Reduction of astringency and evaluation of bioactivity of compounds produced by enzymatic glycosylation of oleuropein

Olive leaves are treated as a waste product during olive oil production. However, they contain a higher concentration of the important bioactive compound oleuropein than the oil itself. Development of olive leaf-based food products has been limited by the astringency of oleuropein. Many methods have been developed to remove astringent substances from food products. However, phenolic compounds have multiple beneficial bioactivities and their removal from food matrices sacrifices health-promoting benefits. This study aimed to reduce the astringency of oleuropein while maintaining its bioactivity by adding glucose residues to its structure. Although glycosylating enzymes have been investigated, no studies have demonstrated the glycosylation of oleuropein to date. In this study, Glycosylation reactions were conducted using four different enzymes, and the products obtained using cyclodextrin glycosyltransferase derived from Bacillus macerans NBRC 3490 were identified. Liquid chromatography-mass spectrometry analysis indicated that the products consisted of oleuropein containing one, two, or three added glucose residues. Structural analysis using nuclear magnetic resonance spectroscopy confirmed that the glucose residues were connected via 1,4-linkages. Taste sensor analysis revealed that astringency decreased in proportion with the number of glucose residues added. Although structural modifications are likely to influence the bioactivities of compounds, assessment of vascular endothelial protective effects demonstrated that the bioactivities of the glycosylated products were comparable to that of oleuropein. The evaluation of the taste properties of oleuropein in this study using a taste sensor provides evidence that the addition of glucose residues effectively reduces the astringency of oleuropein while preserving its bioactivity. These findings highlight the potential of discarded olive leaves for practical applications and contribute to the development of innovative strategies for astringency reduction. • Cyclodextrin glycosyltransferase from Bacillus macerans NBRC 3490 glycosylated oleuropein. • The glycosylated products were much less astringent than oleuropein. • Bioactivities of the glycosylated products were comparable to that of oleuropein.