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I have a sourdough starter which is now close to 15 years old. It has survived everything - including long pauses where it stood in the fridge for over a month or longer. It is incredibly stable and very hard to kill (at least I was not successful so far). Presumably you can also make a backup by drying it completely and storing it long-term (I have made such a backup, but have not had to use it yet).

Sometimes I also like to make yogurt by sterilizing milk at 90-95°C, then cooling to 45°C, adding existing yogurt, and keeping it at 45°C for about 6 hours.

This always works very well for the first 3-4 generations (starting with storebought natural joghurt). The first generations are extremely nice, just like the original.

But after some generations, it inevitably does not continue to work as well. The next generation will not become as thick, and will be more of a soup; clumps of yogurt mixed with the clear watery substance you get in these processes (sorry, don't know the word in English - is that whey?). I'm quite careful to keep the whole process as identical as possible.

Do you know the reason for that, or if it is possible to get a long-term, stable yogurt running?

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  • personal anecdote: I revived a dried sourdough backup after 4 years in a cupboard, and it's still going. – Andreas Grapentin Apr 25 '20 at 4:24
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First, we are talking about different microorganisms when comparing sourdough to yogurt. Generally, a sourdough starter is populated by numerous strains of yeast and bacteria, while store bought yogurt contains a small number of isolated strains of bacteria. However, there is no reason that you can't maintain a yogurt culture that lives on. According to the linked article, the isolated strains in store bought yogurt are generally only viable for a couple of generations. You might look into heirloom starter cultures, as they suggest. That way you can develop a yogurt starter that is as viable as your sourdough starter.

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    Evidently there must be a way - as the yoghurt companies are managing to keep their starters going somehow. – Tim Apr 25 '20 at 14:16
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    @Tim clearly there is a way to keep a starter going at home. That is what my answer describes. Yogurt companies have labs that isolate strains of desired bacteria for their product. They regenerate these strains so that their product remains consistent. – moscafj Apr 25 '20 at 20:03
  • @moscafj, I have gotten a "heirloom" starter back then, and am propagating that weekly without fail. So that works. The answer is old and accepted, but if you find more info on why that is, that would be interesting (the question was more about the "why" then the "how")... – AnoE Mar 8 at 11:32
  • @AnoE I believe it is because the commercial starters are isolated, to single (or just a couple of) strains for product consistency. Heirloom starters are likely comprised of multiple strains of bacteria, which survive and evolve over time. – moscafj Mar 8 at 11:58
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One trick I use for yogurt starter is freezing yogurt in an ice cube tray. Every time I make a new batch, a cube goes in. When I run out of starter cubes, I freeze some more from the current batch. I've been making 3 quarts of yogurt weekly for several years and haven't noticed any changes in the resulting yogurt over the many generations of starter.

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  • Thank you! That's a good point. Sourdough starter holds well if simply cooled (not frozen), but good to know that yogurt starter survives freezing. – AnoE Apr 27 '20 at 9:06
  • This is really interesting. Thanks for submitting the idea. Do I understand correctly that one (frozen) yogurt cube is enough to culture 3 quarts of milk? – Arlo Apr 28 '20 at 17:51
  • Yes, one cube is plenty. Sometimes I cut the cubes into roughly 1 cm^3 fragments. I've actually had a couple batches where I totally forgot to add the starter, but it made perfectly good yogurt anyway (taking somewhat longer)--I think this was due to residual bacteria on the thermometer I use. The bacteria population can grow exponentially at the beginning, so a little starter goes a long way. – Vulcan Apr 29 '20 at 18:26
  • That's really interesting about residual bacteria being enough to make yogurt! Because of the long incubation needed, was the yogurt more tangy than usual? – Arlo May 1 '20 at 4:41
  • This is very cool! Two questions: what did you use in the beginning? A store bought yougurt? Also, how do you de-freeze yur cubes without giving it enough time to spoil? – josinalvo May 1 '20 at 14:32
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It depends on the culture you use. The problems you describe do happen with cultures from store bought yogurts. My family has a yogurt culture which we've been using for decades and it still produces great yogurt every time. It came from overseas decades ago. I got mine from my dad. If you have friends from south Asia or the middle east, ask them if they have a good yogurt culture from the old country.

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  • Thank you for your answer, @NikD! What you describe is what I have witnessed past then (meanwhile I have gotten a "heirloom" starter which reproduces generation after generation with no issue at all). The question was why that is (not so much how to circumvent it, although I'm grateful for the hints and they are surely helpful for others as well)... can you expand on the reasons behind it? – AnoE Mar 8 at 11:30
  • Sorry, don't really know the answer to why. But based on my limited knowledge I would guess that there are other microbes in the yogurt which are able to outcompete the desired one, over time. Is it something the yogurt companies are intentionally adding? Or is there some special process required to ensure that the desired culture dominates? I don't know the answers to those either. – NikD Mar 9 at 13:00

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