Why Does the Same Species Taste Different? Strains and What They Produce

A species is a group of organisms that interbreed, and a strain is a particular lineage within it, descended from a single isolate and maintained separately, differing from other lineages in its genetics and its behaviour while remaining the same species. In yeast, strains of the single species used for baking, brewing and winemaking differ enormously in how fast they ferment, what temperature they tolerate, how much alcohol they survive, how much they flocculate and settle out, and above all in which flavour compounds they produce alongside the alcohol and carbon dioxide. Thousands of strains are maintained in culture collections and by commercial suppliers, and a brewery or winery's choice of strain is among its most consequential decisions.

The practical differences are specific and measurable:

• •Attenuation, meaning how much of the available sugar is consumed, which sets sweetness and strength
• •Temperature range, which distinguishes strains fermenting warm from those working cold and slowly
• •Alcohol tolerance, which sets how strong a product can be before fermentation stalls
• •Flocculation, meaning whether cells clump and settle, which determines clarity and ease of separation
• •Ester production, giving fruity aromas, which varies enormously between strains and with temperature
• •Phenolic compounds, giving clove and pepper notes, which some strains produce and most do not

Genomic work over the past fifteen years has established that brewing and baking strains are domesticated populations with a history parallel to that of crops. Strains used in beer form distinct genetic groups separated from wild populations, showing reduced diversity consistent with a bottleneck, and carrying changes that suit the environment humans provide, including improved uptake of the particular sugars in malt and the loss of the ability to reproduce sexually, which does not matter in a vat that is pitched from the previous batch. Wine strains form their own group associated with vineyards. Divergence estimates place the separation of brewing lineages several centuries ago, consistent with the period when brewing moved into dedicated facilities that reused their yeast deliberately.

Maintaining a particular lineage indefinitely is a practical problem with several solutions. Repitching, meaning collecting yeast from one batch and using it for the next, is the traditional method and works for many generations, with breweries maintaining lineages for decades, but the population drifts as mutations accumulate and as the fermenting environment selects, so it cannot continue indefinitely. Culture collections hold reference strains frozen at very low temperature or freeze dried, which halts change entirely, and commercial suppliers propagate from those references rather than from working cultures. Contamination by wild yeasts and bacteria is the constant threat, which is why breweries test their yeast and why a contaminated pitching yeast can be an expensive problem. Some producers deliberately embrace the drift.

Several products depend on species other than the main one, and the distinction matters. Lager brewing uses a different species, a hybrid formed from the common yeast and a cold-tolerant relative, which was identified only in 2011 when the wild parent was found in South America, resolving a long-standing question about where the cold tolerance came from. Spontaneously fermented beers and many wines involve a succession of wild yeasts and bacteria, with different organisms dominating at different stages as alcohol and acidity rise, which produces complexity no single strain achieves and much less predictability. Bread made with a starter involves yeasts and lactic bacteria together. And several specialised products depend on organisms considered spoilage in other contexts, which is a recurring theme in fermentation.

Strains of one species differ in fermentation speed, temperature range, alcohol tolerance, settling behaviour and above all in which flavour compounds they produce. Brewing and wine strains form distinct domesticated groups with reduced diversity and adaptations to the environment humans provide. Lager uses a different species entirely, a hybrid whose cold-tolerant wild parent was identified in 2011.