The Bark of Abies alba as a Source of Phenolic Compounds: Optimization of Deep Eutectic Solvent and Microwave-Assisted Deep Eutectic Solvent Extraction Processes

Silver fir (Abies alba) bark is an abundant forestry by-product with considerable potential as a renewable source of phenolic compounds. This study investigated the extraction of phenolic compounds from silver fir bark using a glucose/lactic acid (1:3) deep eutectic solvent (DES) as a sustainable extraction medium. Extraction conditions were optimized through a planned experiment by evaluating the effects of extraction temperature and extraction time. DES extraction yielded extraction efficiencies of 6.00–10.46%, total phenolic contents of 120.43–338.09 mg GAE/g of extract, and radical scavenging activity values of 54.06–77.11%. Microwave-assisted extraction with the DES reduced the extraction time from 60 to 180 min to 1–30 min, while providing extraction efficiencies of 3.94–7.75%, total phenolic contents of 14.44–282.14 mg GAE/g of extract, and radical scavenging activity values of 34.71–97.71%. Although conventional extraction generally produced higher extraction efficiencies and total phenolic contents, microwave-assisted extraction achieved higher radical scavenging activity and substantially shortened the extraction process. The identified phenolic profile included procyanidins, catechin, taxifolin, gallic acid, and vanillic acid. The glucose/lactic acid DES represents a promising green solvent for the valorization of silver fir bark and the sustainable recovery of bioactive phenolics from forest biomass.