What is a stuck fermentation? When making hard cider, a stuck fermentation refers to a situation where the juice or partially fermented cider has sugar present but the yeast is unable to process it. Sometimes, this might have been your plan. Other times, this is your problem. This hard cider tip is intended to demystify why a fermentation can become stuck, how you can address it, and how you might want to plan for it in some hard cider recipes.
Let’s start by providing a short summary of yeast.
“Yeasts are single cell microorganisms that are part of the fungus family. In the process of multiplying or budding, yeast convert sugar into alcohol and carbon dioxide (CO2). There are over a thousand known varieties of yeast and most are harmless. They are used for making bread rise and creating alcoholic beverages. They are generally 3-6 microns in size with some being as large as 40 microns while some are smaller.”
The Art & Science of Cider
Thomas J Chezem
Yeast can be tricky little organisms. When you want them to be happy and active, they sometimes aren’t. When you want them to slow or stop, they sometimes don’t. There are a few key elements that drive whether your yeast are actively turning that apple juice into hard cider. To avoid or encourage a fermentation from becoming stuck, you should consider each of the following:
- Yeast feed on sugar and as long as your juice has sugar, they will want to keep feeding and multiplying. They don’t ever get sated. They will try to convert every last piece of yummy sugar until it is gone. That is often called dry and your specific gravity reading will be around 1.0. In fact, it will probably be just under 1.0. As I mentioned in my book, the process of yeast reproducing converts the sugar into alcohol and carbon dioxide. There are some sugars that are not fermentable. These include sorbitol found in pears, as well as lactose, stevia, and many diet sweeteners. By definition, a stuck fermentation means you have sugar available, but the yeast are not processing it. That means fermentation isn’t only about sugar and yeast.
- Yeast need nutrients to process the sugar and reproduce. These include things like nitrogen, amino acids, and fatty acids. Generally, apples will be full of these nutrients. This is especially true if you buy apples or juice from commercial orchards because they fertilize. The trees absorb the nutrients from the soil and deposit it in the apples. If you are using wild, unfertilized apples, you may be lacking in nutrients but probably not enough to cause a problem. Where you might be nutrient deprived is if you added a lot of plain simple sugar or even honey to your hard cider recipe. This is where a hard cider recipe would need nutrients. Simple sugar will not supply any nutrients so you could run out before your yeast convert all the sugar. If that happens at a specific gravity of 1.003, your plan worked or you got really lucky. Either way, bottle that hard cider and enjoy. If that happens at 1.018, you have a stuck fermentation. However, even if you have plenty of sugar and nutrients, you still might get a stuck fermentation.
- The amount of yeast you use can impact the fermentation rate and might result in slow or stuck ferments. You are always better off adding more yeast than too little. Most dehydrated yeast packets will ferment 5-6 gallons (19-23 liters) of wine, beer, or hard cider. Using that packet on 1-4 gallons (3.8-15.5 liters) won’t hurt you. However, if you use half a package on a 3 gallon batch, it might. For example, if you do a direct pitch of the yeast into the juice, you might shock and kill a fair amount of the yeast. This is especially true if the juice and yeast are at different temperatures. If this happens, your yeast will take time to reproduce and it may be competing with other yeasts or even bacterias. The result can be a slow or even stuck fermentation.
- Temperature also plays a big part in the fermentation rate and whether your fermentation might get stuck. Yeast are like people, some like living in Michigan and others Phoenix. For example, lager yeast, which are bottom fermenting like colder temperatures. They can take weeks to ferment. Ale and wine yeast, which are top fermenting tend to like warmer temperatures. In fact, some can work in temperatures as high as 90F (32C) without issue. If you are fermenting your hard cider in a cellar or garage where your temperature might be in the 50’s (10C), your fermentation could become sluggish or stick. Generally, warming it up will fix this problem. I advocate trying to sustain a consistent temperature and using a yeast designed for that range. If you are below a yeast’s range, it can slow and become stuck.
- If you buy juice and is has preservatives in it, it may never ferment. Pasteurization kills the yeast in juice but the juice will still be fermentable if new yeast is added. Sulfites and other preservatives like Potassium Sorbate and Sodium Benzoate don’t kill but suppress yeast and bacteria from reproducing. It’s why you don’t want to buy juice with preservatives because it could be permanently stuck. Some hard cider recipes and books advocate using sulfites like Campden tablets, sodium or potassium metabisulfite, before commercial yeast is pitched into the juice. The goal is to suppress the wild yeast and bacteria and give the commercial yeast a clean slate. Too much or adding the yeast too soon after treatment could result in the suppression of the yeast you just added. This could result in slowed or stuck fermentation.
Others: Acidity and Alcohol
- There are other factors that can impact the fermentation of your hard cider and make it stick. However, the top two are acidity of the juice and the amount of alcohol present. These really are yeast dependent factors. Some yeast work fine in highly acidic juice but others may never start fermenting or stall. This happened to me with a grapefruit hard cider. I first tried an ale yeast but the juice was too acidic, and the ferment couldn’t start. I reinoculated it with a yeast that works in acidic environments and that solved the problem. The same is true for alcohol level. Wine yeast will usually ferment to a higher level of alcohol than beer yeast. Even with sugar present, the high level of alcohol starts killing the yeast and stopping fermentation. This can work in your favor if you are making a dessert cider like ice cider. This is also how they fortify wines. They add spirits to a wine that still has some sugar. The added alcohol kills the yeast culture and stabilizes the wine with this residual sugar.
If you pitch your yeast and find your juice isn’t turning into hard cider or it started but stopped, I’m hoping you are better prepared to identify and potentially rectify the situation. Is the juice too cold? Did you buy a juice with preservatives in it? Maybe, you didn’t inoculate with enough yeast or a yeast that doesn’t like acid. Did you add a lot of simple sugar and run out of nutrients? Or, maybe you’re just being a little impatient. Slow and steady is great for hard cider. It builds flavor and maybe this hard cider will be your finest ever.
If you found this tip useful, check out all my hard cider recipes and tips at my website. Also, follow me so you get copies of my blogs and latest news from Prickly Apple Cider.
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