Question

Why can foods such as pineapples, pickles and tomatoes be treated with temperatures less than 100?

Answer 1

The reason why foods such as pineapples, pickles and tomatoes can be treated with temperatures less than 100 is because these foods are highly acidic (pH<4.6) which provides a hurdle to microorganims.
Low-acid foods have pH values higher than 4.6, while acid foods have a pH of 4.6 or lower. They include fruits, pickles, sauerkraut, jams, jellies, marmalades, and fruit butters.