I stopped growing modern paste tomatoes such as San Marzano a few years ago, when I began finding furry mold growing in fruits that appeared perfect on the exterior. So, while talking about preserving tomatoes to the Multnomah County Master Gardeners the other night, I asked if others had had the same experience. Indeed they had: At least three people in the audience had found mold inside their paste tomatoes.
I would guess that extended heating in a boiling-water bath would kill the mold, but I’m not sure. What I’m sure about is this: I would never recommend preserving tomatoes of this type without cutting them open first.