Abstract
This study was aimed at explanation of evolution process of bound phenolics from IDF through structural modifications after alkaline hydrolysis along with free phenolics and functional characteristics. SEM results depicted that IDF with bound phenolics exhibited denser, more compact and tubular shape whereas, the surface features of alkali-hydrolyzed residue exhibited clear fragmentation, wrinkling and porosity on outside surfaces. Microstructure changes due to alkaline hydrolysis caused disintegration of linkages among hemicellulose and cellulose microfibers. As evident from the FTIR spectra, both IDF samples (with phenolic) and alkali-soluble residue (IDF without phenolics) exhibited resemblance in FTIR spectral features and characteristics bonds. Overall, the peak positions of IDF samples and alkali-hydrolyzed residue did not show any significant change, which was indicative of the fact that no major alterations were occurred in crystalline structures of IDF due to alkali-hydrolysis treatment. IDF-bound phenolic complexes showed the highest TPC, ABTS and FRAP than SDF. In IDF alkaline extracts, total 6 phenolic compounds were detected through HPLC. The identified phenolic compounds were as; salicylic acid, chlorogenic acid, syringic acid, epigallocatechin, p-coumaric acid and ferulic acid. These results demonstrated that higher antioxidant activity of dietary fiber would be related to bound phenolics.
Highlights
Non-digestible food ingredients exhibits significance as a broad category and dietary fiber (DF) is classified as one of them
All detected compounds are of insoluble dietary fiber (IDF)-bound phenolics of grape pomace, each phenolic compound varied in terms of content and composition. These results were in agreement with the findings reported by Xu et al (2020) for 11 identified bound phenolic compounds from IDF fraction of lychee pulp subjected to alkaline hydrolysis
IDF with bound phenolics exhibited denser, more compact and tubular shape whereas, the surface features of alkali‐hydrolyzed residue (IDF without bound phenolics) exhibited clear fragmentation, wrinkling and porosity on outside surfaces
Summary
Non-digestible food ingredients exhibits significance as a broad category and dietary fiber (DF) is classified as one of them. Decreasing tendencies have been reported regarding risk factors and incidence of metabolic syndrome and cardiovascular diseases. As it is ascertained previously that DF plays significant role, phenolics have been reported to play pivotal role in prevention of aforementioned ailments (Xu et al, 2020). Phenolics are classified in two types; bounds phenolic and free phenolics. There is a need to be considered both free and bound phenolics in commutative manner to avoid the under-presenting of total phenolic contents (TPC) in reality. When TPC are measured only free phenolics are taken into account as established in previously published reports (Xu et al, 2020)
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