Most of the food is acid anyway, so it's not enough to get and acid mixture, it has to be acid enough to kill the spores. And the acidity varies with time, so I'm not sure how much acidity the initial mixture must have, IAMAFSE.
From Wikipedia: https://en.wikipedia.org/wiki/Clostridium_botulinum
> [...] type-B strains were isolated from slightly acidic soils (average pH 6.25).
> C. botulinum is a lipase-positive microorganism that grows between pH of 4.8 and 7.0 [...]
