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Understanding attenuation, flocculation and temperature

Three numbers on a yeast data sheet decide most of the outcome: attenuation for dryness, flocculation for clarity, and the temperature range for clean fermentation.

Attenuation: how dry it finishes

Attenuation is the share of fermentable sugar the yeast converts, given as a percentage. A high-attenuating strain leaves less residual sugar, so the beer finishes drier and thinner. A low-attenuating strain leaves more body and sweetness. An English ale strain such as the one behind WLP002 is known for lower attenuation and a fuller finish, which suits malty British styles.

Read attenuation against the style you want: dry and crisp calls for high, rich and malty calls for lower.

Flocculation: how well it clears

Flocculation, rated low, medium or high, describes how readily the yeast clumps and settles once fermentation is done. A high-flocculating strain drops bright quickly and leaves a clean, clear beer. A low-flocculating strain stays in suspension longer, which can help a hazy wheat beer look right but means a clean style needs more time or a cold crash to clear.

Match flocculation to the look you want and to how patient you can be with clearing.

Temperature range: where it stays clean

Every strain has a band it ferments cleanly in. Push an ale strain too warm and it can throw harsh fusel alcohols and heavy esters. Run a lager strain in its cool range and it stays crisp and clean. Some strains, such as certain saisons, are prized for holding up at high temperatures, while lager strains demand cool control.

When substituting, an overlap in temperature range is essential, because a strain you cannot hold at the right temperature will not give the result you are after however well it matches on paper.

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Sources: White Labs · Fermentis (Lesaffre)