Methoxypyrazine Biosynthesis in Cabernet Sauvignon

This morning, I am reading the article Determining the Methoxypyrazine Biosynthesis Variables Affected by Light Exposure and Crop Level in Cabernet Sauvignon in this month's issue of The American Journal of Enology and Viticulture.¹ Somewhere along the line, I learned that methoxypyrazines are responsible for the herbaceous, green notes that can plague the flavor of Cabernet sauvignon and Cabernet franc, as well as Sauvignon blanc. 3-Isobutyl-2-methoxypyrazine or IBMP is the chemical culprit responsible for this herbaceous character and studies have shown that the concentration of IBMP in grapes is influenced by many variables:

• Maturity
• Sunlight exposure
• Temperature
• Water Status
• Vine vigor
• Yield

The last step in IBMP production is known to involve the methylation of 3-isobutyl-2-hydroxypyrazine, IBHP. A methyltransferase gene (VvOMT3) was recently shown to be responsible for most of the differences in methoxypyrazine production among grapevine varieties. The chemical structures of IBHP and IBMP are shown below:

Two variables, sunlight exposure and grape yield, have a direct influence on IBMP concentration in grape berries. Both sunlight exposure and yield are easily and commonly manipulated in the vineyard by practices such as winter pruning, leaf plucking, and bunch thinning. The authors of this research conducted experiments with sunlight exposure and crop level to determine which variable had a more pronounced impact on IBMP production.
The primary aim of the sunlight exclusion experiment was to determine:
(1) if the treatments caused changes in IBMP concentrations
(2) if these changes were due to the alteration of either VvOMT3 expression or IBHP concentration. The authors found that light reduced the expression of VvOMT3 and the concentration of IBHP, suggesting that a combination of these factors reduces IBMP concentration when fruit has greater light exposure. Berry shading has a greater influence on IBMP concentrations in preveraison berries, when methoxypyrazines accumulate.
Cabernet Sauvignon vines with low crop level produced grapes with higher IBMP concentrations than vines with high crop level, however, the effect of crop level manipulation on IBMP concentration was not as great as the effect observed by the elimination of light exposure on bunches.
In order to minimize the impact of IBMP accumulation in Carbernet sauvignon, the authors suggest that canopy management regimes that result in exposed bunches very early in fruit development should be implemented. Altering canopy management after veraison will have less of an effect on IBMP concentrations.
References:
1. Jake D. Dunlevy, Kathleen L. Soole, Michael V. Perkins, Emily L. Nicholson, Suzanne M. Maffei and Paul K. Boss, Determining the Methoxypyrazine Biosynthesis Variables Affected by Light Exposure and Crop Level in Cabernet Sauvignon, Am. J. Enol. Vitic., December 2013, vol. 64 no. 4 450-458.
2. The structure of IBMP was found here: Chemical Book 3-Isobutyl-2-methoxypyrazine(24683-00-9)
3. All structures were drawn by the freely available drawing program from ACD Labs called ACD/ChemSketch Freeware.