The potential of selected industrial food wastes from juice and nut production including apple peel, apple pomace, pomegranate peel, pomegranate seed, chestnut shell, and black carrot pomace as resources for natural antioxidants was investigated. Soluble free and insoluble-bound phenolics were extracted from the wastes and analyzed for total phenolic and flavonoid contents, phenolic profile and antioxidant activity. Total phenolic and total flavonoid contents of wastes were positively correlated with their antioxidant activity. The highest total phenolic and antioxidant activity were determined in soluble fraction of pomegranate peel due to a significant amount of punicalagin derivatives. Pomegranate peel and seed had the most phenolics and flavonoids in soluble form while other wastes had more than 45% of total phenolics in insoluble-bound form. Chestnut shell showed more antioxidant activity in insoluble-bound fraction compared to that of its soluble fraction. These findings showed that not only soluble but also an insoluble-bound fraction of the industrial wastes has good potential for valorization as a source of natural antioxidants.
Agro-industrial wastes have gained great attention as a possible source of bioactive compounds, which may be utilized in various industries including pharmaceutics, cosmetics, and food. The food processing industry creates a vast amount of waste which contains valuable compounds such as phenolics. Polyphenols can be found in soluble (extractable or free), conjugated, and insoluble-bound forms in various plant-based foods including fruits, vegetables, grains, nuts, and legumes. A substantial portion of phenolic compounds in agro-industrial wastes is present in the insoluble-bound form attached to the cell wall structural components and conjugated form which is covalently bound to sugar moieties. These bound phenolic compounds can be released from wastes by hydrolysis of the cell wall and glycosides by microbial enzymes. In addition, they can be converted into unique metabolites by methylation, carboxylation, sulfate conjugation, hydroxylation, and oxidation ability of microorganisms during fermentation. Enhancement of concentration and antioxidant activity of phenolic compounds and production of new metabolites from food wastes by microbial fermentation might be a promising way for better utilization of natural resources. This review provides an overview of mechanisms and factors affecting release and bioconversion of phenolic compounds in agro-industrial wastes by microbial fermentation.
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