Quel est le rôle de la levure dans le processus de fermentation ?

Yeast has been known and loved by humans since ancient times and has been an important ingredient in baking, winemaking, and brewing. During fermentation, yeast cells convert grain-derived sugars into ethanol and CO2. At the same time, hundreds of secondary metabolites are produced that affect the aroma and taste of beer. Differences in these metabolites among different yeast strains are what allow yeast to affect beer flavor so.

Yeast: starter

Fermentation is a natural biochemical process carried out by the enzymes of microorganisms that transforms raw food. Most of the time, this process is anaerobic: it occurs in the absence of oxygen. It is activated by bacteria or fungi such as molds and yeasts, such as those that cause bread to rise and produce gas. In the absence of oxygen in the dough, yeast does convert sugars into carbon dioxide and ethanol, while producing some other metabolites, including organic acids.

Yeast in beer

In beer, yeast plays a different role due to different processes. In bread, the yeast is exposed to air, trapped in the dough, and baked. In beer, yeast is added to a grain liquid, which may also contain fruit and other additives, and is usually kept in an airtight container. In fact, if you add yeast before boiling, you kill the yeast and it won’t ferment for you. Remember, it’s alive and we want to keep it alive for as long as possible.

Yeast affects beer flavor

Yeast is used to start fermentation, producing gas and alcohol. Yeast is also an ingredient that is sometimes considered a “secret” but is crucial to the aromatic character of beer and wine.
Bananas, for example, are appreciated in Beaujolais wines and exhibit special flavors. By acting on acidity or producing antioxidants, yeast can improve the smoothness of some white wines and even prevent them from turning brown.
Yeast produces aromatic compounds that affect the flavor of wine. When they ferment, these yeasts produce alcohol, but they also produce what are called « fermentation aromas, » which bring out floral and fruity notes, such as rose, banana, or peach. Other so-called « varietal aromas » are released by certain yeasts during fermentation, such as the grapefruit and passion fruit notes in Sauvignon Blanc.

Brewing

Many people are a little unsure of the exact role yeast plays in brewing beer, so we wanted to give you a brief introduction. Yeast is a small, single-celled organism that is one of the simplest forms of plant life and handles the fermentation process of beer. Yeast absorbs simple sugars like glucose and maltose and produces carbon dioxide and alcohol. Brewing beer requires several steps. You grind fresh sprouted grains (usually malted barley, but wheat is also common). The grinding process releases the sugars in the grains. Water is then added to the cracked or ground grains and boiled to get those sugars into the liquid. The liquid you now have is called « wort ». Hops are added to this wort. Hops are added because your wort is very sweet at this stage, and hops are known for their bitter taste. Hops add balance. The liquid can be boiled again to sterilize it, killing any potential wild yeast and bacteria that are not needed in the beer. Now, after a cooking time, you add brewer’s yeast to the wort, at which point your wort becomes beer.

The role of yeast in beer brewing

Yeast can also contribute to the classification of beer styles. Brewers choose yeast based on the recipe or style of beer they want to brew. The yeast was identified as ale yeast or brewer’s yeast. Ale yeast is a top-fermenting strain that does best at warmer temperatures, while lager yeast, but, is a bottom-fermenting strain that does best at cooler temperatures.
Due to temperature differences, each yeast strain produces distinct flavor and aroma characteristics, creating different beer styles. Yeast, combined with different fermentation processes and ingredients, can also bring fruity and other flavor characteristics to beer.

Choose yeast

It’s up to the brewer to decide which yeast strain they want, based on flocculation and flavor and aroma characteristics. Don’t be afraid to experiment with various types of commercial brewer’s yeast, add a bit of wild yeast to some sour beers, or even get bacteria into the wort for some wild flavor. As long as you have some yeast, you’ll get some beer. It’s up to you what type of beer you want.

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