Latest developments in polyphenol recovery and purification from plant by-products: A review

Background: In recent times, research has highlighted the bioactive roles played by non-extractable polyphenols (NEP) in human health, especially from by-products of grains, fruits and vegetables. Most studies have reported on the total phenol content of samples without adequate attention paid to a major class of polyphenols referred to as NEP. This limits the scope of polyphenol studies and the extent of NEP incorporation into medicines, nutraceuticals, cosmetic and foods among other applications. Scope and approach: This review introduces NEP and gives an update on recent advances in free and bound polyphenol extraction from plant by-products. Owing to the fact that most researchers are already familiar with the extraction of free polyphenols, this review emphasizes the extraction of NEP and provides a basis for the selection of appropriate techniques and applications for different plant by-product sources. It also presents suggestions for future studies into NEP extraction efficiency from complex plant sources using alternative physical approaches. Key findings and conclusions: NEP from by-products have high potential for use in industry, especially in pharmaceutical, nutraceutical and cosmetic applications. To ascertain the total phenol content of extracts, both free and bound phenols must be accounted for with special attention directed at appropriate sample pretreatment and extract hydrolysis. Higher bioactive properties of NEP, coupled with their cheaper by-product sources makes them economically viable and medicinally potent. Also, structure-function relationships and the possibility to use physical means such as slow-freezing-thawing to increase hydrolytic yield of NEP is recommended.