The Impact of Yeast on Wine Aroma and Flavor: Part One
The Impact of Yeast on Wine Aroma and Flavor: Part One
A recent article by Daniel Pambianchi in WineMaker Magazine inspired this Carl’s Corner post on some key impacts of yeast on the development of aromas and flavors in wine. If you would like a deep dive into biochemistry, take a look at the original article. What follows is a less scientific synopsis of key points relating to aromas and flavors that are produced or influenced by yeasts while doing their primary job of converting sugar in the grapes to ethyl alcohol and carbon dioxide. A few other actions that create aromas and flavors, not necessarily primary functions of yeasts, are also mentioned. Please enjoy the read.
Most wine these days is produced with a commercially developed strain of Saccharomyces cerevisiae wine yeast. Research over the past 25 years has developed much greater understanding of yeast form, function, and productivity during wine making. Companies like Lallemand (Lalvin), Red Star, White Labs, Wyeast, Chr. Hansen, and Laffort, with research support from key enology labs like UC-Davis and the Institut Coopératif du Vin (ICV) in France, have isolated, developed, and tested hundreds of yeast strains, of which over 200 are now commercially available to winemakers. These yeast strains have been developed for different grape varieties, ranges of fruit quality, preferred wine styles, and fermentation conditions. Yeast can be used to enhance or accentuate varietal or process-driven character (as in big, complex, oaky reds); or it can basically just do the work of fermentation and then get out of the way (as in light, crisp, approachable whites).
In his article, Pambianchi points out that yeast selection is very important. Many feel that using natural or indigenous yeasts that come with the grapes from the vineyard is the “sexy, popular” method for today’s wines. However, unless a winemaker is intimately familiar with indigenous microflora, particularly from the vineyard which supplied the grapes, this may not be the most suitable condition for fermentation. These indigenous yeasts can unpredictably react with various compounds in the grapes to produce undesirable aromas and flavors. They can also be very slow at fermentation allowing bad actors more time for mischief. As noted, “In general, non-Saccharomyces yeasts do not possess enological characteristics favorable to fermenting juice into wine.” It has often been pointed out that commercial yeast strains are just native yeasts that were selected and isolated for having superior fermentation characteristics and flavor, then propagated for sale.
So, most winemakers study the characteristics of available commercial Saccharomyces yeast strains that match grape, wine style, and preferred fermentation conditions. All Saccharomyces cerevisiae yeast strains convert glucose and fructose, the primary sugars found in grapes, into ethanol with carbon dioxide and a trace amount of glycerol as by-products. Water, alcohol, and glycerol make up about 99% of the wines we enjoy. Yeasts are also responsible for producing many of the metabolites that make up that other 1% of the wine to differentiate them and make them special and enjoyable. These metabolites include esters, terpenes, ketones, higher alcohols, acids, aldehydes, and volatile sulfur and nitrogen compounds. Let’s consider some of these now.
Esters are chemical compounds that create fruit and flower aromas, and hundreds of them have been identified in wines. Key examples include isoamyl acetate (banana) and phenylethanol acetate (rose petals). Lactones are cyclic esters that have different properties and most often provide wood or oaky aromas and flavors. For example, the lactone sotolon imparts nutty and spicy aromas of toasted almonds and maple syrup. Octalactone results primarily from oak aging and smells of toasted coconut. Another class of compounds called terpenes provide a wide range of aromas – rose petals (geraniol), citrus (limonene, citral), and orange and lavender flower blossoms. 1,8-cineole and alpha-pinene are volatile terpenes that smell like evergreens - eucalyptus, juniper, and rosemary.
The two key acid compounds in grapes are tartaric and malic acids. Tartaric acid is typically not affected by yeast during fermentation, but winemakers study these acids and their transformations carefully. This is especially the case if malolactic transformation is part of the process to convert malic acid, tart apple acidity, into more mellow lactic acid, like in butter or yogurt, creating a gentler creamy mouthfeel in the wine.
Other acids and some higher alcohols can be formed to the detriment of the wine. Often poor sanitation or extreme conditions can result in the formation of acetic acid (vinegar) and others, along with their esters, that can mask desirable aromas and flavors. Winemakers must be constantly vigilant to manage their equipment, processes, and products to prevent undesirable rogue yeasts and bacteria from creating spoilage in their wines.
Ketones are another class of aroma compounds produced in small quantities. Probably the most common ketone found in wine is a diketone called diacetyl, which gives a most pleasant buttery character (think microwave popcorn). Diacetyl is actually formed during the malolactic transformation of red wines and some whites, like Chardonnay. Monoterpenols and norisoprenoids can have a strong impact on wine aromas. Monoterpenols can impart floral, rose, and lilac aromas. Norisoprenoids are known to impart aromas of honey, violets, spices, wood, and the classic petrol or kerosene thought to be a positive attribute in Riesling. And, finally, the notable compound rotundone imparts peppery aromas to varieties like Grüner Veltliner, Syrah, Mourvèdre, and Cabernet Sauvignon. Rotundone is a sesquiterpene that gives a pungent aroma of peppercorns, particularly white pepper. If you get a whiff of peppercorn from your wine, you are smelling rotundone.
It is amazing that yeast strains do so many things for our wines. And this post covers only a small portion of the subject. A follow-up Carl’s Corner will add more info about what impact yeasts have on wine aromas and flavors. For now, though, it is important that we appreciate our winemakers for learning about, managing, and using these yeasts to produce the wines we enjoy. The next time you visit with a winemaker, give him or her a pat on the back for all the effort put into managing yeasts in their winemaking process.
Yeast Impact on Wine Aroma and Flavor, by Daniel Pambianchi, WineMaker Magazine, Digital Edition, 16-Aug-2022
Selecting Yeast Roundtable, written by Dawson Raspuzzi, 13-Sep-2022, https://winemakermag.com/article/1497-selecting-yeast-roundtable featuring panelists Shea A.J. Comfort, founder of MoreWine! and contract winemaker for Lallemand, Michael Dawson, former Brand Manager at Wyeast Laboratories, Inc., Pat Henderson, Senior Winemaker at Kenwood Vineyards, Sonoma Valley, CA, and Kevin Lane, Technical Sales Manager at Fermentis (Red Star).