As summer winds down, many gardeners are saving the bounty of their vegetable gardens by canning tomatoes. And many of them are using the water bath canning method to process their jars. Water bath canning is only safe for ″high acid″ foods, which have sufficient acidity to prevent the growth of bacteria that produce deadly toxins. Folks used to assume that tomatoes were naturally high enough in acid to make water bath processing a safe method for canning tomatoes. But it turns out this isn't so.
According to the University of Minnesota August 2012 Home Food Preservation Newsletter, recent research has shown that a number of factors, from the variety of the tomato to the soil and weather conditions it was grown in to the stage of ripeness when harvested can all affect the acidity level of tomatoes. ″Tomatoes are not consistently 'high' in acid and today's canning recommendations require that acid be added to ALL canned tomato products even if they are pressure canned.″ the newsletter notes. It also points out that this recommendation applies equally to heirloom tomato varieties, as some of these old-time varieties are lower in acid than modern hybrids.
For safe canning, the newsletter recommends adding citric acid to tomatoes at one-half tsp per quart or one-quarter tsp per pint, or bottled lemon juice at 2 TBSP per quart or 1 TBSP per pint. Don't use fresh-squeezed lemon juice as its acid levels can vary. Vinegar (5% acidity) can be used at 4 TBSP per quart or 2 TBSP per pint, but will cause a noticeable change in flavor.
To read current and past Home Food Preservation Newsletter issues and to subscribe to this informative, free, online newsletter, go to: UMN Extension Food Safety.