I f a wine seems “green”, “herbaceous”, or “vegetal”, aroma compounds called methoxypyrazines (MPs) could be responsible. MPs are particularly important due to their ubiquity in grapevines and their extremely low aroma thresholds. These low thresholds make MPs some of the most odor-active compounds found in wine. Occasionally, they can have a positive impact by providing varietal characters– classic examples are bell pepper in Cabernet Sauvignon and grassy/gooseberry in Sauvignon Blanc. However, higher concentrations can mask fruity aromas and damage wine quality.

MP concentration can be such a make-or-break parameter that it’s often a major factor in harvest decisions. Harvesting later can reduce MP concentrations, but sometimes doing so can sacrifice other elements of fruit quality. However, MPs are very stable during fermentation and aging—so once they’re there, they’re there to stay. A number of studies have explored methods to lower MPs post-harvest, but these “fixes” can sometimes have negative effects on wine.

MPs in wine can come from a variety of sources, and understanding their creation is crucial to prevent high levels. As with most flaws, remediation of high MP concentration can be risky, so prevention is key.

Types of methoxypyrazines

There are at least four MP compounds that are currently considered significant in wine, but we will focus on the two most abundant (and best studied):

• IBMP (3-isobutyl-2-methoxypyrazine)
• IPMP (3-isopropyl-2-methoxypyrazine)

These two compounds are also found in vegetables and nuts. In particular, bell peppers have high levels of IBMP while peas have high levels of IPMP, and these compounds give them their distinctive aromas. An easy way to remember the difference is IBMP (Bell pepper) and IPMP (Pea). In different wine matrices, these two MPs can take on other characters as well, as discussed below.

Molecular structures of IBMP (3-isobutyl-2-methoxypyrazine) (left) and IPMP (3-isopropyl-2-methoxypyrazine) (right).

IBMP

IBMP is generally the most abundant MP found in wine. It plays a major role in Sauvignon Blanc, where it accounts for about 80% of total MP content. IBMP character has been described as fresh vegetables, bell pepper, gooseberries, herbaceous, leafy, and vegetative in wine. The aroma threshold is about 10x lower in white wines than in reds (see Table 1).

IPMP

IPMP is the second most abundant MP. It is generally found at the highest levels in stems of grapevines, with lower levels found in the grape seeds and skins. Typically, it is only seen in wines that were fermented with the stems. Its aroma has been variably described as green pea, earthy, asparagus, leafy, and green beans in wines. Although concentrations are usually not as high as IBMP, the threshold for IPMP is extremely low (see Table 1.)

Table 1. Methoxypyrazine compounds in wine, their thresholds, reported concentration ranges, and aromatic characters.

Sources of methoxypyrazines

Grape Biosynthesis

Why are MPs produced in the first place? They are believed to deter seed dispersers (e.g., birds), which find MPs unpalatable, from consuming the fruit before the seeds are viable. Early in fruit development, while the seeds are immature, the plant produces MPs to help ward off animals. Around the time of veraison, the seeds become viable and the plant begins to make the fruit much more appealing: sugar accumulates, acidity decreases, and of course, MP concentration plummets. This pattern can be seen in Figure 1.

Figure 1. Concentrations (w/w) of IBMP and IBHP in California Merlot over time. Dpv = days post veraison. Data from Harris et al. 2012.

The amount of MPs produced by a vine is genotype-specific, meaning certain varieties—particularly those associated with Bordeaux—produce higher levels than others. However, growing conditions and viticultural practices can have dramatic influences as well. These conditions include: sunlight, temperature, water status, vine vigor, and yield. The impact of each of these is shown in Table 2.

Table 2. Grape vine growth conditions and impact on MP concentration.

Sunlight exposure has received a great deal of attention in MP studies. Though it’s generally understood that more sunlight equals lower MPs, there are actually two mechanisms by which sunlight impacts MPs. To make things more complicated, these mechanisms are mediated by the maturity of the fruit. Essentially, pre-veraison sunlight exposure controls the amount of MPs made, while post-veraison sunlight impacts the amount lost. Understanding the difference between the two mechanisms is critically important, because as we will see, one of them is much more impactful than the other.

To quickly review, the grapevine produces and accumulates MPs while the fruit is unripe, but as the fruit matures, it ceases production. The gene responsible for this production, VvOMT3, is sensitive to light. Sunlight exposure down-regulates this gene, so more light causes it to make a smaller amount of MPs. However, this gene is only active pre-veraison. It is naturally “deactivated” around the time of veraison, after which the pool of MPs it created starts to degrade. Sunlight can greatly impact the amount of MPs produced. However, it cannot be overstated that this mechanism is only relevant pre-veraison, while the gene is still active.

The second mechanism occurs post-veraison, when the gene is inactive. Sunlight helps to degrade MPs through a process called photodegredation. However, a study by Dunlevy et al (2013) found that photodegradation accounts for only a small portion of the steep decline in MP concentration seen after veraison. The authors posited that the rest of the degradation might also occur via metabolism, non-enzymatic decomposition, or volatilization of IBMP out of the berry.

The takeaway point is that sunlight early in fruit development will have a larger impact (by lowering the total MPs made) than after veraison, since the effects of photodegredation are minimal. To help visualize these trends, below we’ve modified the curve seen in Figure 1 to show the theoretical effects of sunlight at different times (Figure 2).

Figure 2. Hypothetical impact of sunlight exposure at different stages of ripeness.

Why would MP production be so dependent on sunlight? The simplest explanation is fruit maturity: vines may have evolved to react to low light (which would slow maturation) by ramping up MP production, thereby buying some time before seed dispersers found the fruit appealing. The same applies for temperature, which shows a similar pattern to light (as temperature increase, MPs decrease). It is worth mentioning that because increasing light often simultaneously increases temperature, it has been experimentally challenging to separate the two effects.

Sunlight effects can be modulated by viticultural decisions like (early!) leaf pulling. Other viticultural decisions, such as pruning and irrigation, also affect MP development and concentration. It should be noted, however, that these often have a cascade of effects on the vine. Much like with the “temperature vs. light” conundrum, studies have found it difficult to isolate these variables. In particular, anything that increases vine vigor can also mean larger, shadier canopies that reduce light exposure on clusters. This may help explain seemingly paradoxical studies that have found higher IBMP in warmer climates versus cool, as the warmer climate may have resulted in higher vigor. Supporting this concept, there is evidence that balanced vines tend to have lower concentrations of IBMP. Though the exact mechanism is not fully understood, IBMP has also been found to be negatively correlated with yield, with lower yields resulting in 19-82% more IBMP per berry. Irrigation and nitrogen fertilization effects may be attributable to increased vigor rather than affecting MPs directly—both have been found to increase IBMP, but it was unclear if this was due to greater canopy cover.

A well maintained canopy in the vineyard. Vine shoots are aligned and leaf removal has been carried out to allow balanced sun exposure among the leaves.

Ladybug Taint (LBT)

Yes, you read that right. Even if MPs are successfully mitigated in the vineyard, crush can still leave you vulnerable to the seemingly unobtrusive ladybug. The issue? A few of the MPs that can ruin wine (IPMP, in particular) are also found in the hemolymph of ladybug species. Ladybugs often gather on ripe grape clusters around harvest time, and if they are not removed before crushing and/or if they make it into the must later, LBT is likely to result. Much like other causes of high MPs, LBT can be perceived as undesirable green aromas. However, since LBT is better associated with IPMP (rather than IBMP), it may also take on “peanut” descriptors (or the more unfortunate “burnt peanut butter”). Botezatu & Pickering (2010) estimate 200-400 ladybugs per ton of fruit can be problematic. While this may seem like a high number, awareness and concern about LBT has been increasing since it was first widely reported in North America two decades ago. Furthermore, LBT may become a larger issue in the coming years, as global warming is increasing the population and habitat range of ladybug species.

To prevent LBT, the only real solution is to simply remove ladybugs from fruit. With machine-harvested fruit, this can be extremely difficult to accomplish. With hand-harvested, however, using shaker tables and sorting have been found to be effective at removing the insects.

Ladybugs are beneficial insects for a variety of reasons, but they have been shown to elevate MP levels in wine.

How do I get rid of MPs once they’re there?

MPs, once present, can be stubborn. So if prevention is no longer an option and remediation is required, the solutions can be aggressive. However, the more “scorched earth” fining agents, like activated charcoal, are typically either ineffective at removing MPs, or remove too many desirable compounds to make their usage justified. Currently, there are a few solutions that don’t involve fining agents that have been shown to be effective.

In white juice, clarification before fermentation can reduce IPMP by up to 50%, with the settling time being positively correlated to the amount of IPMP removed. However, this is not an appropriate measure for red wines. Must heating has also shown some success, with reductions of 25-50% IPMP, but only 9% of IBMP. Must heating also risks sensory changes and loss of desirable volatile compounds. Yeast strain selection has also been a topic of interest, but is ineffective at lowering actual MP concentrations; rather, it produces additional fruity volatiles to offset the masking effects of MPs. Lalvin D21 has been shown to successfully lower the perception of MPs, however there is evidence that some yeast strains (Lalvin BM45, for example) can actually increase IPMP. For this reason, if yeast strain selection is to be used for MPs, it is strongly recommended to conduct ample research before selecting a particular strain. Oak chips work similarly to beneficial yeast strains, as they can mask MPs but will not reduce their concentration. Oak is not suitable for all wine styles, however.

Plastic polymers work by actually removing MPs through sorption, though many can contribute unwanted aromas or strip desirable compounds. Currently, the most favorable plastics include a biodegradable plastic of polylactic acid and silicone. These plastics are encouraging as they haven’t been found to contribute undesirable aromas nor significantly strip non-MP compounds, and polylactic acid and silicone are both already FDA approved. Silicone is especially effective; after 24 hours of contact with wine, IPMP was reduced by 96% and IBMP by 100%. Further studies are needed to confirm the selectivity of these polymers, and the method is still being developed for adoption in commercial wineries.

Conclusion

MPs are extremely important aroma compounds in wine that are easy to come by, but hard to lose. As we learn more about the sources of MPs, it is becoming easier to prevent them. Recent discoveries have helped elucidate not only the significance of viticultural decisions on MPs, but also their timing. With more informed viticultural practices being employed, we can help remove the troublesome variable of MPs from the (already complicated) decision of when to harvest. And when these improvements fail, new methods of remediation are improving the outcomes of wine sensory profiles. Promising new technologies, such as plastic polymers, may become a silver bullet for MPs. Even for winemakers who are a bit more reticent to adopt “unnatural” fixes like plastic fining, more passive options are available that may help.

Works Cited

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