May 26, 2020

Butter and Cream . . . by Joel Mann, Staff Wine (And Beer) Tasting Guy

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Buttery, creamy tastes in wines are quite popular, particularly when it when it comes to Chardonnay. The flavor adds a richness to wines, and generally is a pleasing and comforting taste sensation for many people. There are numerous instances where buttery flavor is highly undesired though. Brewers consider the flavor a defect in most beers. Whether sought after or cursed as a flaw, the buttery, creamy flavor of fermented beverages all come from the same source – diacetyl. This month I wanted to delve a little deeper into diacetyl and better explain what the buttery flavor is, and where it comes from.

Diacetyl is responsible for the full range of buttery, creamy flavors. If you’ve ever tasted imitation butter or products that have butter flavoring added, then you’re familiar with diacetyl. It’s a naturally occurring compound found in most fermented foods, which includes dairy. In low concentrations it has a butter-like aroma. As concentrations increase, the intensity of the butter character increases, first becoming exceptionally creamy, then turning to butterscotch, and eventually in high concentrations it begins to take on a somewhat rancid movie popcorn butter character.

The combination with lactic acid, which is the acid form of the common milk sugar lactose, brings in the full range of dairy aroma and flavors which are particularly related to cream and yoghurt, which the latter flavor is often described with Champagne and other sparkling wines.
While diacetyl occurs in both wine and beer production, the beverages require differentiation as there is a key microbiological difference between the two. Buttery flavors in beer are a byproduct of the normal alcoholic fermentation, and the chemical pathways are end controlled by the yeast used in brewing.

Wines however control formation through a secondary bacterial fermentation separate from alcoholic yeast fermentation, which is either intentionally induced or inhibited depending on the style of the finished wine.

Diacetyl is produced in small quantities during beer fermentation by the conversion of a compound called alpha acetolactate (AAL). The conversion is chemical and occurs during aging through fermentation and the green beer phase, with higher temperatures accelerating the process. Brewer’s yeast does not metabolize AAL, but as the compound is converted, the yeast is able to absorb the resulting diacetyl and further break it down to relatively tasteless compounds.

This is what happens when brewers age beer on active live yeast during the green beer rest. The process happens over two weeks for ale style beers, but can take longer for lager beers.

The formation of diacetyl in wine is a metabolic process controlled by bacteria. Grapes contain large amounts of malic acid, which is a biologic substrate that several microorganisms can use for energy. As a result, winemakers often induce a bacterial fermentation which converts the malic acid to lactic acid, resulting in a low energy substrate that is microbiologically stable. During the ML fermentation the bacteria also utilize small amounts of citric acid in the grapes as an energy substrate that gets converted to diacetyl.

The process is enhanced with warmer temperatures and oxygen exposure for the bacteria. Winemakers desiring buttery flavor formation will perform sur lie aging where the barrels are stored at slightly warmer temperatures and the lees containing the spent yeast and malolactic bacteria is stirred in the barrel to give a very small amount of air exposure. Diacetyl is not the final end point of the chemical pathway in malolactic fermentation, so timing is key is achieving the ideal buttery flavor.

The buttery flavor character can be overly intense if stopped too early and wines can have that intense butterscotch character tending towards the rancid popcorn butter. Wait too long and the bacteria will eventually convert diacetyl to a compound called butanedione, which does not produce the desired flavor. Note that there are certain wine styles, particularly young, crisp whites such as Sauvignon Blanc where the creamy ML flavors are not desired. Winemakers will intentionally prevent the ML fermentation by the use of sulfites and sterile filtration of the finished wine during bottling.

Diacetyl has received a bad rap as a chemical additive to food. There is some debate about its use as flavoring agent and its effects. Understand though that diacetyl is a naturally occurring food product and is the compound behind every pleasing butter flavor you’ve ever craved.

Drink responsibly.

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