Once the yeast has done its job, it would form a “carpet” on the bottom of the fermenter. After primary fermentation is over what you’ve got is “young beer”. It’s already an alcoholic drink (yay!), not just a sweet syrup, like you had at the end of the boil. In a perfect setting the yeast would have consumed around 70-75% of the sugars and would have turned them into alcohol and carbon dioxide (CO2). The CO2, however, was released from the airlock and the contents of the fermenter almost completely lack carbonation.
Depending on the chosen beer style, brewers would follow different paths. When doing ales, that would be the moment when most home-brewers would transfer the liquid top part (leaving the sedimented yeast carpet on the bottom) out of the fermenter vessel and into bottles. During bottling, some sugar syrup is added to the young beer, usually 2 – 8g of sugar per liter of beer. The sugar gets dissolved in some water and heated to boiling point ahead of time, so that it can cool down before being added to the young beer. The idea is to provide some additional food for the remaining yeast in the bottles, which it would turn into a bit more alcohol and carbon dioxide – a very pleasant and natural way of carbonating your beer. Others would move the beer to kegs and then carbonate them by plugging in a CO2 tank, leaving it under pressure for couple of days.
When doing lagers, it’s almost necessary for most recipes for the beer to undergo lagering, which is, in essence, transferring it to other vessels (without the yeast on the bottom) where it would stay for at least several more weeks at very cool temperature (1-3 °C). This way the young beer will clarify before being bottled or kegged.
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