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basically a pressure canning process has superseded the boiling water method for just about everything.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.

[sachieauckland]

Foods that are low in acid like meats and vegetables; must be pressure canned.
The amount of acid in tomatoes is highest in unripe fruit and reaching the lowest point as the fruit reaches maturity.

Researchers now know that tomatoes are not consistently high in acid
and current canning recommendations require that acid be added to all canned tomato products: add 2 tablespoons of bottled lemon juice
or ½ teaspoon of citric acid per quart of tomatoes.

Canning and preserving low-acid foods — such as green beans — requires pressure canning to kill microorganisms that are deadly if not destroyed.