How do I read the specifications of a yeast?

While the warm side of brewing and the ingredients at that stage, such as malt and hops, traditionally receive a lot of attention, the choice of yeast and control over the course of fermentation is at least as important! Choosing a yeast strain therefore involves much more than just ‘top-fermenting or bottom-fermenting’.

The specification sheets we put with each yeast on the website under ‘downloads’ are a goldmine of information - at least if you know what to look for. We'd like to guide you through the key terms you'll come across on a yeast sheet, and explain what they mean for your beer.

Yeast type

As a brewer, a basic knowledge of the most common yeast types is a foundation on which tasty beers are brewed. Most hobby beers are fermented with yeasts from the Saccharomyces genus, but there are still many interesting yeasts to be found in our range. The best known are:

·        Saccharomyces cerevisiae: the classic for top-fermenting beers such as IPA, tripel or stout. Works quickly, usually ferments at room temperatures and is quite robust.

·        Saccharomyces pastorianus: the bottom fermenter on duty, used for lagers and pilsners. Needs lower temperatures and a longer maturation time, but produces wonderfully tight beers.

·        Brettanomyces: slightly less conventional. This ‘wild’ yeast adds funky, earthy or fruity flavours. Think lambic or farmhouse ales.

·        Lachancea thermotolerans and Torulaspora delbrueckii: slightly more eccentric choices, but ideal for sours or hybrid styles.

The strain determines not only the fermentation behaviour, but of course also the aroma and flavour profile of your beer.

Be careful with var. diastaticus and Brettanomyces: risk of infection!

Some yeasts are just a bit more adventurous than others. So you have Saccharomyces cerevisiae var. diastaticus - often called ‘diastaticus’ for short - and Brettanomyces (or Brett for short). These yeasts have one thing in common: they break down sugars that ordinary baker's or brewer's yeast leaves alone.

Diastatic yeasts produce an enzyme, glucoamylase, which converts complex sugars (such as dextrins) into fermentable sugars. This allows them to ferment much further than a normal yeast, resulting in a lower final SG and often a drier, ‘stripped-down’ beer. Fine, if you choose to do that. Less fine if it happens accidentally, for example in bottles you have already filled: there it can lead to overcarbonisation or even exploding bottles.

Brettanomyces is also notorious. This wild yeast species works slowly but steadily, and can keep working for months. The result is a funky, sourish or horse-deck-like aroma - desirable in some styles, such as lambic or saison, but absolutely undesirable in a classic IPA or lager. Brett thrives in pores of plastic, rubber and wooden barrels, making it difficult to get your brewing equipment really clean.

So both yeasts require extra hygiene if you want to work with them. And if you don't intend to: watch out for cross-contamination. One incorrectly cleaned siphon or fermentation vessel can be enough to give your next batch a completely different character from the one you had in mind.

Final density

The final density (Eind-SG or Final Gravity) tells you how many sugars remain after fermentation. The lower this value, the drier your beer will be. So a yeast with a low final density is ideal for a tight saison or brut IPA, for example. Do you prefer to work towards a slightly sweet tripel? Then choose a yeast with a higher final density.

Alcohol tolerance

Not every yeast likes a party with lots of alcohol. Some yeasts give up as early as 5%, while others happily ferment to 14% or more. If you want to make a heavier beer, choose a yeast with a high alcohol tolerance - otherwise you end up with a half-fermented sugar bomb.

Fermentation temperature

Every yeast works optimally within a certain temperature range, usually somewhere between 12 and 24 °C. At lower temperatures, fermentation is slower, but you usually get a cleaner, more neutral profile. Higher temperatures often provide more esters and phenols - think fruity or spicy aromas. Note that fermenting too hot can also produce unwanted side flavours, such as solvent or exaggerated banana notes (unless that's just what you're looking for, of course). If you want to achieve the flavour profile set by the manufacturer, it is best to follow the recommended temperature.

Important to know: fermentation is an exothermic reaction. That means the yeast gives off heat during its activity. The temperature in your fermentation vessel can therefore be a few degrees higher than that of the room in which you are fermenting. That extra heat can further activate the yeast, unintentionally leading to an explosion of esters, phenols or even brewing failures. This is why good temperature control - measured on the liquid in your fermenter - is crucial for consistent results.

Fermentation rate

The degree of attenuation (or attenuation) indicates how many sugars a yeast converts into alcohol. This is expressed as a percentage. A yeast with a high attenuation (80% or more) makes dry beers, while a lower attenuation (60-70%) may leave some residual sweetness.

Flocculation

Flocculation says something about how well the yeast settles after fermentation. Highly flocculating yeasts have cells that are better able to clump together and sink to the bottom. This can be influenced by the presence of specific proteins or polysaccharides in the yeast's cell wall. These yeasts produce clear beer faster and are easier to remove, but can sometimes stop fermenting if there are any sugars left. Low flocculating yeasts, on the other hand, stay in suspension longer, but may give some cloudiness.

POF-positive or negative?

POF stands for ‘Phenolic Off-Flavour’. A POF-positive yeast produces phenols, leading to spicy flavours such as clove or pepper (think saison or wheat beer). A yeast that is POF-negative has a genetic mutation that makes the enzyme PAD1 (Phenylacrylate decarboxylase) inactive. This enzyme is responsible for converting ferulic acid, which comes from malt, into 4-vinylguaiacol, a phenolic compound. POF-negative yeasts are consequently neutral or fruity, and thus ideal if you just want to avoid those spicy notes.

Nitrogen demand

Yeast needs nitrogen to grow and ferment properly. Some yeasts have a higher nitrogen requirement than others. Especially in honey- or sugar-rich solutions (like mead or high-gravity beers), it is important to adjust with yeast nutrients.

Pitching

Each yeast has an ideal dosage, usually expressed in grams per litre. An incorrect yeast pitch can have noticeable effects on the quality of the beer.

Underpitching requires the yeast to multiply heavily first, which increases the risk of infections and unwanted flavours such as diacetyl, sulphur compounds, higher alcohols and unwanted esters. Fermentation can also stall, especially at high densities, making the end result sweet and unbalanced.

Overpitching is generally less harmful, but can lead to extremely fast fermentation, reduced esters, a thinner mouthfeel and in some cases autolysis, where yeast cells break down and give off an unpleasant yeasty flavour.

What is organic yeast?

Organic yeast is produced without synthetic aids and fits within a certified organic brewing chain. Ideal if you want to brew with 100% organic raw materials, or if your customers value sustainability.

Var. diastaticus

Saccharomyces cerevisiae var. diastaticus is a notorious yet much-loved yeast capable of breaking down complex sugars, such as dextrins, thanks to the enzyme glucoamylase. This allows the yeast to ferment much further than other yeasts, resulting in a lower final specific gravity (SG) and a drier, ‘stripped-down’ beer. This can be an advantage if you want to brew an extra dry saison, for example, but it can also cause unintended problems, such as delayed refermentation in the bottle. In that case, the yeast can cause unexpected restarts, leading to overcarbonisation and even exploding bottles. Unfortunately, you don't notice this until it's too late. So it is important to pay close attention to whether your yeast is actually diastatic, unless you specifically choose the characteristics it brings.

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Voilà! With these terms in your pocket, from now on you will read through all our yeast specification sheets with ease and choose the right yeast for the right result every time.