Milk thistle (Silybum marianum) seed cake, a by-product of oil extraction, is increasingly recognized as a valuable source of bioactive lipids [1]. This study aimed to compare the physicochemical, thermal, and antioxidant properties of oils extracted from two Polish milk thistle seed cake samples (MTC1–2) using Soxhlet extraction (SE) and microwave-assisted extraction (MAE). The two seed cake samples originated from distinct Polish producers, allowing for a comparative assessment of regional variability.

Soxhlet extraction was carried out for 6 hours, while microwave-assisted extraction was performed under 400 W power for 15 minutes with intermittent stirring, both using hexane as the extraction solvent. Oil yields ranged from 4.17% (SE) to 8.53% (MAE), demonstrating the improved efficiency of MAE. Antioxidant activity, determined by DPPH radical scavenging, was approximately 18% for SE and 24% for MAE, indicating better retention of lipid-soluble antioxidants with MAE. Fatty acid profiles confirmed linoleic acid (C18:2 n-6) as predominant (~45%), followed by oleic (~35%), palmitic (~7%), and stearic (~6%), consistent with typical nutritional patterns of milk thistle oil. Thermal behavior, evaluated using DSC, revealed differences in oxidative stability between seed sources; MAE-extracted oil showed longer induction times (122.89 min) compared to SE oil (65.91 min), indicating superior oxidative stability.

In conclusion, MAE is recommended for milk thistle seed cake oil recovery due to its higher yield, better antioxidant retention, and greater oxidative stability, supporting its application in functional food ingredients and the sustainable valorization of oil industry by-products.