Demi-John

(c) 2000
Harris Filters

Home Winemaking
Stuck Fermentation
  • Fermentation may have completed.
    Many of the stuck fermentation’s reported to us have in fact been found to have already finished. Check the SG and/or taste and refer to the instructions on the label or with the recipe. Certain concentrates contain glucose syrup and have higher than normal finishing SG’s. If fermentation is complete, continue as directed in the instructions.
  • Sugar content too high.
    Some country wine recipes require more sugar that the yeast can tolerate. Adding the sugar in two stages will help, but an initial SG of 1.115 or over (representing more than 1.4Kg 3lb sugar per gallon) is likely to cause a stuck fermentation. Dilute the must with water (up to 1 pint per gallon) and leave in a warm place for fermentation to continue.
  • Sugar added too late.
    If extra fermenting sugar is added too late, the yeast will have fermented all the existing sugar and lost all activity. It may then be able to restart fermentation when the sugar is added. Shake well to re-suspend the yeast and if fermentation does not start within 2-3 days, use a Refermentation Method.
    When making high strength wines such as Port, Vermouth or Sherry types, the extra sugar should be added when the SG is about 1.025. If the wine is fermenting in a warm place, this stage may be reached surprisingly quickly, and if in doubt it is better to add the sugar a day or two early rather than leave it too late.
  • Sugar not completely dissolved.
    If the sugar is not thoroughly mixed into the must, a layer of dense syrup will settle to the bottom of the jar and can stop fermentation. Shake well to re-suspend the yeast, and if fermentation does not start within 2-3 days, use the  Refermentation Method.
  • Fluctuating temperature.
    Wide variations in temperature, for instance when central heating goes off at night, can stop fermentation. Stir the fermenter contents well to re-suspend the yeast sediment and place in a more constant warm temperature. If fermentation does not start within 2-3 days, use Refermentation Method.
  • Too cold.
    Slow, cool fermentations are favoured particularly for delicately flavoured white wines, but in general they are very difficult to manage and quite likely to result in infection and off-flavours. The best advice is to move the fermenting vessel into a constant warm temperature (21-25C).
  • Extra sugar added.
    Occasionally winemakers are advised to add sugar to “speed up” slow fermentation. This is quite wrong as the extra sugar will mean that the wine will take even longer to ferment to dryness and may stick. Stir the jar well to re-suspend the yeast and move to a warm place to ferment out.
  • Wine syphoned during fermentation.
    Syphoning a wine off its deposit reduces the concentration of yeast and nearly always slows  fermentation. This greatly increases the risk of off-flavours, It is always best to ferment wines reasonably quickly in a warm place without racking. Then move the jar to a warm place and leave to ferment out.
  • Malo-lactic fermentation.
    This is a bacterial fermentation that occurs after the primary fermentation in some commercial table wines. It reduces acidity, but also turns the wine hazy and may make corks to blow out. It will not occur with products having negligible malic acid content. However, it can be prevented with correct use of campden tablets at the end of fermentation.