It happens. Sometimes, after a promising start full of froth and vigor, the yeast tap out and leave a partially fermented must/wine. It’s not drinkable and you certainly shouldn’t bottle it, but it’s also vulnerable to spoilage. So how do you get the yeast going again? I’ll explain how I do it, and use my oregano wine as an example.
Find out, and correct, the problem
It’s not always apparent what went wrong, but it’s worth the effort to try and find out. If there is some underlying problem that is inhibiting the yeast, then just adding more yeast, even a vigorous starter, won’t help. This is where good measurements and careful note-taking pay off. Nobody ever plans for things to go wrong, and that’s why good habits should become habits. My oregano wine suffered from a pH problem, and that brought fermentation to a halt. I suspected the pH after reviewing my notes, and confirmed it with a measurement.
Other potential problems that you might consider are temperature, preservative in store bought juice, nutrient deficiency, or lack of oxygen. Review your notes, take measurements, and do your best to discover why your yeast stuck.
The yeast began fermenting again, albeit very slowly, after I corrected the problem. It probably would have fermented out, eventually, if I had left it alone. A long slow fermentation like that is risky, however, so I decided to treat it as though it were stuck and hurry things up a bit.
Gradually add the stuck wine to a vigorous starter
After fixing the underlying problem, you should make a starter. This builds up a large population of actively growing yeast. Add some of the stuck wine to the starter. I like to double the volume every four hours or so, and since my starter was about a cup, I added a cup of wine to it. Four hours later: two more cups. Four hours after that would have been well into the wee hours, so with about a quart (close to a liter) of fermenting wine, I went to bed. In the morning I added another quart.
Nutrient: Not too much, not too little
Without enough nutrient, the new yeast may have trouble growing and fermenting the wine. If there’s more than the yeast can consume, some nutrient will remain in the fermented wine. That can cause off flavors all by itself, and it can also support spoilage organisms. So there’s no way I can tell you how much, if any, nutrient to add at this stage. The best way to decide is to measure the available nitrogen in the must, but it’s pretty unusual for home wine makers to run such an involved test.
If you haven’t got a chemist and a state of the art lab handy, gather up all the information you have about how much nutrient was in the must, how much you added, and how much yeast activity there was. Did I mention the part about good notes? If you started with a lot of nutrient and the yeast didn’t get very far, then you shouldn’t add much (or any) nutrient. If, for whatever reason, you’re starting nutrient level was low then you should add some. I realize that “a lot”, “much”, “low”, and “some” are a little vague, but the only way to get precise answers is with that chemist and the state of the art lab that we haven’t got.
My Oregano Wine recipe called for 1 tsp/Gallon of diammonium phosphate, which isn’t a lot. Since the must was basically a sugared oregano tea, it had virtually no nutrient except for what I added. So I decided to add another tsp of DAP along with the yeast starter.
I’d like to say that you won’t have to deal with stuck fermentations, but if you make wine regularly you’ll probably have to face an unmoving hydrometer sooner or later. Your best bet is to start dealing with it before it happens with good procedures and meticulous note taking. If you do that and use your head, you’ll have a good shot at saving your wine.
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