Acidic food stuffs like tomatoes have traditionally been thought of as safe as far as the risk of botulism was concerned. However, an outbreak of six cases, one of whom died, in Azerbaijan, which has subsequently been linked to home made pickled tomatoes, has brought that conventional wisdom into question. More of this outbreak can be read on the Azer News article.
Botulism results from the presence of the botulinum toxin, which is a neurotoxin produced by the bacterium Clostridium botulinum. Optimal storage conditions, clean and hygienic preparation and proper pH and salting is known to avoid the development of this bacterial toxin in pickled foods. Using pressure techniques to make pickles out of food matter with low acid content guards against the development of this toxin and using boiling or other conventional techniques maybe dangerous.