A cold-water extract of olive leaves (Olea europaea L.) is useful as a texture-improving agent for food protein gels. In this work, the compound contributing to the improvement of gel properties was investigated by using the egg white gel (EWG) as a model for food protein gels. Adding 1.0% (w/v) cold-water extract (OLEx) greatly improved the elasticity (2.1 times), viscosity (4.5 times), and breaking stress (1.4 times) of the EWG. Chemical analyses of the protein revealed that the enhancement of physical properties by OLEx was attributed to protein cross-linking activity of polyphenols. LC/MS and NMR analyses indicated that a major protein cross-linker is the dialdehydic form of demethoxycarbonylelenolic acid linked to hydroxytyrosol (3,4-DHPEA-EDA), which is an aglycone derived from oleuropein, a major polyphenol of olive leaves. These results suggest that 3,4-DHPEA-EDA generated by cold-water extraction from the leaf improves the physical properties, that is, texture, of protein gels.

Olive leaf is known to have the high polyphenol content of 6-9% in dry weight. We investigated the effects of olive leaf powder (OLP) supplemented to fish feed on muscle protein of red sea bream (Pagrus major). Fish reared with feed containing 8% OLP for 40 days had 1.4 times higher myofibril content and 2.2 times higher acid-soluble collagen content than fish reared with control feed for the same period. On the other hand, sarcoplasmic protein content and collagenase activity of the muscle were almost the same between the control fish and OLP-diet fish. Microstructure observation of fish muscle showed that OLP-diet fish has more rigid endomysium structure than that of the control-diet fish. Since collagen fiber in endomysium is responsible for the texture of the muscle, feeding OLP to aquaculture fish will lead to a harder muscle texture. The present study suggests that OLP is a useful feed additive to enhance the texture of aquaculture red sea bream muscle through strengthening of the collagen structure in the muscle.

Yellowtail Seriola quinqueradiata raised on a diet supplemented with 2 olive leaf powder (OLP) was subjected to an analysis of the breaking properties of ordinary muscle. The force-strain curve showed that the slope of the OLP-dietˆsh curve was higher than that of control-dietˆsh that were raised on a diet without OLP. There was no signiˆcant diŠerence in breaking force value, while the breaking strain value of OLP-dietˆsh muscle was 24 lower than that of the muscle of control-dietˆsh. Chemical analysis showed that collagen content was 22 higher in OLP-dietˆsh than in control-dietˆsh. These results suggest that the diŠerence in collagen content is the cause of the diŠerence in textural property between OLP-dietˆsh and control-dietˆsh. The breaking force of the muscles refrigerated for 3 days was 40 higher in OLP-dietˆsh than in control-dietˆsh, suggesting that muscle softening of OLP-dietˆsh was slower than that of control-dietˆsh. Electron microscopic observation of the mus-