Stuck fermentation

What?

• The specific gravity is still much higher than 1 -- the fermentation is not supposed to be done -- and nothing is happening. Or the fermentation does not start.

Why?

• Too low or too high a temperature.
• Lack of nutrients.
• Excess of sugar.
• Excess of sulphite.
• Unhealthy yeasts.
• It can also happen that the fermentation is actually done: calculate the alcohol content and compare to the level the yeast is supposed to reach.

What to do?

• If the problem comes from the fermentation temperature, move the must to a location thermally more adequate and wait until the fermentation spontaneously restarts.  If the temperature was too high, yeasts may be dead and the must has to be repitched. If it was too low, yeasts are "hibernating" but are not irreversibly harmed.
• If it is caused by a lack of nutrients, add some. If the fermentation does not (re)start, there must be another cause.
• To repitch: Prepare a new starter and pour it in the must after solving the initial problem (if the temperature is too high, the new yeasts will die as the other ones did).

How to prevent?

• First of all, one must know the yeasts. Go to the web site of the maker or obtain a catalogue or a booklet showing the yeasts and their specifications. White wine yeasts are supposed to ferment between 15 and 20°C (60 to 70°F), red wine yeasts around 25°C (80°F). Some yeasts are "rustic" (bayanus for example) and they can tolerate unfavorable conditions (lack of nutrients, high alcohol content), but some others absolutely need nutrients. It must also be kept in mind that not all honeys provide the same amount of nutrients, so if one uses a different honey, it can be necessary to modify the amount of nutrient to be added.
• Generally speaking, yeasts do not like too much sugar in their environment (osmotic pressure). But a high alcohol content requires a lot of sugar, so it can sometimes be necessary to add a lot of honey. In this case honey should not be all added at once, several additions can lead to the same high alcohol content but at no time will the sugar content be too high. This prevents the Crabtree effect: when there is too much sugar, yeasts skip the respiratory phase and directly go to fermentation whereas they are still weak. Is there such a risk when a starter is used? No there should not be such a problem (as long as the specific gravity in the starter is not too high). But the osmotic pressure remains.