Fermentation is broken down into two stages: Primary fermentation and an optional secondary fermentation.
Primary fermentation is the process whereby yeast is permitted to consume the natural sugars present in the fermentation liquid (apple juice in our case) in an anaerobic fermentation vessel.
During this process, the yeast breaks down the sugars and produces alcohol as a byproduct. The amount of alcohol present in the final mixture is dependent on the initial amount of sugars present, and the fermentation concludes when the yeast is unable to consume any more sugar - either because the environment is no longer conducive to fermentation or because there are no more fermentable molecules present in the mixture.
During this process, the vast bulk of the alcohol in the final product is generated. The exact amount of alcohol present can be calculated through some maths and the use of a hydrometer to measure the specific gravity.
Once Primary fermentation is complete, the cider can be left to stand at a cooler temperature. This does several things:
- it permits the yeast to settle out of the mixture
- it permits certain undesirable fermentation byproducts to dissipate or convert, improving the general taste of the final cider
After standing for a period of time, the mixture can be bottled or moved to a secondary fermentation vessel, and a secondary fermentation can be done if you desire sparkling cider rather than still. Moving the mixture from the primary to a secondary container has the additional benefit of separating it from the lees - the solid waste products of fermentation that settle out to the bottom of the primary fermentation vessel.
Secondary fermentation involves adding a charge of additional sugar to the mixture during bottling. This provides additional food to the yeast, which then begins fermenting it again. If fermentation takes place inside a closed bottle, the carbon dioxide dissolves in the mixture, carbonating it. This in turn then gives a sparkling cider in the style of most commercial ciders.