Since the last batch seemed like it might have had too little yeast activity and too much bacterial activity, I decided to try adjusting things. When I added chestnut tannins to cider, it seems like it pushed the fermentation towards yeast, but I decided to try a different tactic this time.
- Jan 31: I dissolved 1 tsp of Yeast Nutrient in 1 cup apple juice and warmed it up until the crystals dissolved. added to 1 gal cheap apple juice (packaged with added ascorbic acid) and 1 scoby.
- Feb 7: Despite having a nice scoby growing on top, pH is > 4.4, and this tasted sweet without any sourness.
- Feb 12: pH is 4.4. The flavor was nice and cidery, with only a slight sweetness, but without any sourness. It seems like the literature is correct, and Yeast Nutrient is only minimally used by bacteria. At this point I can only wait for the bacteria to catch up.
- Feb 18: pH 3.6. It tasted sweet, but it was also starting to taste sour. I decided to bottle it before it got too sour for my daughter to enjoy.
By the time I’m publishing this, I’ve already made two posts that mention DAP as a microbial nutrient. But why use it here? Simply put, while it’s used by yeast, it isn’t used by acetic acid bacteria or lactic acid bacteria. Wikipedia describes it well. However, while the yeast nutrient I used is very simple (containing only DAP and urea), many others are more complex, to provide a more complete set of nutrients. Those additional nutrients may be usable by the bacteria (see same article), so different brands may produce different results.
If you decide to try using a yeast nutrient, be careful not to use too much.
Because inorganic nitrogen, such as the ammonium salts in DAP, are toxic to yeast in high levels, it is never added during inoculation when the biomass of the newly re-hydrated yeast is low. Many winemakers split up the dosage of DAP…
Kombucha fermentation is a little different. For example, home fermenters don’t generally use a clean, isolated, and preserved culture that needs to be “reconstituted” each time. Possible exceptions include when using a scoby that has been stored in a refrigerator, or when starting a new culture from a bottle of commercial kombucha. But if you do decide to use a yeast nutrient that includes DAP, you may want to start with some trial runs to ensure you don’t kill off your culture. I suppose I’ll eventually run a series of tests to see how much is too much (for me).
Another interesting tidbit in that article is that the amount of required nitrogen is increased by an increase in temperature, and by an increase in oxygen exposure. Since Kombucha ferments at a higher temperature than many (most) other types of fermentation involving yeast, we can expect it to need more nitrogen. Similarly, since the acetic acid bacteria in kombucha actually requires oxygen, we can expect nitrogen needs to be high. My interpretation for the the most simplistic kombucha fermentation is to be sure that you don’t skimp on the tea.
It also makes me wonder if the increased nitrogen needs means that other nutritional needs are increased. But what sort of nutrients are actually needed by the kombucha culture? I’m not aware of any studies specific to kombucha, but there are a ton of studies about the nutritional requirements of saccharomyces cerevisiae, which is the yeast responsible for primary fermentation in most beer, wine, and distilled liquor, as well as being common in the production of foods which undergo spontaneous fermentation (like chocolate and coffee). There’s a good chance that my kombucha culture contains s. cerevisiae; it’s everywhere. We also know that kombucha cultures contain another yeast, brettanomyces, which has recently become popular as an alternative yeast in beer. I haven’t seen any evidence that brett’s nutritional needs or growth factors are particularly different than sacch, and I expect that the data is interchangeable for the purpose of home-scale kombucha production. Wine research is also potentially useful because they specifically look at acetic acid bacteria (as a spoilage factor) and lactic acid bacteria (as a desirable part of malo-lactic fermentation).
There’s also been some commercial/academic interest in the production of bacterial cellulose, which kombucha brewers will call the scoby. These studies primarily look at acetobacter xylinum, which has been found in kombucha. This study lists potassium (K), sodium (Na), magnesium (Mg), calcium (Ca) and phosphorus (P) as minerals needed for a. xylinum. It also lists yeast extract and casein as good nitrogen sources, molasses as a carbon source, and 82.4°F – 87.8°C as the ideal temperature range. Below that temp bacterial growth is stunted, but over that the bacteria dies. Many yeasts prefer a lower temperature, so while too warm is a serious problem (death), cooler temperatures will create a different microbial balance in the culture. Lower temps should mean less acetic acid, and less scoby growth (but not less health). I may try batches of kombucha made with specific mineral supplementation, and see if it has any effect.
All this discussion of nutritional needs raises the question of how to know when you have enough. I’m not sure that a definite answer is possible. It is simple enough to use quantitative or objective measures to tell that something is happening, but without microbial skills or laboratory analysis, it’s hard to know exactly what microbes are present or gauge their health. If there’s no drop in pH, or sweetness doesn’t decrease, the kombucha culture probably isn’t happy. This is a fairly clear case of killing off the culture, or at least severely inhibiting it. But if the nutrients are only a little too few, it may be malnourished and not be viable long-term. Or if you have a little too much, it may be slightly toxic, and may be facing a slow death. Or maybe those conditions will cause the microbes to adapt, and you’ll be fine. Or maybe the normally-dominant microbes will diminish and other microbes will flourish in the new conditions.
This is the way of kombucha, and why everybody’s is different. Open-air fermentation means that your culture is being constantly inoculated with whatever is in the air. You’re constantly introducing new microbes when you add sugar, tea or water, unless it’s complete sterile (is isn’t, ever). Or variations in the micro-nutrients causes subtle shifts like we’ve discussed here.
Where do I go from here?