Abstract
Our previous study revealed that aqueous extract of grape pomace obtained from a winemaking process could exert bactericidal action upon photo-irradiation via reactive oxygen species (ROS) formation. In the present study, we focused on chemical composition and prooxidative profile of the extract. Liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS) analysis showed that polyphenolic compounds including catechin monomers, dimers, trimers, and polyphenolic glucosides were contained. The polyphenol rich fraction used for the LC-ESI-MS analysis generated hydrogen peroxide (H2O2) upon photo-irradiation possibly initiated by photo-oxidation of phenolic hydroxyl group. That is, reduction of dissolved oxygen by proton-coupled electron transferred from the photo-oxidized phenolic hydroxyl group would form H2O2. The resultant H2O2 was then photolyzed to generate hydroxyl radical (•OH). The prooxidative profile of the extract in terms of •OH generation pattern upon photo-irradiation was similar to that of grape seed extract (GSE) as an authentic polyphenol product and (+)-catechin as a pure polyphenolic compound, and in all the three samples •OH generation could be retained during photo-irradiation for at least a couple of hours. The prooxidant activity of the photo-irradiated extract indicated by •OH yield was more potent than that of the photo-irradiated GSE and (+)-catechin, and this was well reflected in their bactericidal activity in which the photo-irradiated extract could kill the bacteria more efficiently than did the photo-irradiated GSE and (+)-catechin.
Highlights
Grape is the largest fruit crop in the world
Our previous study revealed that photo-irradiation of the aqueous extract from grape pomace obtained from a winemaking process exerted bactericidal effect on Staphylococcus aureus due to highly reactive hydroxyl radial (OH) formation [4]
Since it has been reported that some polyphenolic compounds such as gallic acid, caffeic acid, chlorogenic acid, and proanthocyanidin exerts bactericidal activity upon photo-irradiation [6,7,8], it is speculated that polyphenolic compounds in the aqueous extract from grape pomace would be responsible for the bactericidal activity upon photo-irradiation
Summary
The annual production worldwide amounts to almost 70 million tons and around 80% is used to make wine [1], indicating that waste materials or byproducts obtained from winemaking process could be a valuable resource to be recycled. For instance, it was reported that over 16 million tons of byproducts were produced [2]. As one sphere of beneficial use of waste materials or byproducts obtained from winemaking process, a recent study showed that grape pomace appeared to have the great potential as source of natural antioxidant and antimicrobial agents [3]. Our previous study revealed that photo-irradiation of the aqueous extract from grape pomace obtained from a winemaking process exerted bactericidal effect on Staphylococcus aureus due to highly reactive hydroxyl radial (OH) formation [4]. Since it has been reported that some polyphenolic compounds such as gallic acid, caffeic acid, chlorogenic acid, and proanthocyanidin exerts bactericidal activity upon photo-irradiation [6,7,8], it is speculated that polyphenolic compounds in the aqueous extract from grape pomace would be responsible for the bactericidal activity upon photo-irradiation
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