Diagnostic Key Sluggish Start: Staying Sluggish

Sluggish Start: Staying Sluggish

1.      Why was fermentation onset sluggish?

Solution: sluggish fermentation onset may be temporary due to factors that can readily be corrected such as low temperature, too high of an SO2 addition, low viability of the inoculant yeast or they can reflect a problem with the composition of the juice or growth conditions. It is important to understand why a fermentation initiated slowly to determine if it is something that will be corrected with time or if it represents a truly inhibitory situation for fermentation.  The continued sluggish rate may mean that the inoculant was over-dominated by resident yeasts that may not be robust fermentors. If the reason for the sluggish start is associated with a low viable Saccharomyces population, reinoculation should be effective at restarting the fermentation. The fermentation should be examined under a microscope to determine the nature of the organisms present. If Saccharomyces is not dominant, a reinoculation with a robust strain should occur.

2.      Were nutrients added?

Solution: in severe nutrient limitation conditions yeast will struggle to adapt to the juice, struggle to produce biomass and never attain a rapid fermentation rate. Nitrogen levels should be assessed to determine if a severe deficiency exists. Multiple nutrient deficiencies occurring simultaneously can have synergistic impacts and lead to enhanced sluggishness. If a severe nutrient limitation exists the ferment should be racked off the yeast lees, have a partial dose of nutrients added along with a new inoculum and get a second nutrient addition once the population becomes established. The starved yeasts will have produced population signals of starvation that will lead newly inoculated cells to simply shut down because they are responding to the starvation signal. Removal of the starving yeasts will be required before the fermentation will restart.

3.      Was the fermentation aerated?

Solution: survival factor limitation in addition to growth nutrient limitation can result in a sustained low rate of fermentation.  The cells simply are unable to adapt the plasma membrane and metabolic rates to optimal levels and continue at a low rate. If plasma membrane composition cannot be adjusted sugar transporter turnover and replacement may not occur and transport may become dramatically limiting. In this situation the cells were not able to exchange the glucophilic transporters and hexokinase for fructophilic ones and never develop a high capacity to utilize fructose. The fermentation should be reinoculated preferably with a strain that has a high capacity for fructose utilization.

4.      What strain was used?

Solution: some strains are poorly tolerant of stressful conditions and if stressed early in the fermentation will not recover and will remain sluggish over the entire course of the fermentation. Other strains recover well under these same conditions. Recovery is associated with clearing the metabolic stress signal from the cells. Retention of a partial stress response will slow growth and metabolism but is also increases the chances of population survival should the stress return. Yeasts vary in their basal level stress responses and in their ability to restore normal growth following stress. It may be necessary to reinoculate with a highly tolerant strain to complete the fermentation. The tolerances and requirements of indigenous yeast can be difficult to predict. Sluggish indigenous fermentations may be dominated by strains that have poor fermentation properties but that are adept at out-competing other strains in the environment.

5.      What is the temperature?

Solution: low temperature fermentations will be slow. Sometimes the anticipated heat generation from metabolism does not occur and the fermentation may be chilled more than expected. Cooler fermentations will be slow.  Strains respond differently to cool temperatures so the tolerances of the strain being used should be reviewed. The temperature can be elevated but this should be done slowly to avoid a shock to the cells.

6.      Did temperature fluctuate?

Solution: dramatic repeated fluctuations in temperature should be avoided but if they occur they can lead to a poorly-adapted culture with slow fermentation rates due to the inability to adapt the plasma membrane to the changing conditions. Outside poorly insulated tanks can be warm during the day but chill dramatically during the evening. The yeast are again going to play it safe and not fully adapt to the newly established conditions banking on a return of inhibitory circumstances. If this is the situation, strains that are temperature tolerant should be used.

7.      Are other microbes present?

Solution: if the fermentation contains competing microorganisms, fermentation may be slowed simply due to the competition: depletion of nutrients and generation of inhibitory substances. This would occur if the strain was not fully dominant or if other organisms became well–established and difficult to displace. The fermentation will remain sluggish as long as the competition thrives. In this case efforts to remove the competing organisms will need to be made followed by reinoculation with a more robust strain.

8.      What was the juice microbial bioload?

Solution: if the microbial bioload of the juice was sufficient to cause a sluggish start of the fermentation, the fermentation may continue to be sluggish due to the lasting influence of the early competition. Inhibitory substances may have accumulated that will become more inhibitory as fermentation progresses and other stresses such as ethanol build up in the environment. Nutrients may have been depleted that were not provided. A complex nutrient should be used to feed the Saccharomycespopulation once it has become established. Reinoculation may be necessary if the nutrient depleted populations have arrested as addition of new nutrients may be insufficient in the presence of other inhibitory circumstances to restart the fermentation.

If there was a significant amount of rot in the vineyard, the vine may have produced inhibitory substances that will impact fermentation rate. If there is significant mold growth on clusters that were harvested, the microbial flora of the rot may have produced compounds that will be inhibitory to yeast. These fermentations will be very difficult to restart and will need to be treated with a fining aging to remove the inhibitory condition. If acetic acid is present at inhibitory levels it may be removed using reverse osmosis or other volatile compound stripping methods.

9.      Were pesticides/fungicides used in the vineyard?

Solution: sluggish onset and continuation of fermentation may be due to high residual levels of fungicides used in the vineyard or of components found in pesticides such as fluoride. These compounds may be present in sub-lethal levels and not lead to loss of viability of the culture but could inhibit cell metabolism resulting in a stalled fermentation rate. In this case the juice or must may need to be treated with a fining agent to remove the toxic component and reinoculated. Vineyard application records should be checked to see if this is a possibility. Juice samples should be taken and treated with a variety of fining agents and preparations to determine which one is the most effective at removal of the inhibitor but that shows the least stripping of juice/wine aroma and flavor.