The ideal vessel is one that is insulated to retain heat and is either a boil kettle or a pre-boil kettle like our mash tun - this is for sanitary reasons.
The steps are as follows:
- Collect your wort from the mash
- Clean your mash tun completely of grain (and sanitize with Starsan)
- Transfer your wort (either boiled for 15 mins or not boiled - your choice) and bring your temp to between 95F-115F
- Pitch your Lactobacillus and wait 12-24 hours until your desired sourness is achieved or you have reached your PH of 3.5
Holding temps are crucial for this process as a low temp under 70F will take months to produce the desired sourness. For sanitation reasons, a mash tun is a great place as grain naturally harbors Lactobacillus so every mash tun in the world has it in a small amount. Technically it could be done in plastic coolers but that produces risks in most people's minds, stainless is just much easier to rid of those bugs.