Yeast Glossary: Brewing Yeast Terms Explained

Brewing yeast has its own language — flocculation, attenuation, diastaticus and more. This glossary explains what each term actually means for your brew day, with links to strains in our range that illustrate each one. For a full side-by-side comparison of strains, see our Beer & Cider Yeast Selector or Wine Yeast Selector.

Attenuation

Attenuation is the percentage of sugars in your wort that a yeast strain converts into alcohol and CO₂. A yeast with 75% attenuation leaves more residual sugar (a fuller, sweeter beer) than one with 85% attenuation (drier, crisper). It's one of the first specs to check when picking a strain for a style — high attenuation for a dry West Coast IPA, lower for a sweeter mild or stout.

Flocculation

Flocculation describes how readily yeast cells clump together and drop out of suspension once fermentation slows. High-flocculating strains (like most English ale yeasts) clear quickly and leave a compact sediment — good for cask ales and quicker turnaround. Low-flocculating strains stay in suspension longer, which some hazy/West Coast styles rely on for mouthfeel, but they take longer to drop bright.

Diastaticus (STA1-positive)

Diastaticus yeast produces an enzyme (glucoamylase) that breaks down dextrins other yeasts can't touch — meaning it keeps fermenting sugars long after a "normal" yeast would stop. This gives extra-dry, high-attenuation beers (it's prized in some saison and Brett-adjacent brewing) but it's also a known contamination risk: a stray diastaticus cell in a non-diastaticus batch can cause bottle bombs or off-target dryness months later. In our range, MJ M41 Belgian Ale, MJ M31 Belgian Tripel, and MJ M29 French Saison are diastaticus-positive — worth knowing if you brew multiple styles with shared equipment.

POF+ (Phenolic Off-Flavour Positive)

POF+ strains carry a gene that lets them convert ferulic acid (naturally present in malt) into 4-vinyl guaiacol — the clove and banana character that defines German hefeweizen and Belgian witbier yeasts. In a strain intended for a clean lager or English ale, the same trait would be considered a flaw; in a wheat beer strain, it's the whole point.

Pitch Rate

Pitch rate is how much yeast (by weight or cell count) you add relative to your batch size and gravity. Under-pitching stresses the yeast into producing excess esters and fusel alcohols and risks a stuck or sluggish fermentation; over-pitching can strip aroma and give a thin, yeasty character. Most dried yeast sachets are sized for an 11.5g pack per 20–30L at standard gravity — check the strain's specific guidance for high-gravity or lager brewing, where rates should increase.

Rehydration

Rehydration is rousing dried yeast in warm (not hot) water before pitching, rather than sprinkling it dry onto the wort. It gives the cells a gentler start and better viability, especially for lager strains and higher-gravity worts. Ale strains are generally more forgiving of a direct dry pitch, but rehydrating is rarely a bad idea.

Krausen

Krausen is the thick, foamy head that forms on top of actively fermenting beer, made of yeast, hop resins and proteins. Its appearance, colour and how long it holds are a useful visual check on how vigorously fermentation is progressing — a healthy krausen usually means a healthy fermentation.

Esters & Phenols

Esters are fruity aroma compounds (banana, pear, apple, bubblegum) produced by yeast during fermentation — desirable in Belgian and German wheat styles, usually minimised in clean American ales and lagers. Phenols are the spicier, clove-like or smoky compounds from POF+ strains. Fermentation temperature has a big effect on both: warmer ferments generally push more ester and phenol production.

Viability

Viability is the proportion of live, healthy cells in a yeast sample. Dried yeast viability drops slowly over time even in good storage (cool, dry, sealed) — check the use-by date, and consider a slightly larger pitch if using yeast that's been stored a while.

Underpitching & Overpitching

Underpitching (too little yeast) leads to slow starts, higher stress-related off-flavours, and greater risk of contamination taking hold before the yeast establishes dominance. Overpitching (too much yeast) can finish fermentation very fast but sometimes at the cost of the fruity, characterful esters some styles rely on. Following the strain's recommended pitch rate for your batch size and gravity avoids both.

Killer Factor (Wine Yeast)

Some wine yeast strains carry a "killer" trait — they produce a toxin that suppresses competing wild yeasts and bacteria in the must, helping the intended strain dominate the fermentation cleanly. It's a useful trait in wine and mead making where wild yeast contamination is a bigger risk than in a sealed beer fermenter.

Frequently Asked Questions

What does flocculation mean for my brew?

High flocculation means the yeast will drop out and clear quickly after fermentation, usually with a compact sediment. Low flocculation means the beer stays hazier for longer and may need extra time or fining to clear.

Is diastaticus yeast safe to use?

Yes, when used intentionally and on dedicated equipment — it's a deliberate style choice in some saisons. The risk is cross-contamination into batches using non-diastaticus yeast, which is why it's worth knowing which of your strains carry the trait.

Do I need to rehydrate dried yeast?

It's not always essential for ale strains, but it improves viability and is recommended for lager yeast and high-gravity worts. Follow the sachet's specific instructions.

What's a good pitch rate for a standard batch?

Most 11.5g sachets are designed for 20–30L at standard gravity. For higher-gravity or lager brewing, increase the pitch rate — check our Yeast Selector for strain-specific guidance.