Is it safe to ferment cooked starches?

Hi all,
I'm trying to ferment cooked starches, but not sure how to do it, or if it is safe. For example, could I cook some beans, and then when they cool down, throw them into a mason jar, add salt water and ferment it? Is there a botulism risk? If so, what other simple ways could I use to ferment cooked starches?

Also, what about other cooked starches in mashy-form, like mashed potatoes or pureed beans. How to go about fermenting something like this safely?

I really want to maximise the bacteria, and the metabolites that feed off of the resistant starches.

Thanks in advance,
Alex.

pretty much exactly what you describe is how you make miso. You need to have a decent amount of salt to avoid the bad things growing. But I do know here I've seen people talk about fermenting cooked beans to eat (rather than to make a condiment with), with less salt. If it were me I'd be adding sauerkraut water or some similar "starter" to make sure I'm using the right kind of bacteria.
it's also how you make beer and sake without any salt. Just being clean and introducing the ferment you want to utilize.
I think it might be helpful to know how you plan to use this stuff.

If using cooked starches, you'll need a way to introduce the exact microbes you want to ferment with. You can buy starter cultures, or take some from a batch already fermented, but any that were in the mix before it was cooked will be killed off by the cooking process.

When making wine, I prefer to cook the fruit first. It gives me better control over the result. And, I tend to use unusual fruits, like squash, which work better if softened by cooking.

Starches are usually fermented with yeast (beer) or mould (miso, tofu, etc).

Once the item is cooked, the wild invisible beasties are killed off, so you get to choose what kind of bacteria, yeast, or moulds you want to use.

here's a good introduction to miso https://permies.com/t/52034/kitchen/miso-home

Thanks everyone for your kind advice.

The easiest, and closest thing to a starter that my local supermarket has is buttermilk, which I have previously tested for live cultures by fermenting milk and worked great. Do you think buttermilk could be used as a started for fermenting cooked/cooled starches?

Also, would I still need to add salt?

I would not use buttermilk in a non-dairy ferment. The bacteria that thrive in a cultured dairy are not the same that would like a non-dairy environment, so you would risk pathogenic inoculation while waiting for the dairy-happy bacteria to get geared up. Temperature is also a factor in dairy cultures and although buttermilk is mesophilic (thrives in the mid 70's F), I think it would struggle unless it has lactose to eat. You could start a sauerkraut with a brine and when it gets active, scoop off some of the brine to inoculate your starchy stuff. But a starchy ferment is often a yeast based deal, like bread, beer or miso (koji isn't a a yeast but it's still not the same as kraut, kimchi etc). You need to be scrupulously clean and also probably airlock the ferment to keep it somewhat controlled in my experience, unless you're doing a short one. Whatever is a happy home for your preferred bacteria is also desired by "bad" bacteria, so getting the ones you want to colonize and out-compete the non desirables is achieved by introducing them to the wort or starch or whatever and then blocking the other wild stuff out until the ferment gets active.