By Thomas Henick-Kling Professor of Enology
A note of precaution to using cold soak
The recent rains and earlier powdery mildew infections have caused damage to some grapes, the grape surface is punctured, microorganisms have entered the interior of the berries. For this to occur, we do not need clearly visible mold infection, even some slight growth of fungal hyphae in the surface layers of the berry is sufficient to open it up to spoilage yeast and bacteria. Studies at Cornell University have shown us that such damage by powdery mildew (and following Botrytis) will bring grapes that have a much higher load of yeast and bacteria than healthy undamaged fruit. The damaged fruit can have more than several million yeast per milliliter of juice compared to healthy fruit which has several hundred or at most several thousand yeast per milliliter. In addition, these yeast are not the friendly wine yeast, Saccharomyces cerevisiae, instead, they are mostly Kloeckera sp (Hanseniaspora sp), Pichia sp, and Candida sp yeasts that can produce large amounts of acetic acid and acetic acid esters. Especially Kloeckera sp yeasts can survive and grow during fermentation, even dominate some wine fermentations. This of course can cause major flavor defects.
Cold soak is very risky with such damaged fruit. Inspect the fruit carefully.
What you can do to minimize your risk when using cold soak
Use SO2 (30 to 50 PPM depending on pH) to help suppress growth of unwanted yeast and bacteria. At 50 to 60°F, Kloeckera yeasts are quite comfortable and grow faster than Saccharomyces yeasts. Therefore, if you use cold soak with white or red grapes, make sure the must is truly cold, i.e. Below 10°C or 50°F. Also, after the cold soak period warm the must quickly to 65 to 68°F to encourage the growth of Saccharomyces yeasts.
To assure the dominance of Saccharomcyes yeasts, it is best inoculate with a yeast
If you choose cold soak, you can add a yeast starter culture to the must immediately after you crush the grapes. Make sure that the starter culture is carefully adapted to the low temperature of the must: lower the temperature of the starter culture slowly so the yeast are not temperature shocked and stay highly viable! This addition of a yeast starter culture at the beginning of the cold soak helps suppress the growth of unwanted yeast and bacteria. Again, at the end of the cold soak period, warm the must quickly to 65 to 68 degrees F to favor the growth of Saccharomyces and start the alcohol fermentation.