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In home canning, we are constantly warned that some tomato varieties, particularly yellow types are low acid and so we need to acid 1/4 cup of lemon juice to each batch. While I have no problem adding the lemon juice for added safety, it now appears that these tomatoes are not actually low acid after all.
They tested 58 varieties (cultivars) of tomatoes, including red, yellow, and orange, hybrids and heirlooms, and both older and new varieties. They also examined data from studies from 23 state agricultural experiment stations and multiple USDA laboratories for another 350 cultivars . They were specifically looking at the acid levels in tomatoes for the purposes of home canning. The danger threshold for botulism is 4.6.
Here is a table from directly from the USDA study:
In this USDA study, the researchers bluntly state
"... the risk (of botulism) to consumers in home canned tomatoes is very small."
Since all tomatoes are at the higher (less acidic) end of the pH spectrum for acidic foods for home canning, adding 1/4 of lemon juice to each batch does still make sense. But if you forget to add it for a batch, there is no need for panic, and if you did everything else properly, they will be fine.
The range of pH variation is actually very small and the data says that there really are no "low acid" varieties of tomatoes. The sweeter ones simply taste less acidic.
The ability of C. botulinum to cause food poisoning in humans is directly related to the production of heat-resistant spores that survive preservation methods that kill nonsporulating organisms.(5) The heat resistance of spores varies from type to type and even from strain to strain within each type; although some strains will not survive at 80EC, spores of many strains require temperatures above boiling to ensure destruction.(6,7) The thermal resistance of spores also increases with higher pH and lower salt content of the medium in which the spores are suspended.(8). ... The minimum pH range for growth of proteolytic strains is 4.6-4.8; the limit is pH 5.0 for nonproteolytic strains.
The bacteria that cause botulism poisoning can grow and produce toxin in sealed jars of moist food at room temperature if the pH (measure of acidity) is above 4.6. Vegetables, meat, fish, etc. are naturally fairly high above pH 4.6 (close to 6.0) and so pressure processes were developed for those to kill the heat-resistant spores of C. botulinum bacteria that are likely to be contaminating them.
Tomatoes also can have a natural pH above 4.6 (at least up to 4.8).
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