I have an ongoing experiment (to described in a later post) in which fermentation stalled, and I wanted to add additional nutrients to see if that would get it going again. I didn’t find any kombucha-specific information on that, so I decided to run an experiment! A while back, I ran experiments trying to ferment with minimal ingredients, like tea-only and sugar-only. The tea-only trial did nothing, which would be great to see if additives help, but since tea already provides the needed nutrients, that wouldn’t make a good starting point for an experiment of nutrient additions. In contrast, the sugar-only trial was lacking nutrients, which makes it similar to my current interest. Unfortunately, it also kinda worked, though the results were weird. Still, it should be pretty easy to tell if I can improve on weird, so I used my sugar kombucha as the basis for this experiment.
I set up four quart jars with distilled water, added 1/4 cup regular white sugar (sucrose), 1/4 cup starter liquid from my scoby hotel, and added a different nutrient to each. It was a bit of a challenge to know how much nutrient to add. I basically guessed, based on available information, previous experiments, and general sense of proportion. I feel confident that the amounts I used are not ideal.
.5 tsp yeast nutrient (DAP+urea).
Since this is a standard ingredient used for the fermentation of wine, beer, mead, etc, I had actual numbers to go by: 1 tsp/gallon. Since I wasn’t adding any other nutrients (tea, grapes, malt, honey, etc), I doubled the standard amount. This will nourish the yeast, but may not nourish the bacteria.
- 8 days: pH 3.6. There was a dry, powdery, cracked film on top. The taste had a dull sweetness and sourness. Not bad.
- 12 days: pH 3.2. The sweetness is gone, and the sourness was still relatively dull. I’m guessing this produced lactic acid? Without a scoby, I think it’s safe to say that something inhibited the acetobacter, but I don’t know what. It’s interesting that pH 3.2 is the lowest pH at which lactobacillus plantarum can grow (and it’s a lactic-acid bacteria noted for it’s high acid-tolerance).
1 tbsp blackstrap molasses
Molasses is fermentable all on it’s own, and is commonly recommended as a nutritional adjunct for water kefir and other fermented foods, so it seemed like it should work here. I guessed on the quantity, but tried to be sensitive to the fact that it also adds significant sugar.
- 8 days: pH 3.6. Flavor was lightly sweet and sour, with a clear but not overwhelming (or unpleasant) molasses tinge. The scoby was 2-3mm thick, and very solid.
- 12 days: pH 3.4. Not a lot of change in acidity, but the sourness was a little intense (though not unpleasant). Although there was a slight vinegariness, the overall sourness isn’t dominated by it, and seemed balanced. Fizzy. Scoby is 3mm and solid. This also seemed to have the most yeasty sediment in the bottom. All-in-all, I’d surmise that, of these experiments, molasses produced the most vigorous yeast, and perhaps due to increased ethanol production, the most vigorous bacterial activity typical of kombucha.
1 tbsp nutritional yeast
Dead (spent) yeast “contains a rich variety of nutrients and minerals that the bacteria can use as a complete nutrient set, and for the majority of most fermentations this is all that they need” (reference). It’s also included in some nutritional supplements for beer and wine. This should nourish the yeast, but may not nourish the bacteria.
- 8 days: pH 3.6. There was some scoby “scum” floating on top of a 1/2″ thick layer of foam. The flavor lacked sweetness, had some sourness, and tasted yeasty.
- 12 days: pH 3.4. Not a lot of change in acidity, but it had a clear vinegar character while still tasting rather yeasty. There was a small section of relatively normal scoby cellulose, but the top was still mostly scum on top of foam. There was a slight bitterness in the aftertaste, which I believe to be a part of the yeasty flavor.
.25 cup inulin
Inulin can be fermented by yeast, and is marketed as a prebiotic (bacterial fermentation), which seems like exactly what I’m looking for, though there isn’t any information on it being used quite like this. Will it work?
- 8 days: pH 3.6. There was a light sweetness and bright (but light) tartness. The scoby was very thin, but stable, floating on large, lacy bubbles.
- 12 days: pH 3.2. With a bright tartness without any vinegariness, I’d guess this produced citric acid? No change in the scoby. It felt like this had a slight zing in my mouth and as I swallowed it.
A little post-experiment research turned up a study in which inulin has been used in place of sugar, resulting in lower residual sugar and vastly increased scoby production. I wonder if my scoby was thin because of a lack of nutrients?
In the end, I found the yeast nutrient and the inulin to be the most interesting in terms of a nutritional supplement for kombucha fermentation. Nutritional yeast and molasses each adds its own strong flavor that would dominate the final flavor. The Yeast Nutrient and inulin do appear to support the microbes (with the YN obviously skewed away from the bacteria and towards the yeast), though they also affect the type of acid that’s ultimately produced. This is very interesting to me, but my not be what you’re looking for.
There are several different brands of fermentation nutrients, and I’ll try this again with different ones. I may also run an experiment with different quantities to inulin to see if I can establish a guideline on how much to add, and one where inulin is used in place of sugar. Of course, when making kombucha with tea, none of this is necessary, so this is only useful when fermenting things that need a little help supporting the culture.