Just as in making wine or beer, the yeast is multiplying rapidly while consuming sugar. The byproducts of its sugar consumption is alcohol and CO2. When making wine you typically ferment for a week, by which time the yeast has converted all of the sugar to CO2 and alcohol. The CO2 is vented out of the airlock and the wine is left with ten or twelve percent alcohol. Temperature effects how fast the yeast acts, along with subtle influences by acid and other nutrients.
In making ginger beer the goal is to get lots of CO2 in a short period of time, trapping it in the liquid. We start fermenting in the gallon jug just long enough for the yeast to get really started. Once we see steady fermentation the liquid is transferred to bottles and sealed. Now the CO2 has nowhere to go and is captured in the liquid. The yeast merrily goes on producing, pressure keeps building, and eventually we decide that it is bubbly enough and put the bottles in a fridge which stops the fermentation or at least slows it down quite a bit.
The process has also produced some small percentage of alcohol. It took less than a day from start to finish to make my last batch of Ginger Beer. I measured the percentage of sugar at the start at 9.5% and after fermenting it was right around 9%. Using the alcohol scale on the hydrometer it roughly looks like a 2% change in sugar reflects 1% in alcohol. Given that I calculate the ginger beer alcohol level at around .25% alcohol. Pretty low.
No matter how much filtering you do while making ginger beer you will see some sediment collect at the bottom of the bottles. This is the cast off yeast and finer ginger particles. It's harmless. You can either drink it, or pour the ginger beer into a glass carefully to disturb the sediment as little as possible.
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