- Know your juice: The key to completion of the yeast fermentation is the availability of sufficient nutrient resources to allow cell growth and sustained metabolic rates in the presence of increasing ethanol concentrations. It is important therefore to have some sense of the starting nutritional content of the must or juice and to supplement up to the needs of the yeast. Over-supplementation while not leading to arrest of the fermentation will impact the spectrum of aromatic compounds produced generally leading to less complexity. Excessive nutrients such as amino acids will reduce the concentration of sugar carbon needed for the biosynthesis of new cells and result in higher yields of ethanol.
- Know your yeast: Match the yeast to the nutritional and environmental conditions. It is important to use yeast strains that have both the temperature and ethanol tolerance levels to complete the fermentation. If the reported ethanol tolerance level is below that that will need to be attained at the end of fermentation the yeast will arrest regardless of the nutritional content of the juice. Similarly if the strain has a low tolerance to swings or extremes of temperature it should not be used if such swings of temperature are likely during the fermentation. Equally important is knowing the nutritional requirements of the selected yeast strain and making sure they are met – is it a high nitrogen-requiring strain? If yes then adequate nitrogen should be present.
- Know your bioloads: Fermentation management recommendations will differ as a consequence of the numbers and types of non-Saccharomyces organisms present. If significant rot has occurred in the vineyard then cold soaks and indigenous fermentations may be precluded. If high bacterial loads are present then oxygen treatments of the juice or must may be risky.
- Monitor the fermentation: Routine fermentation monitoring is important in order to spot an impending problem. Even if indigenous fermentations are conducted, following sugar loss will allow a historical reference data set to define the parameters of a normal successful fermentation. Although the variation may be large fermentations outside of that expected variation can be identified. If a commercial strain is used then fermentation progression should be more uniform across vintages, but keeping a historical record for a basis of comparison is still a wise practice.
- Manage yeast stress: if stressful conditions exist, such as high bioloads resulting in significant competition or the production of inhibitory components, yeast will have a more difficult time completing the fermentation. Judicious use of SO2 can help mitigate these impacts as well as avoiding pre-fermentation processes that lead to blooms of non-Saccharomyces organisms.
- Invest in a microscope: Although to a certain extent all yeast look alike microscopic observations of juice and fermenting must can provide an assessment of the vitality of the fermentation. The emergence of bacterial populations can be used as an index of stress.
- Smell everything: An off fermentation aroma is generally the first indication of a problem. It is easy to become familiar with the range of normal yeast esters produced during fermentation. The profile will change if fermentation conditions become inhibitory or non-Saccharomyces populations start to emerge in large numbers.
