Understanding Fermentation
It has long been known that to brew beer you needed three ingredients: water, malts and hops. By mixing hot water with the cereals, you would eventually have sugar then the beverage. Some residues would develop and either fall to the bottom of the fermentation tank or reside on top of it. Scientists used to call this phenomenon spontaneous generation. It was only known later that those residues were actually microscopic beings that are invisible to the eye until a certain number agglomerates and form a larger floating mass. The first observation of microorganisms was by Antoni Van Leeuwenhoek around 1680, but it’s Pasteur that concluded their role in the process of fermentation in 1857.
They reproduce asexually, which means a single cell divides itself in two & this form of multiplication is called budding. This way, a colony can build from one individual given the right environment. The yeast acts by consuming & fermenting the sugars of the cereals to create energy. The waste produced by this process is mainly alcohol and carbon dioxide, CO2, the molecule responsible for effervescence. Ethanol is the major component formed but traces of different alcohols like methanol and propanol may also be present in minimal quantities. The bacteria behave in a similar fashion, although provides much less alcohol and instead generate acidic substance that varies from strain to strain. The yeast can survive in an environment high in its waste only to a certain point, to which it then becomes lethal. At this point some will initiate sporulation, a form of protection that puts the cell in a dormant state while the other die. Most brewers will reactivate the spores through a germination process which includes the creation of a new nutrient-rich habitat, exempt of toxic wastes.
Domesticating the Wild
For many years, brewers let the yeasts present in the environment do the job of converting sugary water to alcohol and dioxide carbon. Some started to confine their strains in their best batches to make steadier products. Nowadays most brewers use lab isolated strains although some of the most search
upon beers are made from traditional spontaneous fermentation which requires a somewhat random mixed of microorganisms coming from the surrounding environment. When working with a specific strain, it is possible to obtain a characteristic range of flavours and a neutral taste to add to the brew. On the opposite, when opting for a spontaneous fermentation, a wide spectrum of flavours and gustatory perception is conceived, although not exactly predictable.
Yeasts in the world of beer are most commonly placed in two categories: Ales and Lagers. This division can be explained by the way these reacts during fermentation. Ales are mostly used at high temperatures and in a short period of time. This, along with the characteristics of the yeast, produces a more flavourful brew with earthy, fruity & spicy notes as well as a usually sweeter profile. They are traditionally in English, American and Belgian-style beer, although also present in a few German ones. Lagers on the other side are fermented at low temperatures and need to be conditioned for longer periods of time resulting in a cleaner profile with less fragrant by-products. They make up the majority of the German and central Europe styles and are also commonly used in America.
The Various Strains
The most common strains are Saccharomyces cerevisiae, the ale yeast that produces a lot of fruity and spicy esters. They are top-fermenting, mainly because they float during the process, they are also efficient in temperatures of 10 to 25 °C (50–80 °F). Conjointly the Saccharomyces pastorianus, the cleaner lager yeasts, which is the bottom-fermenting strain, due to the fact that it sinks. This yeast is probably the most neutral of all letting most of the place to the other ingredients in the beer such as the malts, the hops & the water. As mentioned earlier, there is also spontaneous fermentation which includes wild & indigenous microorganisms. One of the most common & beloved undomesticated yeasts is Brettanomyces, which can be mostly declined in the lambicus, bruxellensis and claussenii strains. These generate a lot of by-products along with various degrees of acetic acid which brings a tart taste to the brews. They also engender a wide array of aromas depending on the way they are employed. In first fermentation, it will usually leave a subtle trace of earthy, spicy and woody notes typical of Belgian strains, and consume all the sugars resulting in a bone-dry beer. When used in secondary fermentation, they can impart more complex flavours ranging from horsey to cherry pie, ripe stone fruits or sweat. These characteristics have led brewers to isolate the strain in ways to gain control over their products profile. Another popular yeast strain is the Saison, an ale strain that generates a very unique bouquet. Peppery, spicy & citrusy with a dry and rich mouthfeel is common with Saison. All traditional region has some specific & historical strains that are a signature of their brews, whether it is Belgian, German, French, English or American which makes it part of the terroir profile of beers.
The Flavours
Alcohol, carbon dioxide and other flavour compounds have been mentioned, but to be more precise here is a small list of the most common and recognizable by-products of yeasts and the way to detect them. Some are wonderful & some are horrible, so pay attention next time you’re tasting a beer & watch out for these flavours.
Acetaldehyde which carries a typical green apple scent & is an off flavour in almost all styles.
Diacetyl which reminisces of butter, popcorn or butterscotch & is usually found in Scotch Ales, Barleywines & others but generally a default in a lot of styles or when overwhelming.
DMS or Dimethyl Sulfide with aromas of canned or cooked vegetables, common when used with pilsner malts & in some stouts or porter. Also considered an off flavour.
Ester brings out fruity odours such as berries, orchard fruits, bananas.
Phenols reminds of cloves, smoke, plastic or even medicinal ones. This molecule can level up a brew by adding some depth and can also destroy it when highly concentrated.
Solvent smell like acetone & are an off flavour in large quantity. When in moderate concentrations it is a bit fruity and smells like pears.
Sulfur which is characteristic of rotten eggs and burnt matches & is not an aroma that lasts long.
The Reference
https://www.beeradvocate.com/beer/101/yeast/
http://www.wyeastlab.com/yeast-strain/french-saison
http://www.exploreyeast.com/article/history-yeast