Phenolic Composition, Antimicrobial and Antioxidant Properties of Apple Wood Extracts

Each year, fruit growers have hundreds of tons of bark and pruning waste. Despite their potential for chemical recycling, residues from this waste are frequently used for applications with low added value. By converting the waste stream into a valuable source, this study contributes to a sustainable innovation. Natural phenolic compounds are an alternative to synthetic antioxidants, bacteria or mould inhibitors in food, cosmetics and possibly even in pharmaceutics. Restraining loses due to waste flow is an overall, ecological and economical inspired tendency. The significant secondary flow that exists in the fruit industry can result into a major ecological and economic value for growers. The aim of the present work is the optimization of the solvent extraction of phenolic compounds from apple trees, more specifically bark and core wood. Solvent extractions, with a varying solvent composition, are assessed based on the extraction efficiency of the total polyphenols and flavonoids (spectrophotometry). The extracts are further characterized by their antimicrobial properties and antioxidant activities. Bark extracts obtained by a 40 v/v% acetone/water mixture presented the highest phenolic (22.84 ± 0.56 mg GAE/g DW) and flavonoid (12.16 ± 0.06 mg QC/g DW) content, and antioxidant activity (1.068 ± 0.005 mM FeSO4.7H2O/g DW for the FRAP assay), while the extract also inhibited growth of the gram–positive bacteria Enterococcus faecalis and Staphylococcus aureus by 100%. Core wood extracts obtained by pure water presented the lowest phenolic (3.45 ± 0.09 mg GAE/g DW) and flavonoid (2.19 ± 0.08 mg QC/g DW) content. The results indicate that warm solvent extraction (WSE) proved to be an efficient way to extract polyphenols with a high reducing ability from bark and core wood of apple trees.