I have been fermenting milk for the last 10 years and used various commercial lactic acid bacteria cultures and starters.  I am well aware that there are thermophilic and mesophilic cultures and that it's important to maintain a certain temperature during fermentation.

Recently, I decided to ferment a lactose water solution. I tried several times with a couple of different LAB starters and was very surprised to see that my lactose water solution ferments very little and much much less than actual milk.

For example, I used Biena YO-CULT (Yogurt Type I) yogourt starter and other starters that ferment milk very well.

For preparing lactose solution, I used distilled water. I tried to get lactose powder from different sources: stores selling beer and wine making supplies as well as health food store brand "Now Real Food" . I tried to dissolve as little as 50g of lactose powder in 1 liter of water to as much as 200g of lactose powder in 1 liter.

The result was always the same: lactose water solution ferments very little and much much less than actual milk. 

For the sake of completeness, a few years ago, i did ferment milk prepared from dried milk powder and it fermented very well as it should. So preparing the solution from a dried source is not a factor by itself. 

So, commercially available lactose powder is not the same as the lactose in milk?

  • 1
    Does the milk powder contain something other than lactose, perhaps? Sep 8, 2023 at 6:00
  • How do you know that it doesn't ferment? What was the final pH, did you experiment with times different from milk fermentation?
    – rumtscho
    Sep 8, 2023 at 6:29
  • @rumtscho, yes of course experimented with times up to 72hr. Final pH was not nearly as low as with milk which you see even by tasting
    – Alex
    Sep 9, 2023 at 7:04
  • @fyrepenguin, this is exactly what I want to hear from diary scientists - in addition to lactose , what else is exactly needed for lactic bacteria to ferment lactose?. For example, you can ferment a plain sugar solution with just yeast and nothing else. But it's not the case with lactose and lactic bacteria. Or, perhaps, the lactose in commercial lactose powders is not the same as the lactose in milk.
    – Alex
    Sep 9, 2023 at 7:06
  • @Alex your bacteria are probably missing the obvious stuff, like nitrogen and trace elements. But you won't hear it from dietary scientists, rather from microbiologists. And there's the crux: why are you doing this at all? For best taste, you don't want your bacteria to develop in a minimal medium, you want them to develop in an optimal medium, and that's milk.
    – rumtscho
    Sep 9, 2023 at 8:49

1 Answer 1


Bacteria can’t survive and flourish on a lactose solution, any more than you can survive and flourish on pure sugar water. Other nutrients are needed for them to metabolise and grow and reproduce. Your dry milk powder is a good example of the “complete diet” necessary to sustain bacterial growth.

  • So, in addition lactose , what else is needed for lactic bacteria to ferment lactose?
    – Alex
    Sep 9, 2023 at 1:08
  • 1
    Presumably electrolytes, amino acids, possibly some vitamins, and trace minerals. But we’re getting outside the realm of “food” here.
    – Sneftel
    Sep 9, 2023 at 5:35
  • I precisely want to get outside the realm of “food”, but are not allowed to post questions in 'biology' as I have fewer than 300 posts:-(
    – Alex
    Sep 9, 2023 at 7:12
  • You mean, you can’t post questions on the Biology site? That doesn’t make sense.
    – Sneftel
    Sep 9, 2023 at 10:24

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