Behavior of 3-isobutyl-2-hydroxypyrazine (IBHP), a key intermediate in 3-isobutyl-2-methoxypyrazine (IBMP) metabolism, in ripening wine grapes.

3-Isobutyl-2-hydroxypyrazine (IBHP) is thought to be a key intermediate in both the biosynthesis and degradation of the herbaceous smelling 3-isobutyl-2-methoxypyrazine (IBMP), but its behavior during the growing season is not well understood. First, an improved method for IBHP quantification was developed. A deuterated IBHP standard was added to samples prior to isolation by mixed-mode cation exchange solid phase extraction. Extracts were silylated prior to quantification by GC-MS. A limit of detection of ca. 20 ng/L could be achieved for a 100 mL juice sample. This method was used to quantify IBHP during the 2010 growing season in berries of two clones of Cabernet franc in the Finger Lakes region of New York and of Merlot grown in the California Central Valley. For all three sources, IBHP was detectable at the earliest sampling point, and its concentration per berry increased to a maximum around veraison, 208-477 pg/berry. On a per berry basis, IBHP peaked and began to decline 1-2 weeks after IBMP, indicating that previous studies that sampled preveraison fruit have missed the true maximum value of IBHP. The highest per berry concentration of IBHP observed was in the California Merlot. However, after veraison, IBHP declined more rapidly in the California Merlot than in the New York Cabernet franc, such that the Merlot had the lowest IBHP concentration at harvest. Thus, IBHP at harvest cannot be used as a proxy for IBMP at veraison as was previously suggested.