This is your beer! You can see that it has just started fermenting, the step where wort (the sweet, grain-based liquid you made on brew day) turns into beer. The yeast that was added to the wort is now eating the dissolved malt sugars and turning them into two main products: ethanol and carbon dioxide. It also makes small amounts of flavor compounds, like fruity esters, that help give each beer style its character.
You can see two signs of an active fermentation in the video. The foamy, tan layer on top of the beer is the krausen, a head of foam made of yeast, proteins and hop residue pushed up by rising CO₂. That gas has to escape, so it travels up through the airlock on the cap. The airlock is the small S-shaped trap partly filled with sanitizer. CO₂ pushes through the sanitizer in a steady bubble, while oxygen, wild yeast and bacteria can’t get back in.
When the airlock is bubbling this fast, the yeast is at its busiest, usually a few days after brewing. As the sugar runs out over the next week or so, the bubbling slows, the krausen falls back, and the yeast settles to the bottom. That’s when the beer is ready to condition and package.
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