Red wine under a microscope, Pinot Noir Precoce. The photo shows crystals of frozen wine in polarized light.
Fruity, floral wines and beers are having a moment in the sun, with eager brewers and winemakers finding ways to incorporate more of these aromas and flavors into their beverages. One way to evoke these delicate notes is by carefully crafting ester compounds. There’s a catch, of course: Like the fruits and flowers whose scents they evoke, the esters responsible for some of these flavors are ephemeral, often surviving less than a year in the bottle before fading beyond human perception.
Esters comprise a vast category of volatile compounds. For fermentation enthusiasts, the most desirable esters are formed as relevant acids and alcohols interact. Differing yeasts added to a ferment produce differing alcohols that combine with the organic and fatty acids present in both beer and wine substrates to develop into many unexpected aromas and flavors. The yeast-produced alcohols and substrate acids combine to form unique esters, and the higher the concentration of a particular ester, the more likely the drinker is to perceive its signature smell and taste.
Aromatic Saccharomyces cerevisiae yeast cells create ester profiles in both wine and beer when they react with various fatty and organic acids.
To further complicate things, certain esters, alcohols, acids, and other compounds interact with one another in ways that affect our perception of each constituent and its particular smell and taste. In beer, low concentrations of certain banana esters — isoamyl and isobutyl acetate — combined with high concentrations of 2-phenylethanol — an alcohol we recognize as having a rose-like scent — can make both the banana and rose qualities more noticeable. Conversely, acetic acid, the main component of vinegar, has antagonistic interactions with red wines’ fruity esters, reducing our perception of their taste and scent without affecting the actual ester concentrations in the beverage.
Choosing an “aromatic” yeast is crucial for creating a high-ester product, regardless of the other conditions of fermentation. Without a yeast that’s already prone to generating esters, the brew will retain a low-ester profile, and the more fruity and floral notes sought by brewers will be faint or imperceptible to the drinker. Brewers and winemakers have tried all kinds of techniques to foster the formation of esters without stalling fermentation or creating undesirable flavors. Finding the perfect balance for optimal ester results is a tricky business. Generally, the best environment for producing esters includes: sufficient sugar and nitrogen to feed yeast; decreased oxygen levels; and a low yeast-pitching rate — or low yeast-to-sugar ratio — which slows fermentation. However, a fermentation that’s too slow risks stalling the process altogether, particularly if the initial yeast-pitching rate is too low to generate a self-sustaining yeast population. On top of all that, low oxygen levels in the ferment prompt greater ester production, but decrease yeast health, while high temperatures promote both ester production and yeast growth. Temperature also affects the types of esters produced, with more floral esters at higher temperatures, and more fruity esters at lower temperatures. Esters are thought to last a little bit longer in the beverage when it’s kept cold during and after fermentation, which might permit a relatively low ester production to reach perceptible concentrations.