POTASSIUM SORBATE

Most home winemakers have experienced refermentation of off-dry wines after bottling.  This can be very annoying, as it can cause corks to blow out; also, the resultant fizzy wines are yeasty and not always to one's taste.  Uncorking the wine and letting it continue its ferment to dryness in a carboy is an unnecessary waste of time, so the usual way of preventing bottle refermentation from happening is to introduce potassium sorbate (sorbate) before bottling.  Understand that sorbate does not kill the yeast; it merely inhibits renewed yeast activity under the correct conditions.

Many winemakers use Wine Art's, R.J.Spagnols' or Brewhaus wine conditioners which contain sorbate. Unfortunately, they contain a standard amount of sorbate that is not always sufficient to prevent renewed fermentation.  It is probably better to use reserve grape juice or sugar as a sweetener or if there is insufficient unfermented sugar in the wine and add sorbate for stability.

 Sorbate addition is dependent upon several interdependent criteria:

Wine pH;
Concentration of  free SO2;
Percent alcohol by volume;
Concentration of sorbate; and 
Viable yeast cell concentration.

While it is virtually impossible for winemakers to be able to determine the viable yeast cell concentration, they can rest assured that the probability is that there will be some viable yeast cells in off-dry wines.  Therefore, the other criteria become very important and are much more easily determined.

Assuming that proper levels of free SO2 are maintained and the pH's are within the desired ranges, sorbate additions can be determined by the estimated alcohol of the wine.  The following table is based upon the percentage of alcohol in the wine:

As can be seen, the amount of sorbate decreases as the alcohol level increases.  This may be due to two reasons: 1) At the lower alcohol levels, there May be a greater volume of viable yeast cells; and 2) The higher alcohols may have an inhibiting effect on refermentation.

Many winemakers hesitate to use sorbate because of the bubble-gum like aroma and taste it can impart.  However, if used properly, sorbate can be very effective without the aroma and taste it can impart.  Average threshold levels for detecting sorbate are about  0.182 g/l, so with the proper amount free SO2  and careful measurement of sorbate, the aroma and taste should not be able to be detected by the majority of people.  Do not use it in any wine that has undergone MLF; if the MLF recurs, the resultant unpleasant odour will be geraniol, a geranium-like smell.