Yeast Factors for a Good Fermentation
There are three principle yeast factors which should be considered when determining if your wort/must has been properly prepared to support a good fermentation. These factors will help consistently achieve a good fermentation activity and results: Quantity, Oxygen, and Temperature.
After selecting the appropriate yeast strain to match your intended fermentation conditions and yeast contribution to the finished product, the first step to achieving a good fermentation is to pitch enough yeast. This can be accomplished by growing the yeast colony via a yeast starter. This is recommended for dried and liquid yeast; smak paks and pitchable tubes. A quart of starter for a 5-6 gallon batch is ideal.
Oxygen can be the most significant factor in determining the quality of yeast. Yeast requires oxygen to synthesize sterols for cell construction during the respiration phase. With insufficient oxygen dissolved in solution, fermentation tends to be under attenuated because the lack of available oxygen is limiting the yeast growth. When sterol levels become depleted, yeast stops bubbling. Boiling wort drives out the dissolved oxygen normally present. Wine kits to which little water is added have had the dissolved oxygen removed prior to packaging. In both cases it is extremely important to dissolve oxygen back into solution prior to fermentation.
Proper aeration of the wort/must can be accomplished in several ways. Wort must be cooled to below 80 degrees F. before aerating to prevent hot-side aeration. The following suggestions are in ascending order of labor and time required to accomplish the task:
- • Shaking the container (e.g. starter jar and fermenter).
- • Inserting the Mix-Stir into the fermenter and letting your drill do the work.
- • Using a stainless steel airstone with an aquarium air pump which has a .5 micron in-line air filter.
- • Using pure oxygen with a stainless steel airstone.
Yeast is greatly affected by temperature, too cold and they go dormant, too hot (more than 10 degrees F above the recommended range) and they indulge in an orgy of fermentation that often cannot be cleaned up during conditioning.
High temperature fermentation encourages the production of fusel alcohols which can have harsh solvent-like flavors. Banana characters are one example of high esters due to high temperature fermentation. High temperatures can also lead to excessive levels of diacetyl, a butter or butterscotch character and slickness on the pallet. If the yeast is pitched when the wort/must is too warm and slowly cools during primary fermentation, more diacetyl will be produced than the yeast can reabsorb during the secondary stag. Remember that the internal temperature of fermentation can be as much as 10 degrees F. above the ambient temperature due to the heat produced by the yeast during fermentation.
Hopefully this will help you produce the best possible products by having sufficient yeast and the right conditions for them to work.