Polyphenol content of fruit greatly underestimated

01 September 2009

While the polyphenol content of fruits usually refers to extractable polyphenols, new research published in the Journal of Agricultural and Food Chemistry reports that the non-extractable polyphenol content is up to five times higher than extractable compounds.

According to studies with apple, peach and nectarine, previous measures to quantify polyphenols may have been limited by the extraction technique. The content of polyphenols in fruits reported in the literature normally refers to extractable polyphenols (EPP) analyzed in aqueous-organic extracts. However, significant amounts of bioactive compounds that are usually not considered in nutritional studies remain in the residue from extraction as nonextractable polyphenols (NEPP). The main objective of this work was to analyze both EPP and NEPP (hydrolyzable polyphenols and proanthocyanidins). EPP were analyzed in methanol/acetone/water extracts, and NEPP were determined in acidic hydrolysates of extraction residue from apple, peach, and nectarine using HPLC-MS and spectrophotometry. Results showed that the NEPP content (112-126 mg/100 g of fresh fruit) was higher than the EPP content (18.8-28 mg/100 g of fresh fruit). Further analyses of NEPP in other fruits and plant foods consumed in diets are needed to compile a complete database of use for nutritional and biological studies.

"These [non-extractable] polyphenols need to be treated with acid to extract them from the cell walls of fruit in the lab," said lead author Sara Arranz from the Spanish Council for Scientific Research (CSIC) in Madrid. "If non-extractable polyphenols are not considered, the levels of beneficial polyphenols such as proanthocyanidins, ellagic acid and catechin are substantially underestimated."

In collaboration with the Institute of Food Research in the UK, Arranz states that this represents the first report on using methanol and sulfuric acid to analyse the non-extractable polyphenol content of apples, peaches, and nectarines.

"In the human body these compounds will be fermented by bacteria in the colon, creating metabolites that may be beneficial, for example with antioxidant activity," said Paul Kroon from IFR.

see full paper: http://pubs.acs.org/doi/abs/10.1021/jf9016652