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November 2009 Issue of Wines & Vines
 
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How Viticultural Factors Affect Methoxypyrazines

Water levels, cluster shading and temperature all play roles

 
by Justin J. Scheiner, Gavin L. Sacks and Justine E. Vanden Heuvel
 
 
MPs
    Wine East HIGHLIGHTS
     

     
  • The two most common methoxypyrazines (MPs) in wine create the aroma characteristics of bell pepper, asparagus, peas and earth. In water, MPs can be perceived at one part per trillion.
     
  • Processes to remove MPs can also alter desirable wine components.
     
  • Rapid vine growth, excessive water and cluster shading have been associated with high MP levels in studies.
     
  • Good canopy management prior to veraison can reduce MP levels at harvest.

Green pepper, herbaceous and vegetal are three terms sometimes used to describe aromas in wines, especially Eastern red wines such as Cabernet Sauvignon or Cabernet Franc. More than 20 years ago, researchers identified the source of these aromas as a potent class of odorants called methoxypyrazines—also known as MPs, or by their chemical name, 2-methoxy-3-alkylpyrazines.

Since then, MPs have received considerable attention from researchers and industry members alike. While MPs may add complexity or typicity to some wine varieties, such as Sauvignon Blanc, they are generally regarded as having a negative impact on wine quality, especially in red wines.

Enological treatments to remove MPs during or after winemaking (such as thermovinification, micro-oxygenation or the use of activated charcoal or extended oak aging) have had limited success, because practices that will cover up or remove MPs also may alter desirable wine components. Since MPs cannot selectively be removed in the winery, the challenge to viticulturists is straightforward: “How can we control MPs in the vineyard?” Experience has shown that the best predictor of MPs in wine is the concentration of MPs in harvested grapes.

In grapes, the accumulation of MPs begins at berry set, but the majority of accumulation occurs between 30 and 50 days post-bloom. Accumulation ceases approximately 50 days post-bloom, around the start of malic-acid degradation. MP concentrations initially decline one to two weeks prior to veraison, due to berry enlargement and dilution.

According to Imelda Ryona, who reported the results of studies done at the New York State Agricultural Experiment Station in Geneva in the Journal of Agricultural Food Chemistry in 2008, degradation of MPs begins around veraison and continues through maturation. At harvest, MP concentrations are generally only 5% to 10% of the maximum observed pre-veraison. MPs also can be derived from exogenous contamination, such as the multi-colored Asian Lady Beetle (MALB), but we won’t consider these events in this review.

While several environmental factors have been reported to affect MP levels at harvest, interpreting the results from these studies can be challenging. A cultural practice could decrease levels of MPs at harvest by either decreasing accumulation of MPs, or by increasing degradation of MPs.

MPs in grapes and wines
The predominant MP in grapes and wine has an aroma characteristic of bell pepper and is referred to as IBMP (chemical name 2-methoxy-3-isobutylpyrazine). Another MP, which has odors characteristic of asparagus, peas and earth, is also occasionally reported at lower levels. This MP is known as IPMP (chemical name 2-methoxy-3-isopropylpyrazine).

Perhaps the most interesting and challenging aspect of studying MPs is their low sensory threshold. In water, MPs can be perceived at concentrations close to one part per trillion. To put that number in perspective, the current population of the earth is approximately 6 billion.

G.J. Pickering reported in the Journal of Food Science in 2007 that the detection threshold of IBMP is higher in red wine than in water (10 to 15 parts per trillion, depending on wine variety, style, etc.), and the threshold of IPMP is reported as 1-2 ppt.

Cultivars and clones
All cultivars studied to date have measurable levels of MPs pre-veraison, but only the “Bordeaux cultivars” (e.g., Merlot, Cabernet Franc, Cabernet Sauvignon, Carmenère, Sauvignon Blanc) are routinely reported to have MP concentrations at harvest in excess of sensory threshold.

Unsurprisingly, these are also the cultivars that are most likely to be described as having “green pepper” flavors. A few studies have reported differences in MP concentrations between different clones, but this data is limited and insufficient for clonal selection. However, the variation in growth between clones would predict differences in MP levels at harvest, for reasons described below.

The presence of MPs has not been extensively studied in non-vinifera or hybrid cultivars. Unpublished data from the lab of one of the authors, Gavin Sacks, has shown sub-threshold levels in many hybrid cultivars at harvest, including Noiret, Traminette, Catawba, Concord, Maréchal Foch and St. Pepin. Levels in excess of sensory threshold have been found in several experimental V. cinerea crossings (including one with more than 200 ppt of IBMP at harvest).

Grape maturity
MP concentrations decline during berry maturation (see chart on page 117), and berries harvested well before sugar maturity are, of course, widely reported to have higher levels of MPs. We have observed that berries afflicted with grape leafroll virus (GLV) have higher MP levels at harvest, likely because of delayed maturation.

Although maturity is associated with MP degradation, the utility of extended hang time in reducing MPs is still unclear. In 2007, Andrea Belancic reported in the American Journal of Enology and Viticulture that MP concentrations plateau before sugars stop accumulating, but data on the subject is sparse. 

Cluster light environment
Cluster exposure has been associated with lower MPs and decreased vegetative aromas in resulting wines. However, it has not been clear if this decrease is due to increased degradation of MPs post-veraison, or to decreased accumulation of MPs pre-veraison. While the anecdotal explanation has been that MPs are photodegraded in the berry during ripening, there is little evidence to support this claim. Instead, there is compelling evidence that cluster shading will increase accumulation of MPs pre-veraison. 

For example, the IBMP concentration of Cabernet Sauvignon clusters was reported by Malcolm Allen in 1996 to increase with increasing leaf layer number (LLN). Clusters were sampled at 0, 1, 2 and 3 LLN (number of leaves between clusters and the outside of the canopy as determined by point quadrant analysis) at veraison and at harvest. IBMP was approximately three-fold higher at LLN = 3 compared to LLN = 0 at veraison as well as at harvest.

If photodegradation was an important factor, then the fold difference in IBMP levels between shaded and exposed fruit at harvest should be larger than at veraison, but this has not been observed. In the more recent study by Ryona in 2008, regions of high and low cluster light exposure were created in individual vines through manipulation of shoot density.

Within the same vine, well-exposed clusters had lower accumulation of IBMP pre-veraison, but the percentage of IBMP degraded post-veraison was similar in both shaded and exposed fruit. Finally, the highest reductions in MP concentrations occur when clusters are exposed as early as possible after fruit set, rather than at veraison. Said Ennahli reported the data this year.

These results indicate that the timing of cluster exposure is critical, with exposure post-fruit set inhibiting MP accumulation. Although little evidence exists for cluster exposure increasing MP degradation post-veraison, it is not implied that post-veraison cluster exposure should be avoided. Exposing red winegrape clusters during ripening results in reduced disease pressure, increased accumulation of pigments and some desirable aroma compounds, and a decrease in malic acid according to Ron Jackson in Wine Science: Principles, Practice, Perception, published in 2000. 

Vine vigor and crop load
Rapid vine growth during the period of MP accumulation has been linked with high MP concentrations. Although high vigor can result in cluster shading, research also shows that vines with excessive growth produce fruit with higher concentrations of MPs pre-harvest, irrespective of cluster light exposure.

In the study by Alan Lakso in 2009, MP concentration was measured in individual exposed clusters and correlated with their corresponding shoot’s length and growth rate. Longer, faster growing shoots pre-veraison produced clusters with higher concentrations of MPs at their pre-veraison peak.

A recent unpublished study on Cabernet Franc by Justin Scheiner suggests that crop load (as expressed by the ratio of yield to pruning weight) is more directly related to MPs than simply vine growth or vigor. Lower yielding vines are usually more vigorous and vice versa. Vineyard managers who strive for low yields but struggle with excessive vigor should consider revisiting their yield management strategies to focus on vine balance as opposed to cropping at a specific level.

Water status and soil
Vine water status also is linked to MPs. Excessive water led to higher MPs at harvest according to Cristina Sala in 2005 (reported in the Journal of the Science of Food and Agriculture). Typically, high water availability results in more vegetative growth, and thus more cluster shading. Both of these conditions are associated with greater MP accumulation. 

In 1995, Ann Noble linked soil type to MP content in grapes and wine, because of its ability to influence vine growth through characteristics such as water holding capacity and nutritional status. Vines grown on soils with high vigor potential—those that are relatively deep, fertile and have a high water holding capacity—are expected to grow more rapidly and also experience more cluster shading, again resulting in higher MPs.

Temperature
Growing degree-days (GDD) and MPs in resulting wines are reported to be inversely correlated, with lower MPs observed in hotter years or regions. However, in several of these studies, GDD is likely correlated with other factors reported to reduce MP accumulation (e.g., more sunlight, lower water availability). Fewer growing degree-days also are expected to result in a delay in maturation, and thus higher MPs at a given harvest date (M.J. Lacey et al., in the American Journal for Enology and Viticulture, 1991). The impact of temperature on MP accumulation and degradation, independent of other viticultural factors, is still unknown.

Control of MPs in the vineyard
As noted earlier, research to date indicates that MP concentrations at harvest are closely correlated with MP concentrations pre-veraison and, as a result, cultural practices can be used to decrease MP accumulation. In particular, more vigorous vines and/or cluster shading yields fruit with higher MP concentrations, likely because these conditions encourage MP accumulation. Unlike growing season temperature, these factors can be manipulated as a possible means of controlling MPs.

Ideally, this begins with vineyard establishment. In a site with high vigor potential (deep fertile soils, high seasonal rainfall, etc.), proper drainage, rootstock selection, vine spacing, training system and nutrient management could have implications with respect to MPs. For example, close in-row vine spacing could lead to inadequate room for vine growth, and consequently to excessive cluster shading and poor vine balance.

In established vineyards, one can apply the same principles of optimizing vine balance and cluster microclimate. Irrigation and nutrient management can be utilized to control vine size, but these may not be an option in areas where rainfall is excessive and soils are naturally fertile. In this case, vine vigor and crop load may be best manipulated through pruning and canopy management.

Balanced pruning of vines (i.e., pruning vines to an adequate number of nodes based on the previous year’s growth) is a straightforward and effective means to manipulate vine size and crop. In situations where vines are highly vigorous, leaving more nodes will reduce vine growth and have a direct, positive impact on yield and crop load.

Increasing shoot densities can, however, lead to crowded shoots and shaded conditions in the cluster zone; therefore it might not be appropriate for all situations, or it might necessitate retrofitting the training system to one (such as Scott Henry) that will accommodate more shoots.

Another canopy management strategy to reduce MPs is basal leaf removal. Several studies have shown that opening up the fruiting zone results in increased fruit quality. With respect to MPs, this should be done early in berry development, before MP accumulation begins.

Conclusion
Although there are additional management possibilities to reduce MPs, the strategies discussed above are considered universal in terms of overall good viticulture management. Optimizing vine balance and cluster light environment should be the goal of any canopy management regime to maximize fruit quality. In particular, we stress the importance of good canopy management practices pre-veraison to reduce MP accumulation, and eventual MP levels at harvest.

Justin J. Scheiner, Gavin L. Sacks and Justine E. Vanden Heuvel conduct research at Cornell University Ithaca and Geneva, N.Y. To comment on this article, e-mail edit@winesandvines.com.


Editor's note: There was a misstatement on page 115 of the November issue, in the Wine East feature “How Viticultural Factors Affect Methoxypyrazines.” The first sentence under the subhead “Cluster light environment” should read, “Cluster exposure has been associated with lower MPs and decreased vegetative aromas in resulting wines.”

 
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