Why using a hydrometer does not work with kombucha

[ffmurray]

Ive noticed in several threads that people keep using hydrometers to try and determine if their ferment is done. While this works for beer and wine it does not work for kombucha.

Kombucha is a very complicated thing, there are many different chemicals including alcohols and acids. If you were to break the kombucha down into each of its different components dissolved in water each would have a very different SG.

So before we go into kombucha lets start with why SG reading work in alcoholic ferments. Pure water has a SG of 1.00, when you add sugar the specific gravity goes up. For the sake of the example lets say we are fermenting plain sugar water. If you put a pound of sugar into a gallon of water, the SG will go up to about 1.046 or so. This is because the sugar water is denser than plain water.

Pure ethanol has a specific gravity of 0.787 When the yeast consume the sugar(remember its denser than water) and turn it into ethanol(less dense than water) and carbon dioxide(leaves the ferment further removing mass and therefore SG) The specific gravity goes down enough to get a good consistent measure of how the fermentation is doing and when the fermentation is complete.

So for kombucha, lets make a theoretical batch that is made with just sugar water(no tea, I know its unrealistic), and produces only acetic acid(also really unrealistic, but for the sake of demonstration its good to simplify) Acetic acid has a SG of 1.052, herein lies our problem. Because acetic acid is much closer to our original specific gravity reading of 1.046 for the pound of sugar in a gallon of water, the change from fermenting the sugar to acetic acid will have a very small effect on the total SG of the "Theoretical Kombucha" as it ferments. The change in density is much smaller, and therefore much harder to calculate how much acetic acid is being produced.

In its simplest terms pure ethanol is much less dense than the original starting SG of one pound of sugar in a gallon of water (0.782 vs 1.046 a difference of 0.259) and pure acetic acid is much closer to the original SG (1.056 vs 1.046 a change of only 0.010) and this small range make it almost impossible to successfully measure the change in SG and calculate the amount of acetic acid produced in our very very simplified example. Because kombucha produces so many other acids and chemicals, and each mother and ferment is different and produced different acids in differnt ratios, you can see why the SG reading does not help at all in the fermentation of kombucha

I hope this helps and I did not ramble on to much

*various SGs came from www.engineeringtoolbox.com

 

[hunter_le five]

Good stuff, thanks for the thorough explanation.

 

[enricocoron]

I believe a refractometer would be preferred as it's only measuring the refraction of light by sugar. It would be a little off because of the small amount of alcohol, but if your goal was to just stop the fermentation with the same residual sweetness each time you could measure a starting sugar content of 10 brix down to say 2 brix. You only need a few drops for the refractometer as well. Unless the acetic acid throws off the refraction of sugar? I wouldn't think so as lactic acid doesn't have much of an effect if you make sour beer using a lacto culture.

I have zero experience with fermented tea but in general refractometers are used in the beer and wine industry to ballpark the original gravity during the brewing process and then a hydrometer is used to establish the final gravity and to make sure that the fermentation is terminal. The refractometer can be used in the presence of alcohol if you have an original gravity and current using a correction equation but, a hydrometer won't work once the fermentation begins because of the density of acetic acid as explained above.

 

[MarkKF]

so..... if I take a initial S.G. before adding the scoby
there is no magical final S.G. for when it's "done"?

 

[Yooper]

so..... if I take a initial S.G. before adding the scoby
there is no magical final S.G. for when it's "done"?

Probably. Because, you are adding sugar to tea (so you'd have a decent OG). But,you aren't really creating ethanol like with wine. Kombucha has a negligible amount of alcohol. Bacteria like aceterobacter "eat" alcohol and produce vinegar, while lactobacillus also produces tartness. The products are gluconic acids as well as the vinegar/ My own SCOBYs don't produce much vinegar at all, and I try my best to keep my culture from doing so. Just like with yogurt, a fermented milk product, the alcohol content is negligible, but there are other products of fermentation in there. While the sugar is gone (fermented), I'm not sure a SG would be at all helpful. Of course since it's different types of fermentation which produce other things besides ethanol, even if you got an SG reading, it may not show you "done".

For example, when you make vinegar, the vinegar bacteria "eat" alcohol and release the vinegar. They will take all of the alcohol, and produce vinegar until it's done. But the Sg may or may not change. Because the presence of alcohol is lighter than water- so when alcohol goes away, the SG may actually increase. That's opposite of what happens in winemaking and brewing. So a hydrometer would be pretty useless in kombucha making.