Wine Making: Stuck ferments
Occasionally a must will stop fermenting before it should. This is known as a ‘stuck’ ferment for which there are a number of causes. The must may have become too hot or too cold, either of which extremes will inhibit a weak yeast colony from activity. The remedy is to change the position of the vessel and to give the jar a good shake.
You may have forgotten to include sufficient acid or nutrient in the must which would inhibit activity. The addition of acid or nutrient crystals will sometimes restart a ferment but not always and you may have to blend the stuck must with a must that is fermenting vigorously.
Lack of sugar will also cause the cessation of fermentation and it is always wise to check your records to ensure that sufficient sugar was added and also the present specific gravity of the must.
Excess sulphur dioxide, from using too much loose sulphite rather than a controlled quantity in the form of Campden tablets, will prevent fermentation from starting. Excess carbon dioxide in the must can inhibit fermentation. The remedy for both of these causes is aeration of the must by pouring not siphoning the must from one vessel to another, two or three times. This allows the gases to escape and encourages the must to take in air which helps the yeast to reproduce itself.
When all remedies have been tried it sometimes happens that fermentation cannot be started again. The best remedy is blending with a similar must that is fermenting well. Alternatively, make up a fresh yeast starter and when it is fermenting vigorously add 300ml / ½ pint of stuck must to it. When this is fermenting, add 1.15 litres/1 quart and when this is fermenting add die rest. Always add the stuck must to the fermenting must in equal measures.