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How to re-start a Stuck Fermentation
We have already discussed the 10 steps to a successful fermentation as well as how to fix failed fermentations. Sometimes, despite all of our efforts, we are stuck with a lifeless fermentation. The first step is to determine if it is worthwhile restarting the fermentation. Re-starting fermentations cost time and money (need to add more nutrients, possibly potassium carbonate and if necessary new yeast). First, determine how much unfermented sugar remains and what amount of alcohol it will add to your fermentation. If you are only going to get a couple of millilitres worth of distillate, it might not be worthwhile.
Once you have followed the steps mentioned in our article "Failed Fermentations and Fixing them", and it still seems like there is no activity, your yeast might have died which means that a fresh batch of yeast should be added.
How to re-start a stuck fermentation - The Steps
When you are re-starting a stuck fermentation, you cannot simply add your yeast directly to your medium. Your mash has already started fermenting which means there is alcohol present. Alcohol is toxic to all micro-organisms and even though Saccharomyces cerevisiae (the typical yeast specie used in alcohol production), has a relatively high tolerance toward alcohol toxicity, adding active dried yeast directly might cause such a shock that the new yeast cells might die soon after. The following protocol should be followed when restarting a stuck fermentation.
1.) Rack your wine from any dead yeast cells that might have dropped to the bottom of your fermenter. This is important since these old yeast cells contain toxic compounds (such as medium-chain fatty acids), which is released into the medium and inhibits yeast growth.
2.) Add Yeast Hulls at the maximum dosage to the stuck fermentation (these can be bought at any yeast manufacturing company). They provide large amounts of important proteins, amino acids and vitamin B, which necessary for new cell production. Another important contribution is their ability to absorb and remove toxic compounds, such as medium-chain fatty acids, present in the medium.
If you cannot get a hold on these Yeast Hulls, you can make your own. Take note, the following method should only be used in times where yeast aren’t available for purchasing. Producing high quality Yeast Hulls requires a specific process with specialised equipment which cannot be mimicked at home.
Harvesting and preparing Yeast Hulls
- Pour the yeast that is left over after racking into a pot. Add water in a 3:1 ration (for each ml of yeast lees, add 3 ml of water). Bring the pot to a medium boil for at least 30 min at 80 degrees Celsius, stirring occasionally (this will kill of the yeast so that they become inactive and kill any spoilage micro-organisms that might be present.
- Allow the yeast-water mixture to cool down. Pour into a sterilised, clear bottle and allow the yeast to settle at the bottom. This process can be accelerated by placing the bottle in the fridge.
- As soon as the yeast has settled, syphon the water off the lees.
- Pre-heat the oven to about 25- 28 degrees Celsius. Smear the yeast on Parchment paper (used to line baking pans during baking) and place it onto a cookie cooling rack and then into the oven.
- Allow to bake for 12 -24 hours, or until completely dry.
- Add the dried yeast to your stuck-fermentation.
3.) Hydrate a new batch of yeast properly with the addition of yeast activator. Keep in mind that you should use a yeast strain that can handle these harsh conditions, such as EC-1118. Yeast activator is a special blend of nutrients to obtain high levels of certain essential vitamins and amino acids required for healthy yeast fermentations. Yeast are most sensitive during the initial stages of re-hydration, infusing yeast with these essential nutrients arms them against ethanol toxicity and optimizes nutrient availability to the re-hydrating yeast culture. For information and instructions on how to rehydrate yeast with the addition of yeast activator, read our article: Active dried yeast: re-hydration or direct additions?
- Since you will be starting a stuck fermentation, you will have to increase the yeast dosage. You can double the amount of the recommended dosage to ensure that you have a big enough starting yeast population, for if some were to die, you will still have enough viable yeast cells left to finish the fermentation
4.) After 20 minutes of yeast rehydration. You have to acclimatise the yeast to the condition of your mash fermentation. This is done by exposing the re-hydrating yeast to a small volume of stuck- fermentation mash and then gradually increasing the addition volume until the whole tank of stuck-fermentation has been transferred into the container containing the re-hydration medium. Depending on the total volume of stuck fermentation, you may have to transfer the re-hydration medium into a bigger container.
- After 20 minutes of yeast re-hydration, add a volume of stuck wine equivalent to 50% of the volume of the rehydration mixture. If your initial re-hydration volume (containing water, re-hydrated yeast and yeast activator) is 10 L, add 5 L of stuck-mash.
- Repeat step 3.1, taking the new total volume (10L + 5L = 15) into account. Therefore add 50% of the total volume = 7.5 L of stuck-mash. Wait 1 hour.
- Continue using the same procedure until all of the stuck-wine has been transferred.
- As soon as there is yeast activity (CO2 being produced/ brix reading decreases), add some yeast nutrients (a mix containing Yeast Hulls, DAP, Vitamins and Minerals).
The whole process should take a day or more. The longer you wait between each addition of stuck-mash the more time (and a better chance) the new yeast has to acclimatise to the increasing alcohol levels. The success of the protocol depends on the time it takes to transfer the stuck- mash to the new yeast.