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Kefir is a fermented milk drink made with kefir grains. It usually is prepared by inoculating cow, goat or sheep milk with kefir grains.

I would like to prepare the drink with soy milk, which worked fine for some months. Then the kefir grain stagnated in its growth. I use bottled soy milk from the supermarket. I do not produce the soy milk by myself.

In "real" milk the kefir grains never stop growing (and dividing), but in soy milk they apparently do.

Why do the kefir grains stop growing in soy milk? Are they lacking a specific nutrient that is only contained in milk from animals?

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Animal Milk. The ideal environment for Kefir Grains is animal milk. Varieties include cow, goat, sheep, and other similar species. Kefir Grains thrive in animal milk due to the chemical makeup of the liquid. The lactose in animal milk provides the most efficient food source for the Kefir Grains and therefore the most efficient and effective culturing process.

Alternative Milks. While animal-based milks are the healthiest for the Kefir Grains, alternative types of “milk” (coconut, soy, rice, nut, etc.) can also be used under certain conditions. Generally speaking, Coconut Milk, Soy Milk, and Rice Milk can be cultured using Kefir Grains as long as a revitalization period is observed (see below). While some people report success culturing Kefir Grains in seed and nut milks (e.g. hemp milk, almond milk, etc.), these varieties tend to yield more inconsistent results. Revitalization Period. When using a non-animal variety of milk, it is important to occasionally allow the Kefir Grains to revitalize in animal milk for 24 hours.

We recommend allowing the Kefir Grains a revitalization period at least once every few weeks and ideally more often. To revitalize the Kefir Grains, simply place the Kefir Grains in 1-2 cups of animal milk for 24 hours. Once the process is complete, the Kefir Grains can be returned to use with an alternative variety of milk.

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While this answer does not explain in detail, which part of the chemical makeup of animal milk is missing in alternative milks, I tend to accept this answer because it simply confirms, that an entirely vegan Kefir is not possible (in the long run). But I am still curious why Kefir is more 'picky' than yeast – which grows well on different kinds of substrates. –  juan topo Jul 21 at 10:02
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Yes, they are behaving differently because of the lack of nutrients, including lactose, and possibly because of the presence of other chemicals in soy milk that aren't in dairy.

Kefir grains are a complex community of different types of lactic acid bacteria and yeasts which metabolize (eat) various nutrients/chemicals in their environment. Because soy milk is a chemically different environment than dairy milk, these communities function in different ways. Over repeated propagations in soy milk, you probably have cultivated a very different community in regards to species diversity and the chemical products.

As Liu and Lin describe here , the kefir community members in dairy milk are eating mainly lactose, whereas in soymilk they are eating sucrose, raffinose and stachyose. As a result, the reproduction of the bacteria and the yeasts differs, as did the alcohol and lactic-acid contents of the end-product.

Because your question is a practical one, you might be interested to know that in that paper, the authors found that adding 1% sucrose (table sugar) or 1% glucose to the soymilk produced an end-product that more closely mimicked that of dairy milk. You could give that a try and see if it improves your kefir growth.

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Thank you for your response and the link! I already tried sucrose (assuming that the kefir grains need a substitute for the energy providing lactose) but in the long term it does not seem to work: the growth stops. That is why I rather suppose the kefir grains are lacking some kind of trace element of the dairy milk than the need of a different type of carbohydrate. But I will give the glucose a try … –  juan topo Jul 14 '13 at 9:34
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As you say, the added nutrients may just be a short term solution: when Liu and Lin added sucrose or glucose (or even lactose!) to the soy milk, they got alcohol and acid contents that were much closer to dairy milk kefir, but the community balance was still different: in soy kefir the bacteria:yeast ratio was much higher than in milk. I imagine that this community balance is what's affecting your long-term growth. –  Oreotrephes Jul 14 '13 at 13:40
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