If you use a mesophilic (rather than thermophilic) strain you don't have to keep it warm: it ferments at room temperature. The most popular strains in the US ar

If you use a mesophilic (rather than thermophilic) strain you don't have to keep it warm: it ferments at room temperature. The most popular strains in the US are all thermophilic, but Finnish viili and a bunch of other (primarily Scandinavian) strains are mesophilic and you can just plop it on your counter overnight. "Bad" bacteria won't have a chance as long as you started with clean materials.