[coronabarb]

Yes, you add it to the jars when you are preparing them for canning. The NCHFP did lots of testing to see which strength would assure that a wide range of tomatoes would be at the safe pH after spending time on the pantry shelf after canning. I just go by their guidelines instead of worrying about testing pH with a meter...although it would be interesting to see how much it varies before canning, after canning and after a month or so later.