Fermentation is a process in which yeast digest sugars, breaking them down primarily into carbon dioxide and ethyl alcohol, along with various byproducts.
There are many variables that will affect how your yeast performs, and therefore, your final product. Optimizing these variables will lead to less off flavors and a better mead.
Yeast – Take particular note of their alcohol tolerance, optimal fermentation temperature, and any available information on nutritional requirements.
Original Gravity – This is a measure of how much sugar is in the mead, determined with a hydrometer or refractometer. A very high gravity mead that produces a lot of alcohol will take longer to age.
Oxygen – Yeast use it during reproduction in the lag phase. A must can be aerated by vigorous stirring, by hand or with a drill and mix-stir attachment. Many mead makers aerate their must until 1/3 of the sugar content has been metabolized.
Nutrients – Honey has very little in the way of yeast nutrition. It is particularly nitrogen deficient. adding yeast nutrient will help fermentation complete without off flavors.
Temperature – Ferment in the lower 1/3 of the yeasts temperature tolerance listed by the yeasts manufacturer. Fermenting too hot will produce off-flavors. For most wine yeasts, 60-65 F is a good fermentation temperature. Remember that fermentation is exothermic - so the ambient temperature will be lower than the temperature in the fermenter.
pH – Sometimes your must is too acidic for a healthy fermentation. You can adjust with potassium carbonate, potassium bicarbonate or calcium carbonate. For a healthy fermentation, start at 3.8-4.6 pH.
These are not necessarily scientific terms. They are guidelines and common terms used to help describe the different stages of fermentation.
Lag phase (approx. 0-12 hours): The period right after pitching your yeast where seemingly nothing is happening. Yeast are absorbing nutrients and adapting to the new environment, and will quickly start to reproduce.
Growth phase (approx. 0-72hours): Period after lag phase where the yeast are budding rapidly to take on the ferment. Fermentation activity shows, but may be limited.
Fermentation (typically 1-4 weeks to complete): Usually broken up by sugar breaks. one-third and two-third can be confusing as they are often used interchangeably. Most of the time, they mean when either 1/3 of the sugars in the must have been converted to alcohol or when 2/3 of the sugars in the must are remaining to be converted (the same thing). Some of the time people actually do use 1/3 or 2/3 as a measure of when there is 1/3 of the sugars remaining and/or 2/3 of the sugars depleted; you'll have to decipher intent from the surrounding content of the message.
Post-Fermentation & Aging (2 weeks - ?? years): Optional step in brewing; not every brew needs to be aged. Aging time is a personal decision. Anywhere from 3 months to 5 years is common. When a batch is clear, it can be bottled, but you can choose to bulk age the mead for extended periods if you desire. If you bottle early while the mead is still cloudy you will get a lot of sediment (lees) at the bottom of all your bottles from the yeast and other solids dropping out. Over time this can impart flavor to your mead. Sometimes this can be desirable, but accidental sur lie aging is not usually suitable for mead. This isn't an issue for meads you plan on drinking quickly, such as a carbonated hydromel, but can be problematic for bottles you plan on aging.