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What is the difference in the making of yoghurt to produce carbonated vs non-carbonated yoghurt?

I'm not a proper cook, and am just researching this 'cos i'm curious..

Yoghurt is fermented milk, and gas would be produced..

So what's the difference in the yoghurt making process between when a yoghurt is carbonated, like a Kefir drink(Kefir is mentioned on wikipedia, it's a carbonated yoghurt).. vs when a yoghurt is like most we are more familiar with, not carbonated like normal yoghurt.

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There are different kinds of lactic acid fermentation which are done by different kinds of bacteria and/or yeasts.

The simplistic picture is that yogurt is typically made with only two types of bacteria (lactobacillus bulgaricus and streptococcus thermophilus) that perform only homolactic fermentation converting sugar (glucose) into lactate only. In contrast, Kefir is made with a larger mix of bacterias that perform homolactic fermentation as well as heterolactic fermentation, which converts glucose into carbon dioxide (CO2), ethanol and lactate.

For this reason, kefir is carbonated and have even slightly alcoholic.

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    +1 I am super impressed to see such a scientific answer. I'd been reading about fermentation a lot and not run into that. – barlop Nov 2 '17 at 11:12
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    Thank you, @barlop! If you are interested in a less sketchy picture keep in mind that while yogurt is somehow easy to understand, Kefir is a much more complex business! Different kefir can have different mixes of bacterias and yeasts that of course interact and compete during the fermentation process. It is easy to get lost in the plethora of scientific articles about kefir... – greedyscholars Nov 2 '17 at 11:21
  • perhaps a correction, when you write "lactic fermentation" you mean "lactic acid fermentation", named that way because it's a fermentation process that is seen in lactic acid bacteria(though can occur elsewhere eg it occurs in humans)..It's not named because of what it produces. It produces lactate. So when you write converting sugar into lactic acids, you mean into lactate not into lactic acid. Interestingly, some human cells do lactic acid fermentation, and like any "lactic acid fermentation" it's lactate that is produced. – barlop Dec 22 '17 at 18:22
  • Thanks for the precision @barlop. It never occurred to me this distinction in English. Interestingly, in the romance languages "Lactic acid fermentation" is simply "Lactic fermentation", e.g. the French "Fermentation lactique", Spanish "Fermentación láctica", Italian "Fermentazione lattica" or Portuguese "Fermentação Lactea"! You are also right for lactic acid vs lactate. I tend to mix them up because they are conjugated! But in solution, it is indeed lactate. – greedyscholars Dec 25 '17 at 12:51
  • one question that has bugged me, you may be the man to ask.. When Louis Pasteur coined the term "anaerobic" in reference to fermentation, did he mean "without air"(which is what anaerobic means etymologically), as in, it occurs in a vacuum. Or did he mean without oxygen. And say he meant without oxygen, did he mean no oxygen in the atmosphere, or did he mean yes there can be oxygen in the atmosphere but it doesn't use any of it? There is no history of science stackexchange site for me to ask that one on – barlop Jan 2 '18 at 0:45

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