This topic focuses on preserving high-acid foods by canning. You will learn to can high-acid foods which does include most fruits, tomatoes, sauerkraut and pickled products. Because tomatoes have pH values that are close to 4.6 they must be acidified before processing to ensure their safety. Adding commercial vinegar or bottled lemon juice or citric acid ensures they will have sufficient acid to be safely processed in a water bath canner (or a steam canner).  You need to use bottled lemon juice rather than lemon juice from a lemon, because the acidity needs to be of a specific PH (the measure of acidity/alkalinity).