Grape Maturity and Light Exposure Affect Berry Methoxypyrazine Concentration

The effects of berry development and light exposure on the concentrations of 2-methoxy-3-isopropyl- and 2-methoxy-3-isobutylpyrazine (isopropylMP and isobutylMP) were examined in grapes. The concentrations of MPs in unripe grapes (at about 30 days after anthesis) of eight cultivars were higher than those in ripened berries, although the concentration was dependent on cultivar. The MPs in unripe Cabernet Sauvignon grapes which had been removed from the cluster and packed in glass vessels increased under fluorescent light. On the other hand, the isopropylMP in grapes sampled at about 50 or 70 days after anthesis, decreased under such light. The MPs in unripe grapes treated with a saturated solution of CaCl₂, which might inhibit the formation of MPs in the grapes, were stable in the absence of light, but decreased in the presence of light. Results indicate that light exposure has two opposite effects on the concentration of MPs in grapes: (a) promoting the formation of MPs in immature grapes; and (b) photodecomposing the MPs in ripening grapes.