Yeast

It is true that the Bavarian rule of 1516 does not mention yeast: only barley, hops and water. But that is not because nobody knew yeast. An older London brewing rule from 1484 simply mentions it: water, malt and yeast. Bavarian and Franconian rules from that time, such as those of Munich (1487) and Bamberg (1489), mention, like the one from 1516, only barley or malt, hops and water.

Other 16th-century texts also treat yeast as a matter of course. Andrew Boorde mentions 'yest, barme, or goddesgood' in 1542, and Christoph Kobrer describes in 1581 how to reuse wet and dried yeast. According to brewer Matthias Trum, who also studied history, there was even a separate profession, the 'Hefener', who harvested the yeast and put it into the next brew. Why yeast is missing in 1516, the text itself does not say. Trum thinks yeast did not count as a raw material, because you get it back after brewing, and even more of it.

What people did not know at the time is what yeast is. Only in the 19th century was it established that fermentation is the work of living organisms. In his book on beer from 1876, Louis Pasteur showed what role micro-organisms play in the spoilage of beer. In Bavaria, 'Hefe' (yeast) was explicitly written into the law in 1910.

What is correct: until Emil Christian Hansen grew the first pure culture in 1883, yeast cultures in Bavaria were, according to the Historisches Lexikon Bayerns, always mixtures of top- and bottom-fermenting yeasts, wild yeasts and lactic acid bacteria. Brewers reused their yeast from brew to brew, so an infection came along with it. Beer could turn sour, especially in summer.

But 'all beer was sour' goes too far. Lars Marius Garshol, who researches old brewing traditions, lined up historical sources. Olaus Magnus wrote in 1555 that beer is brewed sweet or bitter, according to taste; according to Garshol it turned sour mainly by accident. A recipe from a Bavarian book of 1598, translated into Danish in 1616, warns that beer quickly turns sour in summer, and gives tips to prevent it. Garshol also points to 19th-century analyses of German and Bohemian lagers: the lactic acid content was on average mostly below the threshold above which you taste something sourish, although quite a few individual beers were above it. His conclusion: most beer from before Pasteur was not sour, but a good share of it was somewhat sourish.

The idea that beer fermented by itself before Pasteur because nobody knew yeast is not right either: in 16th-century descriptions of brewing, adding yeast is simply normal. There are scientists who write that originally all beer was at least slightly sour. How sourish the average beer was exactly, we do not know.

The names are not made up. Ale yeast (Saccharomyces cerevisiae) rises to the surface in traditional open fermenting vessels; brewers skimmed it off there to reuse it. Lager yeast (Saccharomyces pastorianus) does not float to the top under any circumstances and collects at the bottom.

But many breweries now ferment in tall cylindroconical tanks. Because of the pressure of the tall column of liquid, ale yeast too ends up at the bottom there, in the cone. According to brewing-book author John Palmer, many current yeast strains do not stick to the division anyway. The important difference is, according to him, the temperature: ale yeast prefers to work at 18–24 °C and becomes inactive below about 12 °C, while lager yeast works fine at 7–13 °C.

These are also two different yeast species. Lager yeast is a cross between ale yeast and the cold-tolerant wild yeast Saccharomyces eubayanus, and owes its cold tolerance to that part of its genetic material.

The words themselves do not mean the same everywhere either. According to the BJCP, Germans prefer to speak of obergärig and untergärig; for them, 'lager' is mainly a way of maturing cold. A Kölsch is therefore called a top-fermenting lager beer there.

Kveik is a Norwegian dialect word, mainly from the west and south of the country, that originally simply means 'yeast'. According to Lars Marius Garshol, who researches the Norwegian farmhouse brewing tradition, brewers there distinguish 'gjær', yeast from the shop, from 'kveik', their own traditional farmhouse yeast. If you want to talk about the beer, he says, you are better off saying maltøl, and even that covers very different styles. The BJCP guidelines also say it explicitly: kveik is not a beer style.

What does make kveik special: farmhouse brewers passed their yeast down for generations and kept it dried, sometimes for a year or longer. Traditionally the yeast went into wort at 28–40 °C, a very high fermentation temperature for brewing yeast. Genetically, the kveik yeasts form a group of their own among the brewing yeasts. The strains examined were not phenolic (so no clove notes), flocculated strongly and fermented fast.

A traditional kveik is also not a single strain but a mixture. In the shop you often buy a single strain that has been grown out of it. According to Garshol, kveiks differ greatly from one another in flavour.

Weizen is made from wheat malt, barley malt, hops, water and yeast. The BJCP explicitly calls banana and clove the fermentation profile of the weizen yeast.

The banana comes from esters that the yeast makes, mainly isoamyl acetate. Researchers at KU Leuven call that the compound that gives beer a fruity banana flavour. The clove comes from 4-vinylguaiacol, which yeast with the genes PAD1 and FDC1 makes from ferulic acid in the grain. Many brewing yeasts have lost that ability, and in most styles 4-vinylguaiacol counts as an unwanted off-flavour. Weizen yeasts do make it, and in this style a lot of it is actually desired. A sensory study from Freising found about thirteen times more 4-vinylguaiacol in a typical Bavarian wheat beer than in a wheat beer in which that typical weizen aroma was somewhat lacking.

A small nuance: part of that compound already forms when the wort is boiled, but most of it comes from the yeast during fermentation. Witbier is the counter-example: there, citrus and spiciness do come mainly from added coriander and orange peel.

With bottle refermentation, some sugar or wort is added to the beer at bottling. The yeast then makes the carbon dioxide in the bottle. The BJCP calls bottle refermentation a common characteristic of the Belgian monastic beer styles, and under the HORAL rules Oude Geuze and Oude Kriek must referment in the bottle. That yeast then sinks to the bottom is normal. A guide from CraftBeer.com (Brewers Association) even recommends storing speciality beer upright, so that the yeast sinks to the bottom.

Whether you pour the layer along differs per style. With weizen the brewer may, according to the BJCP, indicate whether the yeast should be swirled up. Australian Sparkling Ale is, according to the BJCP, usually poured with the yeast. Witbier is, according to the Brewers Association, traditionally poured cloudy. With tripel the BA, on the contrary, writes that you are not supposed to swirl up the yeast on purpose.

One caveat. Contamination with wild yeasts or bacteria can, according to the Brewers Association, cause cloudiness and flakes, and in addition off-aromas and off-flavours, sourness or a bottle that foams over when opened ('gushing'). But in a beer that has been refermented in the bottle, a layer of yeast at the bottom is not in itself a sign of spoilage.

In most beers bacteria are indeed unwanted, because they can cause spoilage. In sour beers, lactic acid bacteria, and sometimes acetic acid bacteria, are among the desired microbes. They make the organic acids that make the beer fresh and tart.

That happens in a controlled way. With kettle souring the wort sours in the brew kettle in one to three days, after which the souring is stopped by boiling or by adding strongly hopped wort. Flanders red matures for up to two years in large wooden vats and is then blended with young beer.

The BJCP style guidelines also describe what is a fault. Berliner Weisse must never be vinegary. In Flanders red a little acetic acid is acceptable, but a dominant vinegar character is not. And in lambic and gueuze a clear vinegar or cider aroma counts as a fault among Belgian brewers.

Sourness is therefore a deliberate flavour choice. A slide into vinegar is what brewers see as a fault.