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There are recommendations online that if you use supermarket yogurt as a starter for home made yogurt then you should not use the resulting yogurt as starter for another batch, or at least not more than a few times. The instructions for my yogurt makers say "If you make yoghurt regularly, save 125ml of unflavoured yoghurt from your last batch to start your next. If you do this, ensure you don’t save it too long or the probiotics can weaken and the yoghurt won’t set. We only recommend doing this for 2-3 batches in a row. Then use a fresh starter.". If you buy a heirloom yogurt starter culture you can reuse it. An example where heirloom yogurt starter is mentioned and perhaps sold is here. These are frequently advertised as not being a monoculture.

Is this true? If so, what is going on genetically? I am aware of the threats that pathogens pose to monocultures, such as that faced by the Cavendish banana, but it is hard to see how this could happen in a yogurt culture.

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  • Not an answer, just a pointer: I used to try to get yogurt working but have switched to milk kefir. The taste is somewhat similar, and from the get-go it has been trivial for me to keep the culture alive. Might be worth a try if yogurt proves cumbersome for you.
    – AnoE
    Jan 31 at 15:02

5 Answers 5

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I had a look at the manual, and there is quite a lot of more or less wrong, or at least misleading information in there, especially around the biology. I am a virologist by training, but have had a fairly varied career, including being involved in teaching of food microbiology - such as yogurt making, so have some experience in this area. I've also been making my own yogurt at home for more than 10 years using the EasiYo Maker system (there's one on the link I have put in the next paragraph).

Also note, they (i.e. the supplier of your yogurt maker) are a company, which sells, among other things, yogurt sachets for making your own yogurt; they are trying to sell you these products!

Many companies make yogurts and similar products with particular strains of the bacterial species, however, in general, unless they have done some extensive optimization and/or selection, they will be using a regular species and probably from a common strain. When they do have something special, they typically ensure that you know it in terms of advertising and price. Have a look at "Yakult", which widely vaunts its "Shirota" strain

The probiotics in yogurt are generally bacteria of the genera Lactobacillus (particularly L. acidophilus and L. casei (now called Lacticaseibacillus casei), and L. delbrueckii* ssp. *bulgaricus) and Bifidobacterium (often B. bifidum). There is also Streptococcus thermophilus, which is apparently synergistic with growth of L. delbrueckii - each providing metabolites the other uses. These are all anaerobic bacteria, meaning they don't grow or don't grow well in the presence of oxygen, that make up a major part of the bacteria in our intestines. The genera are diverse having lots of different species and have genomes that are quite malleable - in that they can acquire or lose genetic components depending on the environment and as such can be difficult to maintain in anything other than the proper conditions. Commercial vendors get around the problem of change of characteristics by having validated stocks that are assessed for the characteristics that are wanted, grown in bulk, frozen down (often at -80 C/-112 F) in small aliquots and then used for inoculating the product. Loss of genetic characteristics is entirely possible, but unlikely - in my experience you are more likely to get additional characteristics. We tend to get a mucilage producing variant over time in plain yogurt. No change in flavour or anything else, just a genetic characteristic that turns up, especially so if we use vanilla for flavouring.

Now - on to your manual. I can see some problems with how it works that makes me think that the largest problems you will have are contamination issues - yeast and normal gut microflora from you or a pet will grow very happily in warm milk. The aim of adding 125 ml of inoculum (starter) to your milk is to overcome this problem to some extent by having a lot of the wanted bacteria from the yogurt in there, using up available resources and turning the milk sour (relatively) rapidly. Contaminants, which usually are added in small amounts accidentally, will get out competed by the good ones and the acidification will create a non-optimal growth environment for the contaminants. The boiling the milk beforehand is also to lower contamination chances.

You can, of course, get away with much less inoculum - we use about 15 ml (1 tablespoon) in our litre (~2 pints) yogurt. However, to do this, you should take this from the yogurt immediately after it has been cultured into a very clean sealable container, and put it into the freezer (-20 C/-4 F), where it will keep reasonably well for a couple of months until all the bacteria die off because of ice crystal formation. The risk you run with the smaller inoculum is that it takes a bit longer to get going. A typical biological system, such as in culturing yogurt, has 3 phases of growth - lag, logistic and plateau (S-curve). The lag is at the start when there is slow growth until the inoculated bacteria create a favourable environment for growth (in this case acidic) and reach a point where they start to divide/replicate logistically, replicating about once every 20-30 min. Once the population reaches a point where most of the resources (in this case nutrients from milk) in the environment are used up, the replication slows and reaches plateau. The smaller the inoculum, the longer the lag phase will be - which gives more time for contaminants to grow.

For the inoculum do NOT use yogurt that has been sitting in the fridge until almost used up and you need to make some more. The reasoning here is that this is more likely to have contaminants in it; the more times the container has been opened and things put in it (e.g. spoons), the more likely contaminants have been introduced. I suspect this is the real reason they say not to use it more than 3-4 times.

If you get a packet mix of yogurt - the first thing you should notice is that it is dry - all it is, is a mix of bacteria and milk-powder and maybe flavourings and colourants. You can replicate this at home with ordinary milk powder and some inoculum. Just follow the instructions on the milk powder packet to get your required milk volume, and add your frozen inoculum and mix. You can also portion the packets out and make up the bulk with milk or milk-powder. Ours is usually done within 6 h of incubation. Longer gives more acidity (e.g. Greek-style) and firmer yogurts, while shorter is sweeter and more runny.

If you notice a non-yogurt smell in your yogurt, or notice changes in texture (no change in recipe) or flavour, then you should throw out your inoculum and get some fresh from a packet or store-bought live culture yogurt.

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  • This is a very thorough answer, but I miss how many "generations" of yogurt you typically gain in your experience until it starts to degrade?
    – Dubu
    Jan 31 at 9:33
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    @Dubu no definitive answer on that - many. I'd guess that we make a litre or two a week and get new starter roughly 1-2 times a year. You could get around this need by making lots of smaller aliquots immediately from a fresh (from starter) batch, freeze those down individually, use 1 until something happens (make batch 1a, 1b...), then get out the 2nd, and so on. This is more or less what happens in commercial supply.
    – bob1
    Jan 31 at 9:57
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    I can't imagine any reason why there would be a limit in terms of generations. Have you seen that there is and, if so, any idea why? I just can't think of any genetic mechanism that would cause this although, granted, I am more familiar with eukaryotes.
    – terdon
    Jan 31 at 20:40
  • @terdon In theory there isn't any limit to the number of generations, it's purely that there is some evolution that goes on in sequential culture - it happens in cell lines too. This might be as simple as faster growth or it might be some characteristic that you don't like - perhaps a byproduct that smells funny. Once you get an odd characteristic, it's easier to get rid of that batch and start again than it is to try and remove the characteristic. Of course contamination would be a re-start too.
    – bob1
    Jan 31 at 20:52
  • Ah yes, fair enough, that makes sense, thanks.
    – terdon
    Feb 1 at 11:12
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I will suspect that it's having low expectations for the average user's microbiological care more than any inherent issue. I doubt there's any genetic time-bomb in the culture.

For instance, if you are using starter from a container of yogurt you've been eating from, it's more likely to get contaminated with something.

Anyway, I have had no issue with many (honestly no idea how many...) generations starting from normal store-bought yogurt. The store-bought would be freshly opened when used as the starter. I keep the starter in its own little glass jar, which gets steam processed (not technically sterilized, but pretty well sanitized) before being filled with the freshly mixed starter (from the other starter jar, they rotate) and cooled scalded milk, then incubated and then refrigerated; and it is only opened once when it's about to be used. I feel that this minimizes potential contamination.

Probably unrelated, but I have also adopted a 24 hour incubation at lower temperature (95°F/35°C) than the typical "try to get it done as fast as possible" 105°F/40.5°C or more for 8 hours - I'm not industry, I don't have a production quota that can be tripled by going faster. The results appear somewhat better to me.

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Most commercial yogurt cultures are not mono-cultures... They are made up of a small handful of isolated cultures. I think it is 2. Strepptocus and Lactobacillus... THoough they might use more.

Each of those isolated cultures helps make the yogurt the right consistency/flavor... But they like growing under different conditions. So one of those cultures will slowly take over your starter, crowding out the others, this ruins the flavor and texture. It is slow, because the difference in growth rates is small which is why you can use commercial yogurt as a starter... But the imbalance gets large enough to significantly effect quality after 4-5 generations. If you pay attention you will notice how every single yogurt you make has a slightly different taste/texture then the original starter.

Heirloom yogurt cultures don't suffer from this problem because they are a mixture of a lot of other bacteria, not just the ones we have identified as producing the flavor/texture desired... These other bacteria act as stabilizers and keep all of the cultures balanced. (Heirloom yogurt starters that weren't self balancing mixes were thrown out) This is why heirloom yogurt starters can be used forever.

Is it possible to use a commercial yogurt starter forever? Sure if you isolate all the strains, growing them independently and then mix in the correct portions to your yogurt growth medium (its what big yogurt does)... Which is out of the wheelhouse of most people... And too much work for anyone who isn't making yogurt at a commercial scale

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  • 1
    Is this also the case with brewing/fermentation? I've heard of home brewers using commercial live beer to start their batches but that you get inconsistent results if you try to use your own product as the source for later generations.
    – JimmyJames
    Jan 31 at 22:14
  • I have never done brewing... But I believe that is the case..
    – Questor
    Jan 31 at 23:13
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    @JimmyJames it’s quite rare to ferment beer with anything other than a monoculture. Drift in the results there is more likely a matter of contamination with other strains of yeast (or bacteria).
    – Sneftel
    Mar 30 at 10:58
  • @Sneftel I just learned something new. Thank you
    – Questor
    Apr 9 at 16:39
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I can't respond to the genetics. I regularly start my yogurt with store bought. Certainly not "heirloom" (I don't think...not sure what that is for yogurt). I use the yogurt I make as a new starter at least 3 - 5 times before I forget, use it all, and have to buy a new container of starter yogurt. I've never notices any degradation in flavor or texture over that period. I assume the bacteria change over time to respond to local conditions, but I don't see how they could become unsafe in the acidic environment of a properly cared for yogurt system.

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The genetic limitations on the longevity of a yogurt starter are primarily linked to the accumulation of mutations over successive generations. As the bacteria replicate, errors in DNA replication can occur, leading to genetic variations that may impact the starter's vitality. Over time, these mutations can accumulate, affecting the starter's ability to function optimally and potentially restricting its lifespan. Regularly introducing fresh cultures helps maintain the overall stability and efficacy of the yogurt starter.

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  • This cannot be the whole story. If an individual acquires a mutation that degrades its ability to metabolise lactose one would expect that line to be out competed by unaffected lines.
    – User65535
    Jan 30 at 11:59
  • nope. Not caused by mutations.. Commercial yogurt uses 2 different strains of bacterium One is a thermophile, one is a mesophile. They like different temperatures. After a couple of generations the imbalance between the 2 cultures significantly impacts the flavor/consistency of the yogurt...
    – Questor
    Jan 30 at 20:21

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