Often shortened to 4VG, 4-vinyl-guaiacol is one of the most important flavor and aroma chemicals in the alcoholic-beverage world.
This compound belongs to a large class of chemicals known as phenols, which are defined by having one or more hydroxyl groups attached to an aromatic ring. While many kinds of phenols occur in beverages, only a relative few are sensory-active. Besides 4VG, other flavor-active phenols include guaiacol, phenol, vanillin, acetovanillone, eugenol, 4-vinyl-syringol, and 4-vinyl-phenol (4VP).
The presence of 4VG is common in many foods as well as perfumes. Many describe it as having a strong clove or spicy smell, while others describe it as smelling of carnations or apples. Part of what makes 4VG important is its extremely low flavor threshold—in the parts per billion (ppb) range, with some picking it up at concentrations as low as 4 ppb.
That low threshold makes controlling the level of 4VG in a finished product extremely important.
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4VG in Alcoholic Beverages
In spirits, we mostly associate 4VG with the distinctively spicy flavor of rye whiskey, but it also plays an important role in the flavors of many other distilled products. Some craft distillers actively seek out rye varieties with higher potential for 4VG production, aiming to increase the spicy character of their products.
That demand has led several universities to take more interest, as they research new growing methods for high-4VG rye. Not all producers share the enthusiasm, with some distillers seeing 4VG as a possible off-flavor in whiskey.
The compound also occurs in other drinks. In beer, its clove-like character is most associated with Belgian and German wheat beers, though it can also occur in other styles. Many wines also include 4VG, and many view it as desirable in certain white wines. It’s also a major flavor component in Chinese rice “wines.” The fermentation starter—jiuqu or qu in Chinese—is rich in enzymes that produce 4VG, and its presence is an important aspect in fermented and distilled versions of such products.
How 4VG Is Made
There are several ways to create 4VG, whether intentionally or otherwise. The most common is via enzymatic decarboxylation by a microbe with an active PAD1 gene.
Short for phenyl acrylic decarboxylase, PAD1 is a nonessential gene commonly found in yeast and other microbes. PAD1 produces enzymes that decarboxylate various phenolic acids into compounds less dangerous to yeast. For 4VG, the most common and important precursor is ferulic acid. During fermentation, yeast that are PAD1-positive (PAD1+) will enzymatically remove a CO2 molecule from available ferulic acid, thus producing 4VG (see diagram below).
Another way to create 4VG from ferulic acid is through a process known as thermal decarboxylation. Thermal decarboxylation simply refers to the use of high heat to break off a CO2 molecule from ferulic acid and convert it into 4VG. Mashing and fermentation don’t get hot enough for thermal decarboxylation, but it can happen when boiling in the still. Studies have found a linear increase in the amount of 4VG in direct relation to how long a product is exposed to high heat. However, it’s difficult to determine exactly how much 4VG this method produces.
Finally, another process that creates 4VG is photo-isomerization, which involves exposure to sunlight. However, this is a rare occurrence.
Increasing 4VG
Many craft distillers—but especially rye distillers—have begun looking for ways to increase the amount of 4VG in their products. There are several options.
Yeast
The first and often most important step to increasing 4VG is to select yeasts that have an operational, highly active PAD1+ gene.
This gene’s level of activity varies from strain to strain. The most common way to measure that activity is by measuring yeast growth on cinnamic-acid media. Cinnamic-acid media are made up of various cinnamic acids that have an antimicrobial effect. These cinnamic acids—which include p-coumaric acid and other close relatives to ferulic acid—must be broken down for yeast to be able to grow on the media. Because only PAD1+ yeasts can break down cinnamic acids, a strain’s rate and strength of growth gives an indication of the amount of PAD1 activity present in the yeast.
That’s a relatively straightforward method, but it’s not always accurate because many other factors can affect yeast growth. For higher accuracy, yeast suppliers often prefer to measure PAD1 activity through a combination of gene editing and high-performance liquid chromatography (HPLC) sampling. These methods are extremely accurate, but they’re also time-consuming and often prohibitively expensive for most distillers.
Besides the PAD1 gene, it’s important for distillers to select yeast strains that can produce their own endogenous feruloyl esterase. Feruloyl esterase is a naturally occurring enzyme in some yeast, and it is key to liberating cinnamic acids from large molecules. The presence of an endogenous feruloyl esterase gives the yeast more substrate to convert using the PAD1 gene, resulting in increased 4VG concentration.
Measuring feruloyl esterase activity in yeast is complicated, often best handled by professional labs. Best practice for distillers is to consult with yeast suppliers about a strain’s PAD1 and feruloyl esterase activity first, before doing any in-house testing.
Raw Materials
You can also increase 4VG production by selecting certain ingredients.
A distiller who wants to increase 4VG should choose grains with significant quantities of available ferulic acid, the precursor. Without ferulic acid—or if the concentrations are very low—the PAD1 pathway won’t be able to produce noticeable amounts 4VG.
The grain most commonly associate with increased ferulic acid is rye, which has amounts frequently measured above 1,000 micrograms per gram of dry matter (ug/g DM). This makes sense considering rye whiskey’s association with spice character, but note that the amount of free ferulic acid can vary greatly among different varieties and growing conditions.
For example, Michigan State University recently released a study in which they planted 21 different varieties of rye in two distinct regional plots. They then analyzed the rye from those plots and found a difference of as much as 8.15 mg/L in ferulic acid concentration among varieties, and as much as 6.3 mg/L difference between plots. A key takeaway here is that you can’t use just any rye and expect increased 4VG. The data also show that the concentration is positively correlated with protein content—so, you can use protein as a predictor for ferulic acid.
It’s also notable that some rye varieties have larger quantities of other phenolic acids, such as sinapinic and caffeic. While these don’t necessarily have decarboxylated flavor compounds associated with them, they can still contribute to the character of a product.
Beyond rye, there are other grains with significant amounts of ferulic acid. Wheat is widely regarded as a good source, and in brewing it’s an important part of the profile in Bavarian weissbier. There are also substantial amounts in barley, where it is connected to lignin and arabinoxylans. The malting process is known to promote the activity of feruloyl esterases, which also can increase the amount free ferulic acid.
Finally, less traditional sources include oats and buckwheat—in the latter, 4VG is one of the main aroma components. While studies have found increased levels of various phenolic acids in both those grains, more research is needed to better understand their ability to add 4VG to distilled products.
Mashing
Distillers can also alter their mashing regimen to favor conditions that increase 4VG and its precursors.
The most important adjustment a distiller can make in that regard is to include a low-temperature protein rest. Enzymes such as proteases and cinnamyl esterase work best at low temperatures, and they’re critical to maximizing the amount of free ferulic acid released into a mash. Over the years, several studies have confirmed that holding mashes at temperatures between 104° and 113°F (40° and 45°C) results in an increase in overall ferulic acid, which in turn leads to increased 4VG concentrations in the final product. Conversely, mashes that skip the protein rest and go immediately to gelatinization temperatures have lower ferulic acid and 4VG concentrations because the higher heat denaturizes important enzymes.
A second adjustment is to alter the mash’s pH. During mashing, distillers often use various salts, acids, and chemicals—whether endogenous or exogenous—to buffer washes to between 5.2 and 5.6 pH. Unfortunately, that’s not the ideal range for cinnamyl esterase and other enzymes critical to creating free ferulic acid. Instead, these enzymes prefer a pH of 5.8 and above. So, distillers who want to increase ferulic acid could try increasing the pH of the overall mash—however, this comes with other risks because pH levels above 6 negatively affect other enzymes that are critical to saccharification.
Also worth noting: At a pH below 4.6, cinnamyl esterase activity appears to stop completely.
Distillation
Finally, a fourth way to increase 4VG concentration is by lengthening the boil in the still.
As I mentioned above, thermal decarboxylation can produce 4VG via high heat. By its nature, the distillation process already includes the required temperatures, and increasing the contact time could increase the production of 4VG. Unfortunately, despite some anecdotal evidence to support this method, no studies have yet confirmed its effectiveness.
Methods for Decreasing 4VG
Conversely, some distillers may want to decrease the presence of 4VG in their distillate—or perhaps dial it in, if the presence is perceived as too high.
For the most part, distillers can accomplish that by considering the above steps in reverse: select PAD1-negative (PAD1–) yeast strains; use ingredients low in ferulic acid and other precursors; go for lower pH and higher temperatures in the mash; and reduce the high-heat time in the still.
However, there’s an additional step distillers can take to reduce 4VG: a high-temperature sanitation step before fermentation. While that may seem counterintuitive, because of the danger of thermal decarboxylation of ferulic acid to 4VG, this extra step helps to ensure the complete degradation of enzymes that can release ferulic acid, and it reduces the overall bacterial load of the wash. Reducing the bacterial load is important; many contaminating microbes (such as wild yeast) are highly PAD1+ active, and they will produce considerable amounts of 4VG.
Indeed, for distillers who want to reduce 4VG as much as possible, cleaning and sanitation are incredibly important at all stages of production. On rare occasion, wild yeasts and microbes have been known to pass the PAD1+ gene to previously PAD1– yeast strains. This means that even a very small infection may eventually result in a noticeable amount of 4VG.
Final Thoughts
Without doubt, 4VG is one of the most significant flavor chemicals in the distilling world. The resurgent popularity of rye whiskey has sparked new and interesting research, and craft distillers are among those leading the charge. As the industry grows, no doubt we’ll be able to expand our knowhow on harnessing 4VG for spirits that meet our flavor expectations.
