[throwawaaarrgh]

It does look like there's no preservatives added to heinz. It appears the lack of mold growth in ketchup comes down to a combination of lower pH from the vinegar, and low water content by cooking it out of the tomatoes. According to some food safety experts, it's possible for mold to grow on ketchup, less so for pathogens. Keeping it cold will definitely extend the time it would take mold to root.

[ricardobeat]

The story is that Heinz specifically developed their recipe to avoid the use of benzoate over 100 years ago. Ketchup at the time was a completely different condiment: much thinner and not nearly as sweet, salty and vinegary, and also absolutely loaded with preservatives.