Examining the Role of Extraction Techniques and Regional Variability in the Antioxidant and Phytochemical Composition of <i>Juglans regia</i> L. Septa

Walnut septa, traditionally discarded as waste in walnut processing because they primarily serve a structural function in the walnut fruit, have recently gained attention for their potential abundance of phenolic compounds, suggesting their overlooked value. This study aimed to optimise extraction parameters to maximise the extraction yield of bioactive compounds and explore regional variations in antioxidant activity and chemical composition of <i>Juglans regia</i> L. septa. The experimental variables included extraction methods (maceration, dynamic maceration, ultrasound processing, and reflux extraction), temperature, solvent type (methanol, acetone, and ethanol), and the percentage of water in the solvent. The optimal conditions were determined based on the total phenolic content—reflux extraction using 60% ethanol as a solvent for a duration of 60 min. Samples from 12 different regions in Lithuania, Armenia, and Ukraine were analysed for their phenolic and proanthocyanidin content and antioxidant activity using the CUPRAC method. The total phenolic content ranged from 131.55 to 530.92 mg of gallic acid equivalents per g of dry weight of plant material (mg GAE/g dw), while the proanthocyanidin content varied from 1.14 to 7.65 mg of (–)-epicatechin equivalents per g dry weight of plant material (mg EE/g dw). Among all the regions studied, the Šiauliai sample demonstrated the highest concentrations of phenolic compounds, proanthocyanidins, and antioxidant activity, with statistically significant differences compared to the other samples (<i>p</i> < 0.05). These findings demonstrate that walnut septa are a valuable source of phenolic compounds and antioxidants, with significant potential for developing natural nutraceuticals and antioxidant products.