–19 g 100 g−1), with low water activity (<0.33), indicating good storage stability. Fatty acid analysis revealed a predominance of polyunsaturated fatty acids (49–52%), mainly linoleic acid (C18:2 n-6), confirming their nutritional value. Differences between industrial sources indicated variability associated with raw material and processing conditions. Extraction solvent significantly affected bioactive compound recovery from the oil fraction. Dichloromethane extracts exhibited higher total phenolic content (up to 8.87 mg GAE g−1) and stronger DPPH radical scavenging activity (up to 28.07%) compared to hexane extracts, which may be attributed to a greater extraction of moderately polar phenolic compounds, including flavonolignan-type constituents potentially associated with silymarin complex. Overall, milk thistle cake represents a promising raw material for the recovery of natural antioxidants and valuable lipids, supporting its application in functional food or feed products and sustainable biorefinery processes.