Correlation of 3-Isobutyl-2-methoxypyrazine to 3-Isobutyl-2-hydroxypyrazine during Maturation of Bell Pepper (Capsicum annuum) and Wine Grapes (Vitis vinifera)

Environmental factors affecting degradation of 3-isobutyl-2-methoxypyrazine (IBMP, green pepper aroma) in wine grapes (V. vinifera) are widely studied, but the degradation pathway is not defined. We hypothesized that IBMP is demethylated to 3-isobutyl-2-hydroxypyrazine (IBHP) during fruit maturation effectively reversing the final putative step of IBMP biosynthesis. A quantification method for IBHP was developed using solid-phase extraction coupled to one- or two-dimensional gas chromatography mass spectrometry with a recovery of ca. 80%. IBMP and IBHP in bell peppers (Capsicum annuum) and V. vinifera (cv. 'Cabernet Franc', 'Riesling', 'Pinot noir') were then measured at different maturities. IBMP and IBHP were inversely correlated in both bell peppers (R-2 = 0.958) and Cabernet Franc grapes (R-2 = 0.998) over a range of maturities. In bell peppers, we observed a significant decline in IBMP (125 to 15 ng/mL) and increase in IBHP (undetectable to 42 ng/mL) during ripening. In grapes, all cultivars had comparable IBHP concentrations preveraison (64 to 88 pg/mL) but differed in IBHP concentration by 2 orders of magnitude at the final sampling point (undetectable to 235 pg/mL). Higher preveraison IBMP was correlated with higher final IBHP across the three grape cultivars, with the order Cabernet Franc > Riesling > Pinot noir for both IBMP and IBHP. Acid hydrolysis resulted in a significant increase (33%) in IBHP in Cabernet Franc, indicating that IBHP exists partially in a bound form in grapes.