Diagnostic Key Sluggish Start: Arrested Fermentation

Sluggish Start: Arrested Fermentation

1.      Did temperature fluctuation occur?

Solution: abrupt arrest of struggling fermentations occurs when conditions have rapidly become intolerable for the yeast. The struggling fermentations may be more susceptible to other seemingly moderate stresses such as a slight temperature shift and more sensitive to an immediate arrest than a fermentation with a normal initiation. Compounded stresses lead to a reduction in stress tolerance overall. Under these circumstances the fermenting wine should be racked off of the existing yeast population, nutrient content analyzed and adjustments made and reinoculated with an adapted inoculant suited to the wine conditions.

2.      Were any additions or compositional adjustments made?

Solution: struggling fermentations may not restart following a compositional adjustment and the adjustment may inadvertently stress the fermentation more than it already was. There are several reasons for this. The yeast may have adapted to the stressful conditions and abrupt alleviation of the stress does not provide the opportunity to re-adapt to more permissive conditions. Changing temperature, pH or acidity level should be done step-wise for struggling fermentations to allow sequential adaptation. If this is not possible then the adjusted fermenting wine should be racked off the yeast lees and reinoculated. Stressed populations will not necessarily behave the same way as non-stressed populations.

3.      Are other microbes present?

Solution: fermentations that are not readily dominated by a robust strain of Saccharomyces may foster the growth of other microorganisms. Nutrient additions then may favor the non-Saccharomyces population or at least not disfavor them. These other populations may then bloom making conditions even more inhibitory to the yeast. Sluggish and arrested fermentations should always be examined under a microscope to determine if apiculate yeasts and bacteria are present as these are the most inhibitory species. If they are present then there are two options: use of filtration to reduce their numbers and reinoculation or reinoculation with a robust strain resistant to the inhibitory impacts of other microbes.