Antioxidant properties of phenolic extracts of murtilla pomace: First report on the importance of soluble and insoluble-bound compounds

Some native Chilean berries, including murtilla, have gained attention for their high phenolic content which renders them attractive for the beverage industry. However, phenolic-rich by-products are generated during the production of murtilla juice (murtilla pomace), and there has been no scientific consideration of this by-product as a source of different forms of phenolic compounds. The aim of this study was thus to obtain phenolic extracts from the soluble fraction (free, esterified, and etherified) of murtilla pomace as well from the counterpart that contains insoluble-bound compounds, and evaluate their antioxidant properties. The fraction obtained from the insoluble-bound form (insoluble-bound phenolic hydrolysates) showed the highest total phenolic content, reducing power and antioxidant capacity, as evaluated by the FRAP and ORAC assay, respectively. The results revealed the presence of different compounds in each fraction, highlighting important levels of phenolic acids (e. g., gallic acid) and flavonoids (e.g., quercetin and its derivatives) as evaluated by UPLC-ESI-MS/MS. The most prominent fraction (released from the insoluble-bound form) exhibited antioxidant activity (in concentrations as low as 0.0025 mu M of total phenolic acids and 0.0002 mu M of total flavonoids) in Caco-2 cells. The necessary concentration of quercetin to protect Caco-2 cells against the hydrogen peroxide-induced oxidative stress was twenty times lower than the concentration needed for gallic acid to exert the same level of antioxidant protection. We can therefore suggest that the quercetin concentration would be a better-quality control indicator for functional ingredients, food supplements, and/or nutraceutical products generated from the insoluble-bound phenolic fraction of murtilla pomace.