Cooking For Engineers

[jradatz]

When canning tomatoes, instructions say they are a low acid food so the addition of lemon juice is needed as an acidifier. When canning green beans, no acidifier is required and the cooking pressures and times are exactly the same. I have never understood that. Any explanations?

[Dilbert]

recommended procedures for canning have changed over the years

here's the latest:
http://nchfp.uga.edu/publications/publications_usda.html

basically a pressure canning process has superseded the boiling water method for just about everything. pressure canning achieves temperatures high enough to kill any nasties - in particular - botulism spores.