To optimize conversion of rutin to isoquercetin by commercial alpha-l-rhamnosidase using high hydrostatic pressure (HHP). The de-rhamnosylation activity of alpha-l-rhamnosidase for isoquercetin production was maximal at pH 6.0 and 50 A degrees C using HHP (150 MPa). The enzyme showed high specificity for rutin. The specific activity for rutin at HHP was 1.5-fold higher than that at atmospheric pressure. The enzyme completely hydrolysed 20 mM rutin in tartary buckwheat extract after 2 h at HHP, with a productivity of 10 mM h(-1). The productivity and conversion were 2.2- and 1.5-fold higher at HHP than at atmospheric pressure, respectively. This is the first report concerning the enzymatic hydrolysis of isoquercetin in tartary buckwheat at HHP.