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We know that in yogurt fermentation, not all of the lactose is consumed by the lactic acid bacteria (primarily Streptococcus thermophilus and Lactobacillus bulgaricus) fermenting the milk, though the remainder of the lactose post-fermentation continues to decrease with storage time.

However, recently I came across this paper that showed an interesting way of hydrolyzing lactose in milk - which was by using a "cell catalyst"-ie, Streptococcus thermophilus (one of the yogurt fermenting bacteria above) treated with nisin A, a peptide produced by Lactococcus lactis, which evidently causes the cell membrane of the Streptococcus thermophilus bacteria to become permeable, thus allowing lactose molecules to enter the cell and get exposed to the lactase enzyme present inside the cell, thus hydrolyzing the lactose.

Now, nisin A is produced by Lactococcus lactis pretty much as it is allowed to incubate in nonfat milk (table 3, entries 1 and 2) which should not be that different from incubation in whole milk.

Additionally, according to this paper, "Two seasonal samples (winter and summer) of raw milk were obtained from six farms in two areas (Bessin and Bocage Falaisien) of Normandy. All of the strains analyzed had a Lactococcus lactis subsp. lactis phenotype" - in other words, Lactococcus lactis is found in raw milk.

Moreover, Streptococcus thermophilus "is ubiquitous in raw milk".

All this should support the hypothesis that since both Lactococcus lactis and Streptococcus thermophilus are present in raw milk, if the raw milk is left to ferment (clabber) at room temperature, as both of these bacterial species grow, the former should produce nisin A to permeabilize the cell membranes of the latter, which should produce cell catalysts with lactase activity thus hydrolyzing the lactose in the milk.

However, from personal experience, being a lactase non-persistent person myself, I have found I get the typical symptoms of lactose malabsorption (bloating, gas etc) after consuming clabbered raw milk left to ferment for 4-5 days. This does not happen if I add a few drops of commercial lactase enzyme drops to the raw milk before the fermentation period begins - indicating that my lactose malabsorption-like symptoms are indeed due to lactose present in the clabbered raw milk.

Thus, the lactose is not likely getting hydrolyzed as the hypothesis above would suggest - which begs the question - where does the hypothesis go wrong?

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The issue is that the microbes have multiple types of enzymes. Some break down lactose to glucose and galactose, but others break glucose and galactose down to lactic acid, which lowers the medium pH. Enzymes work only within a specific pH range, sometimes very narrow. Once the microbes have produced enough lactic acid, the medium gets too acidic and the lactase enzyme stops working. If the microbes haven't processed all the lactose by this point yet, it will remain in the final product.

That's a bit simplified; the activity of lactase doesn't immediately go down to zero in low pH, but is slows down significantly. That's why the remainder of the lactose post-fermentation continues to decrease with storage time.

However, if you are adding just the commercial lactase enzyme to milk, you are adding exclusively that single enzyme, thus cleanly converting all the lactose to simple sugars without creating any acids. This way, the enzyme is able to break down all the lactose.

The pH of fresh milk is around 6.4 to 6.8, pH of sour milk is 3.8 to 5.0, and pH of yoghurt is 4.4 to 4.8. The optimal pH for the lactase enzyme is around 6.6. Important to note that the pH scale is logarithmic, so even small number differences mean a lot: going from 6.8 pH down to 4.8 pH means that the medium gets 106.8-4.8 = 102 = 100 times more acidic.

Disregard the other answer because it is incorrect (and mildly condescending as well, which is quite disappointing).

Similarly, just because you have bacteria which turn on a process that consumes lactose, this doesn't mean that you'll suddenly have all the lactose disappear from your system. The bacterial colony will establish itself, and convert some of the lactose, but there is no reason to assume that it will use up the full amount of lactose present.

There absolutely IS reason to assume that microbes could deplete the full amount of the bulk nutrient present, hence the question. That's basic microbiology. No need to be condescending there with the infantilizing argument that "Cats eat mice, but this doesn't mean that the world has no mice left in it.". This type of argumentation is void, because cats would absolutely exterminate all the mice if they were both contained within a small and closed system (like microbes within the milk medium are), and if the mice were the limiting source of nutrition for the cats.

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  • This is a good answer, but the last couple of statements are incorrect - there's plenty of evidence that yogurts contain lactose and the bacteria don't metabolize it all, despite the closed nature of the system - so while they could get close to 0, they don't. The question even has a link in the first paragraph that measures these levels - around 2.3 g lactose per 100 g yogurt (2-4 g/100 g seems to be the range in the literature)..
    – bob1
    Jul 20, 2023 at 9:08
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There's a very simple place where it goes wrong: thinking that the presence of these bacteria must remove all the lactose completely.

Ecosystems rarely work that way, and you see it all the time. Cats eat mice, but this doesn't mean that the world has no mice left in it. Beavers fall trees, but this doesn't mean that there aren't any trees left, even in forests full of beavers. It's rare that a species will consume until it has used up all of an available resource, and in cases where this happens, there will be one resource that gets consumed fully, and once this happens, a level is reached where other resources don't get consumed, because the species is limited by the scarcest resources.

Similarly, just because you have bacteria which turn on a process that consumes lactose, this doesn't mean that you'll suddenly have all the lactose disappear from your system. The bacterial colony will establish itself, and convert some of the lactose, but there is no reason to assume that it will use up the full amount of lactose present.

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  • The problem here is that the bacteria (permeabilized S. thermophilus) isn't exactly "consuming" the lactose-rather, it just houses lactase enzyme inside its cell, which hydrolyzes any lactose that enters the permeable cell membrane & comes in contact with the lactase. The resulting by-products of the hydrolysis-glucose and galactose are released into the medium and are not consumed by the bacteria at all. This is a cell catalyst, and functions just like lactase enzyme drops. Hence it does not make sense to posit that equilibrium is established even when lactose hasn't been completely depleted. Jul 6, 2023 at 12:45
  • Typically when lactase enzyme is added to milk, the equilibrium is established only when an extremely small amount of lactose is remaining, which is insignificant. The cell catalyst that I have referred to in my question functions just like lactase enzyme and hence should not leave behind any significant amount of lactose. Jul 6, 2023 at 12:47

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