Science

The Science of Pyrazines in Wine

Understanding the divisive aroma compounds best known for creating green notes like bell pepper and grass in Bordeaux grapes

Methoxypyrazines are present in the well-known Bordeaux family of grapes, but viticulture and vinification impact pyrazine levels in finished wines. Photos by Adobe Stock and Alex Russan.
Methoxypyrazines are present in the well-known Bordeaux family of grapes, but viticulture and vinification impact pyrazine levels in finished wines. Photos by Adobe Stock and Alex Russan.

Methoxypyrazines (MPs)—often just referred to as pyrazines by wine professionals—are some of the most distinctive and divisive aroma compounds in wine. They cause many “green” aromas in wine: bell pepper, jalapeño, grassiness, and earthy or herbal aromas. Their management is a particular consideration with a widespread and genetically close group of grapes: Cabernet Franc, Sauvignon Blanc—as well as their progeny Cabernet Sauvignon—Merlot, Carménere, and Sémillion. 

MPs are polarizing, and not just among wine drinkers: most winemakers want nothing to do with them, while others appreciate what they bring to the glass. The vineyard is where MP impact is largely determined, although red wine structure can influence how they’re expressed in the glass. MPs can be affected by, and have an effect on, a myriad of other aromas in a wine.

Nitrogen is the nutrient most associated with green plant growth, so it’s no surprise that MPs, which cause green, plant-like aromas, are nitrogenous. The MP that most affects wine is 3-Isobutyl-2-methoxypyrazine (IBMP), as it is found at concentrations above sensory threshold levels most frequently. There are “other MPs, similar in character and potency, to IBMP, but usually lower in abundance, and mainly related to inclusion of grape stems during fermentation,” notes David Jeffery, Ph.D., an associate professor in wine science at the University of Adelaide, and the co-author of Understanding Wine Chemistry. The other MPs that may reach sensory threshold concentrations in wine are IPMP (3-isopropyl-2-methoxypyrazine, which also results from ladybug taint), IBHP (3-isobutyl-2-hydroxypyrazine), and SBMP (3-sec-isobutyl 2-methoxypyrazine).

Pyrazines in the Vineyard

MPs are found in many parts of the grapevine, including leaves, roots, and shoots. In grapes, they accumulate early in berry development, with a sharp decline at veraison. Most degradation happens between veraison and 20 Brix, slowing significantly thereafter. Their early, intense presence in grapes prior to veraison may play an evolutionary role: their green flavor may deter frugivores from eating grapes before veraison, when seeds become viable.

What causes the reductions in MP concentration? This is part of what Dominique Roujou de Boubee, the owner of vineyard consulting company Terroir en Botella in Galicia, Spain, set out to find when he was part of a University of Bordeaux team tasked with understanding MPs in 1995. “We knew it was a molecule sensitive to light,” says Roujou de Boubee, who is almost ubiquitously referenced in the MP sections of wine textbooks. “We showed that removing shoots and leaves, creating more light exposure, all reduced the concentration of MPs in grapes.”

Differences in light exposure on clusters can happen in a myriad of ways—whether the result of the land, the vine, or how either are tended by humans—although many factors boil down to vine vigor’s impact on canopy size, and as a result, how much shade is created on clusters. Dr. Jeffery was part of a study that showed significant variation in MP concentration in Cabernet Sauvignon grown on eight different rootstocks—and not only the concentration in the grapes, but the stems as well. However, the difference seemed to result from differing vigor imparted by the different rootstocks, causing more or less canopy growth, and therefore more or less shade on clusters. Jeffery does not rule out the possibility that other factors may be at play. “Considering MPs are produced in the roots and potentially translocated to other parts of the vine,” he says, “there could be a genetic effect of rootstock on MPs, but more work is required to resolve this.” 

David Jeffery, Ph.D., has been part of several studies related to methoxypyrazines. Photo courtesy of David Jeffery.
David Jeffery, Ph.D., has been part of several studies related to methoxypyrazines. Photo courtesy of David Jeffery.

Soil can affect MP concentration as well. Contrasting the near-mystical powers often attributed to soil in the discussion of terroir, its effect here comes down to vigor and light. As the Handbook of Enology, Volume 2: The Chemistry of Wine Stabilization and Treatments states: “In the Bordeaux climate, soil has a decisive influence on MP concentrations … due to its effect on vegetative growth … grapes grown on well-drained, gravel soils have the lowest concentrations. On limestone or clay-silt soils [which retain more water than gravel, encouraging vigor], Cabernet Sauvignon has a higher MP content.” The effect of low soil water retention can be mimicked in irrigated vineyards by reducing irrigation at specific times during the growing season to inhibit shoot growth.

Much less research has been done on the impact of temperature on MPs, and it’s notoriously difficult in wine research to determine what separately comes from light versus heat, as sunlight inherently increases temperatures. Additionally, vineyard temperatures do not get hot enough to break down MPs. 

Despite this, many winemakers and wine drinkers observe a correlation between warmer temperatures and lower MP intensity. “We work with a Cabernet Franc from quite a cool site, the furthest south in Napa,” says Duncan Meyers, the cofounder and winemaker of Arnot-Roberts in Napa, California. “In cooler years, it has higher pyrazines, and it always has more pyrazines than the Cabernet Franc we get from a warmer site in the Chalk Hill AVA of Sonoma County. It’s the same for warmer parts of the Loire—you taste less pyrazines in those wines.” In the HortScience article “Light and Temperature Independently Influence Methoxypyrazine Content of Vitis vinifera Berries,” authors Cassandra M. Plank, Edward W. Hellman, and Thayne Montague write “there appears to be an effect of ambient air temperature on IBMP” in both their own study and numerous others. Just how this may be occurring, however, remains a mystery.


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Topography also has an impact—the angle and direction of a slope changes how much light reaches clusters. In the Northern Hemisphere, south-facing slopes are the most exposed (the opposite is true in the Southern Hemisphere). “We work with a Cabernet Sauvignon from the Fellom Ranch in the Santa Cruz Mountains,” says Meyers. “It’s planted on a knoll that wraps around 180 degrees. The south-facing grapes move through pyrazines and taste pretty ripe every year. Once we pick the east and north-facing slopes, they still have pyrazines.” Higher light intensity also results from lower latitudes (Southern California versus Champagne, for example) and higher altitudes (such as much of Argentina and California’s Sierra Foothills).

How Vinification Impacts Pyrazine Levels

“Pyrazines extract very easily,” says Roujou de Boubee. “In reds, you get the vast majority of what’s in the grape before fermentation even starts. With fermentation and pump-overs, you get the rest. In whites, you get nearly 100 percent after a two to three-hour pressing.” For white wines, he notes that decanting after pressing and racking off the solids slightly lowers pyrazine concentration. In reds, which ferment on the solids, one cannot. 

Thermovinification and flash détente—techniques that involve high temperatures and are generally used to shorten production times—lower MPs significantly because the temperatures reached happen to volatilize them; some winemakers do use them for this purpose when dealing with underripe grapes, although these practices impart their own impact on wine flavor.

Interestingly, for reds, higher MP concentration doesn’t exactly equate to more intense MP aromas in the glass. How red wines are structured appears to have a significant impact on how MPs are expressed in reds. 

Dominique Roujou de Boubee's name is often referenced in discussions of methoxypyrazines. Photo courtesy of Dominique Roujou de Boubee.
Dominique Roujou de Boubee’s name is often referenced in discussions of methoxypyrazines. Photo courtesy of Dominique Roujou de Boubee.

“If a good structure can be created, then pyrazine aromas can be integrated,” writes Clark Smith in Postmodern Winemaking. For Smith, “good structure” considers a variety of complex factors, including anthocyanin to tannin precursor ratios, tannin polymer length, and their impact on colloidal structure. “A properly formed tannin colloidal structure is capable of providing a home within the wine for [varietal fruit, oak, MP, and microbial] aromatic compounds,” he writes. “The shorter the tannin chains, the finer the colloids, and the greater the interactive surface area for intercollating these compounds.”

Stem inclusion can greatly increase pyrazine concentration in a wine. A 2002 study led by Roujou de Boubee showed Cabernet Sauvignon stems to have 53 percent of IBMP content of the cluster itself. A 2023 study that Jeffrey worked on notes that while “Shiraz appears unable to synthesize [MPs] in the berry … MPs are readily extracted from the rachis during fermentation, producing Shiraz wines with uncharacteristic ‘green’ flavors.” 

Pyrazine Perception and Stylistic Decisions

MPs reach sensory threshold at extremely low concentrations (billionths of a gram per liter), and are very stable in a wine over time. As with many wine aromas, other components in a wine may affect our perception of MPs. Jeffery points out that “[the norisoprenoid] damascenone can mask it, perhaps by boosting fruity aromas” and that yeasts that “produce an abundance of fruity esters or release bound monoterpenoids could also have a masking effect.”

MPs can also affect other aromas in wines. According to a study Jeffery worked on, even below sensory threshold, “IBMP can … increase perception of ‘smoky’ and ‘tar’ notes … or decrease … ‘red berries’ and ‘floral violet’ aromas.”

What role should pyrazines play in a wine, if any? The answer is subjective. Many—perhaps most—winemakers do not want their wines to have MP aromas. Roujou de Boubee feels that “the noble expression of Bordeaux is when the Cabernet Sauvignon does not smell like green bell pepper,” but this is not simply about MPs. “If we had the green character, we didn’t reach the perfect ripeness, the tannins were still astringent, the noble aroma of Cabernet Sauvignon wasn’t there yet,” he recalls of his time in Bordeaux. “So for us the absence of pyrazines was an indicator of maturity.” He notes the Bordelaise were offended when an article by Claude Bayonove, who first identified pyrazines in grapes in 1975, stated that green bell pepper was identified as a typical aroma of Cabernet Sauvignon.

Duncan Meyers samples Trousseau for Arnot Roberts. Photo courtesy of Arnot Roberts.
At Arnot Roberts, Duncan Meyers thinks pyrazines add an “intriguing component” to a Cabernet Sauvignon or Sauvignon Blanc wine. Photo courtesy of Duncan Meyers.

The common practice of entirely stripping leaves on one side of the fruit zone—which is usually the default preventative practice by vineyard management in the name of disease control—has served to diminish or negate the presence of pyrazines in wine. Sun exposure on clusters also encourages more impact-driven, ripe fruit character, and fewer earth, floral, and mineral aromas. 

Meyers notes that they only de-leaf in vineyards where disease pressure is the highest, and even then, they do so minimally, looking for the more subtle style that results from more shade on clusters. “A lot of winemakers are concerned that you can’t plant Bordeaux varieties in a cool site, as you won’t ‘move through’ the pyrazine flavors, but I like a little—like old Napa, when you had larger canopies and less leaf pulling,” he says. “It adds a spice and an intriguing component to a wine. Sauvignon Blanc too; I’m happy with more green than tropical.”

MPs are powerful compounds, and can exert a significant impact on a wine’s character, both directly and indirectly. Regardless of a winemaker’s position on MP aromas, MP management for certain grapes can be key for stylistic choices, both through site selection and how that site is tended.

Dispatch

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Alex Russan, based in Los Angeles, is a former winemaker, importer, and sherry bottler.  He writes about viticulture, enology, tasting and the nature of wine

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