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While brewing Kombucha sugar is transformed to carbon dioxide, alcohol, acid, etc. But what does that mean to the calories? Is there less? How much less?

Here is my setup:

  • tea: 25 ltr
  • sugar: 1000 grm
  • temperature: 18°C
  • duration: 4 days

Can you tell how much calories I have in my Kombucha?

I hope for an answer like: "Calories go down by 20% to 30%".

  • What was OG before and after fermentation? This is quite important to tell how much sugars got processed. – Mołot Apr 14 '16 at 15:17
  • What do you mean by "OG"? – Witek Apr 14 '16 at 16:15
  • original gravity. And final gravity. Measured with hydrometer. – Mołot Apr 14 '16 at 16:19
  • Do I realy have to measure OG? Can't it be calculated from the 1000grm sugar in 25 ltr water? – Witek Apr 16 '16 at 16:33
  • starting one can. Final gravity can not, it depends on yeast or bacteria strain, temperature, condition, on avaliability of nitrogen in brew, on pitch size... And if you are going to measure final gravity, you can as well measure original one, too. For simplicity. – Mołot Apr 16 '16 at 16:36
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Well in your specific kombucha I won’t be able to tell you but I can give you some math that should help you figure it out and should be able to be used by any brewer to figure out the caloric effect of fermentation on your brews. So just to hit you with the math right now and ill break it down later the equation to figure out the change in calories caused by fermentation you will use the equation.

OG = Original Gravity

FG = Final Gravity

V = Volume of ferment in ml

Version for Sucrose fermentable used by OP

((((-(OG - FG) * 131.25*0.789*(V /100))/46.07)/4) *342.3*4) +(((OG - FG) *131.25*0.789*(V /100)) *7)

Version for maltose fermentable in beer (treats Maltose as glucose for math purposes)

((((-(OG - FG) * 131.25* 0.789*(V/100))/46.07)/2) *180.16*4) +(((OG - FG) * 131.25* 0.789*(V/100)) *7)

If you replace the variables with your values and paste it into https://www.wolframalpha.com it will output the answer.

Now to break down what is going on here.

(OG - FG) * 131.25 = ABV – this is the equation for estimating the ABV of your brew using original and final gravity.

ABV*0.789 = ABW – this changed the ABV into ABW which is a weight as opposed to volume measurement of the alcohol in your brew.

ABW*(V /100) – this is the total grams of alcohol in your brew – I’ll refer to this as AT from here on out.

AT/46.07=Moles of Alcohol – this converts the grams of alcohol into moles of alcohol which is a count of the number of molecules of alcohol in your brew.

Moles of alcohol/4=Moles of Sucrose Fermented – the process of fermenting sucrose (which you used) creates 4 molecules of alcohol for each molecule of sucrose fermented so by dividing by 4 we get the number of sucrose molecules fermented.

Moles of Sucrose Fermented*342.3*4 = calories of sucrose fermented- by taking the moles of sucrose and multiplying by the molar weight we change the moles of sucrose into grams of sugar then multiply those my calories per gram we get the total amount of calories that those sugar molecules possessed.

Now for the second half this will be quick I promise.

You will notice that the other half looks very similar that is because it is just calculating the total weight of alcohol in the brew and multiplying that by the calorie content on alcohol per gram giving us the calorie content of the alcohol.

Last part bringing it all together.

So once we do all this math we get

(- calorie content of sugar that was fermented) + (calorie content of new alcohol)

This gives us the total change in calories created by fermentation.

  • Isn't there a thumb rule like "after 4 days calories go down by 25%"? – Witek Apr 15 '16 at 23:34
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Matter cannot be created nor destroyed

So really the only calorie loss during fermentation is that in which the yeast burn and what is concentrated from the loss of c02 escaping.

Ethanol has 6.9 calories per gram Sugar has 4 calories per gram

Here is a calculator http://realbeer.com/spencer/attenuation.html#calories

If you play with the numbers for calories to simulate fermented and unfermented , you'll notice that don't change much in fact there is about 5% more calories after fermentation.

This is because calories are calculated by actually burning the test matter.

All that being said alcohol is digested but takes more energy to digest than sugar since its converted to sugar first but still has a positive net.

So all in all, pre and post fermentation has little change in actual calories.

Edit: It seems acetobacter in SCOBY could potentially consume most of the calories. I've not found any formulas to calculate calorie loss from acetobacter. But 5 grams of red wine vinegar has only 1 calorie, 5 grams of red wine has about 4 calories.

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    But Kombucha has not much alcohol. And what about the SCOBY? Doesn't it absorb some energy? – Witek Apr 16 '16 at 16:30
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    @Witek I guess it really depends on the amount of acids produced, which would require before and after PH readings. It seems like it could be signifigant since red wine vinegar has 1 calorie for 5 grams, much lower than red wine. – Evil Zymurgist Apr 16 '16 at 19:00
  • Just found this thread, but these numbers seem highly suspect to me. Even ignoring the lack of info on the acetobacter part, your conclusion is that when fermenting from sugar to alcohol, in a net-exothermic process (which also produces substantial (discarded) byproducts (CO2)), in an otherwise effectively closed system, that the total energy of the system (calories) increases by 5%? You seem to be forgetting another golden rule: You can't create energy from nothing. – Foogod May 18 '17 at 0:45
  • I suspect at least part of the problem is you're assuming that the mass of ethanol produced equals the mass of sugar consumed, but in actuality, in that process around 49% of the mass is actually lost as escaping CO2. As such, I would expect the caloric content of the ethanol produced to end up being somewhere around (0.51 * 6.9/4) = 88% of the calories in the starting sugars. If we assume (very roughly) a similar calorie reduction from the acetobacter as in the red wine vinegar example you mention, then the end result should be somewhere around 0.88 * 0.25 = 22% of the original calories. – Foogod May 18 '17 at 1:13
  • @Foogod I think where you're getting confused is with mass and volume. The mass loss from c02 is negligible. But when sugar is converted to ethanol the mass is basically the same but the volume of sugar in water is higher than the volume of water when the sugar is converted to ethanol because the ethanol molecule fits between water molecules while sugar does not. The reason alcohol has more calories than sugar is because how calories are measured, by litterally igniting the sample. But is a poor gauge to calculate what goes on in digestion. – Evil Zymurgist May 18 '17 at 12:51

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