Subcritical water extraction of bioactive phenolic compounds from distillery stillage

Distilleries generate huge amounts of by-products that have a negative impact on the environment, so the management of wastes generated by this sector should be improved. Because distillery by-products are a source of bioactive compounds, the recovery of these compounds not only reduces issues with environmental protection but also provides the basis for a waste-to-profit solution. Following the latest trends in the search for so-called green extraction techniques for recovering valuable products, this study investigated the effect of subcritical water extraction (SWE) conditions (temperature (25-260 °C), time (5-90 min), and solid-to-solvent ratio (1:5-1:50, w:v)) on the efficiency of recovery of bioactive compounds, i.e., polyphenols from distillery stillage, and on the antioxidant activity of the extracts. The highest extraction yield was obtained with 30-min SWE with a solid-to-solvent ratio of 1:15 at either 140 °C (for total polyphenol content and phenolic acid content) or 200 °C (for total flavonoid content), as indicated by the Response Surface Methodology analysis. Phenolic acids in the extracts were present mainly in free forms (up to 88% of the total content). The antioxidant activity, which was measured using several assays, correlated positively with the content of phenolic acids, which confirmed their significant contribution to the bioactive properties of the extracts. The antioxidant effects of the extracts were mostly due to hydroxycinnamic acids (especially ferulic and p-coumaric acids). Principal component analysis showed that the temperature and time of SWE were the factors that can explain the greatest amount of variation in the extraction yield, composition, and bioactive properties of the polyphenols. These results will influence the design of further processes, such as purification and concentration, which are necessary before using the extracted compounds as substrates that are applicable in various industries. Based on the analysis of the elemental composition, the biomass remaining after SWE was evaluated to consider the possibilities of its further utilization.