TL;DR: Does E418 (gellan gum) inhibit lactic acid fermentation of organic soy milk?

/cc [ @fermentation | @vegan ]

For a couple of a years I have been making my own vegan yoghurt by emptying the contents of two probiotic capsules into a litre of organic soy milk, mixing thouroughly, and then letting the milk sit in a yoghurt maker (or a pressure cooker set on the yoghurt program) for 14 hours. The effective working time is less than 5 minutes, and the result is a wonderfully tangy and creamy plantbased yoghurt. It's one of the easiest things you can make in your kitchen — or so I thought until yesterday:

For my own convenience I used a different brand of soy milk, because I thought that organic soy milk was organic soy milk. However, when the incubation periode was over yesterday, the soy milk was still runny — it didn't seem to have coagulated at all. No problem, I gave it 6 hours more (for a total of 20 hours): still runny, and not the slightest sour. WTF?

To exclude the possibility that I — in a fit of distraction — had taken some other capsules from the fridge than the probiotics, I repeated the procedure with a fresh carton of the same brand of soy milk, making sure that I used the probiotics this time. Same result: after 14 hours of “fermentation” the soy milk was still runny and not tangy at all (the taste was just like lukewarm soy milk and it didn't have even the slightest resemblance to yoghurt).

Both brands (we can call them Coop365 and Spir) of soy milk are of the sweetened type with added vanilla flavour. Coop365 (the one I usually use) is made from 8.5% soy beans and has 3.7 g protein per 100 g. Spir is made from 8% soy beans and has 3.2 g protein per 100 g. Both are organic, and both are sweetened with organic cane sugar. Coop365 has a slightly lower salt content than Spir — 1.2‰ vs 1.8‰ — but in such low concentrations it should not have any effects on lactic acid fermentation.

The only real difference between the two soy milks according to the fact box on the cartons is that while Spir contains E418, also known as gellan gum, Coop365 doesn't. Gellan gum is said to be “inert”, and according to the woodchuck book it is often used in “plant-based milks to keep plant protein suspended in the milk”:

🔗 https://en.wikipedia.org/wiki/Gellan_gum#Food_science

Both soy milks coagulate readily when adding a small amount of e.g. lemon juice. According to a page I read yesterday, the curdle point of soy milk is around pH 5.5.

The probiotic capsules that I use contains three different species of lactics: Lactobacillus acidophilus, Bifidobacterium lactis (syn.: B. animalis), and Lactobacillus rhamnosus (syn.: Lacticaseibacillus rhamnosus). All three of them are found/used in dairy products such as fermented milk.

Now my question is simply: Do any of you fedizens have _experiential_, or otherwise solid, knowledge that E418 is able to inhibit lactic acid fermentation of soy milk as described above? Or, when reading my description, does your _experience_ tell you that “Hey, the Spir didn't ferment because of …!”?

Thanks in advance! 🙏

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