Release and possible mechanism of extractable and non-extractable polyphenols from highland barley bran using chemical and physiological methods

Highland barley bran (HBB) is rich in extractable polyphenols (EP) and non-extractable polyphenols (NEP). Previous studies have generally used chemical solvents to extract EP and NEP and assess their functional properties. However, EP and NEP obtained using chemical method do not indicate phenolic compounds profiles released in the body. This study used chemical (acetone and acid hydrolysis) and physiological methods (in vitro gastrointestinal digestion and colonic fermentation) to treat HBB and investigated the differences between the two methods in releasing EP and NEP. Results showed that chemical method yielded a higher total phenolic content (EP + NEP = 718.25 mg GAE/100 g) than physiological method (634.62 mg GAE/100 g). More phenolic compounds, particularly ferulic acid oligomers, were released by chemical method and exhibited stronger total antioxidant capacities than those released by physiological method. Moreover, structural analysis indicated two methods used different mechanisms to release EP and NEP, with chemical method releasing polyphenols by drastically disrupting the cellular structure of the aleurone layer and physiological method releasing polyphenols by degrading cell walls. Therefore, a possible release mechanism was proposed. These findings highlight the importance of physiological method in accurately reflecting polyphenol bioaccessibility, which is critical for evaluating their potential health benefits.