Open kettle was filling the jars with boiling tomatoes and sealing the lids . Mom always said they spoiled when she did them while pregnant . Body chemistry she claimed . I think she worked slower while pregnant = cooler contents = spoilage .OK, same thing. Yes I've heard lemon juice too.
A few years ago I bought a steamer canner, uses less water. I think that would be similar to the open kettle canning method? except it's covered so that may be the only difference. IDK, if the ingreds. in the jar are boiling and the lids seal I'd think that is what matters.