German Brewing

This is going to be a really stupid question. Is it possible to brew a decent Gose with lactic acid and Sauermalz only? Asking for a friend

Probably not. Milk the funk is a good resource in general.
Why do you want to avoid lactobacillus?

Is it possible? Yes, but sauergut would be much preferred for its overall digestibility and flavor profile.

I recommend skipping the mineral lactic acid and simply adjusting the percentage of sauermalt if you are not going to invest the time in making sauergut. Weyermann has some information on making berliner weisse using only sauermalt, which you can use as a guideline for making gose as well.

https://www.weyermann.de/usa/faq.asp?um ... sprache=10

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It's a competition in the Netherlands organized by a macro brewer. They don't want any vicious stuff in their brewery.
Not that I'm going to participate. It's literally for a friend

The Weyermann link is interesting. But I guess a Berliner Weisse is more sour than a Gose, right? Haven't had a real German one yet...

Apart from that I've become interested in brewing a gose, but then with a proper method

A kettle sour is a bit better, the bacteria get killed in the boil.

Why not try? If you really want to get to the bottom of it then I'm sure you will--given enough time.

Is there a lower limit to the acidity of the mash in terms of fermentability? I.e. how far below 5pH can I (he, this is literally for a friend) go? And is there a rule to calculate the final pH of the beer based on the pH of the wort?

Try.

Fear is never a good driver in all scientific and engineering activities.

Beer generally ferments down to the 4.x pH scale. I sometimes, erroneously, went below 5.1 in my boil ph adjustment back in the early days. I cannot say I didn't regret it and it's not investigated enough imo.

Try and share to get feedback quickly. Its a proven engineering method of the 21st century in many disciplines

Courage.