Restarting Arrested Fermentations Guide
Step 1: Aerate the fermentation:
The first thing to try with an arrested fermentation is to expose the culture to air. If survival factors are limiting this is often enough to encourage the yeast to finish the fermentation.
Step 2: Check Ferment Chemistry: measure pH, ethanol content, nitrogen levels.
Is the pH within permissible range? >3 and < 4?
Is nitrogen depleted? If yes a modest addition should be made.
What is the ethanol level? Compare to strain tolerance if a commercial strain was used. If ethanol is above 12%, nutrient additions may not be effective.
Step 3: Evaluate a sample of the fermentation under the microscope:
Are cells ovoid like Saccharomyces with evidence of budding?
Are cells refractile with nothing or only a vacuole visible in the cytoplasm?
If the answer to these two questions is yes then the culture still appears to be healthy and a restart may be successful.
Are the cells bloated, elongated or have lost refractility?
Is there evidence of small vesicles in the cytoplasm indicating loss of cytoplasmic integrity?
Are apiculate, unusually tiny cells or bacteria in evidence?
If the answer to any of these questions is yes then the culture is in stress and a reinoculation is warranted.
Step 4: Add yeast ghosts:
Yeast ghost addition can be beneficial in restarting arrested fermentation. The ghosts can add yeast fatty acids and sterols and be a source of nutrition. They can also absorb inhibitory fatty acids that may have accumulated in the arrested fermentation. The addition of ghosts may also provide additional oxygen to the fermentation.
Step 5: Rack off the lees:
If the fermentation has a settled yeast population or will be reinoculated racking off of the existing yeast biomass is a good idea. This will prevent further synthesis of alarm signals by that population and will make sure that added nutrients are not absorbed by this arrested population.
Step 6: Add nutrients:
If culture appears healthy from step 3 and step 2 indicates a nitrogen deficiency, nitrogen should be added. The dose does not need to be that high, just sufficient to complete the fermentation.
Step 7: Inhibit non-Saccharomyces biomass:
If the assessment in step 3 reveals the presence of bacteria or non- Saccharomyces yeasts the ferment should be treated to eliminate these organisms. Lysozyme or sulfite can be used to inhibit the bacteria as long as low sulfite levels are used. The yeast could be eliminated by treatment with DMDC, following manufacturer’s instructions and allowing enough time between DMDC treatment and reinoculation for the DMDC to hydrolyze.
Step 8: Start an adapted reinoculant culture:
If step 3 indicates the culture is in distress then reinoculation should be attempted. Successful reinoculation will require adapting the new inoculant to the conditions of the arrested fermentation and selecting a strain that has sufficient tolerances to complete the fermentation. Commercial strains specifically designed to restart arrested fermentations should be considered. Rehydrating the same way as for initial inoculation or dumping the dry yeast into the wine will not allow the yeast to gradually adjust to the ethanol level of the ferment. If the ethanol level is above 10% the cells will have great difficulty adapting. There are highly ethanol tolerant strains (19% and above) that may be able to reinitiate fermentation under these conditions, but this is not always reliable.
The yeast should be rehydrated in the presence of nutrients so that absorption of those nutrients can occur. This should make sure the inoculant culture has the necessary micronutrients to complete fermentation.
Step 9: Mix the rehydrated culture with the arrested fermentation:
The rehydrated culture should be mixed with the arrested wine. If fresh juice is available mix the rehydrated culture with the juice in a 1:1 ratio, wait roughly 1 to 2 hours then add an equal volume of wine. This will drop the ethanol content to below 7-8%. Wait until evidence of fermentation occurs, a drop in Brix value. The use of juice will provide some sugar for the starting culture that can be used to allow cell growth and adaptation to the ethanol levels of the mixture.
Step 10: Monitor Brix of reinoculant culture:
The Brix of the reinoculated culture should be monitored. Measuring the ethanol content would be a good idea too, if possible. It is critical at this stage to not let the reinoculant culture go dry. When the reinoculant culture has started fermenting and has increased the ethanol content by 1 to 2%, add an equal volume of the arrested wine. Fermentation should continue.
Step 11: Monitor for evidence of continued fermentation:
At this point the fermentation is at least 75% of the arrested fermentation and the cells in 2 to 4 hours will be adapted to the higher ethanol concentration. Again you do not want the fermentation to go dry.
Step 12: Slowly add reinoculant culture to rest of arrested ferment:
Once a healthy adapted reinoculant culture has been created it can be added tothe rest of the tank and fermentation should complete.