As most avid canners know, 4.6 is the pH dividing line between acid foods that can be safely water bath canned and less acidic foods that need to be canned in a pressure canner. Most fruits have a lower, i.e. more acidic pH and can be water bath canned.
Tomatoes, on the other hand, are often near the 4.6 pH level and USDA tested recipes will call for adding either bottled lemon juice or citric acid (I prefer citric acid as the taste is more neutral).
I used to think that this issue was because different tomato varieties vary in their acid content. It turns out that it’s more about when tomatoes are harvested, not to mention what the weather was like during the growing season. Add this variability to other factors, such as how many cans you put in your canner, the material your pot is made out of and the type of heat source and you end up with a tricky question for the food scientists who test home canning recipes. All of these factors are why the recommended hot water bath canning time for raw packed tomatoes is 85 minutes.
I’ve hot water bath canned tomatoes and got great results (especially with San Marzano tomatoes). But 85 minutes is a long time. You can cut the processing down considerably and get better results by pressure canning tomatoes. Here’s a raw pack recipe that includes both hot water and pressure canning instructions. Note that you still need to acidify.
Thanks to Linda Harris, UC Cooperative Extension Specialist, Food Safety and Microbiology who gave a lecture at the Master Food Preserver conference where I gleaned these factoids.