I want to make jelly from a mixture of 50% fresh apple juice and 50% juice of another fruit. The resulting pH must be less than 4.6, otherwise the jelly will not be safe from C. botulinum.
Based on my research, apple juice generally has a pH between 3.3 and 4.0 (Let's say 4.0 to be conservative), so it's safe for home canning. The other juice is less acidic (a pH of ~ 6.0).
I plan on mixing equal parts (by volume) of the two solutions (pH 4.0 and 6.0); I need to know if the resulting pH will be less than 4.6.
I found a claim that if you mix equal parts of 2 acids, the resulting pH will always be the lower pH plus 0.3. So if that's true, the answer to my question is 4.3 (safe from botulism, but I need to know if this claim (and my subsequent result of pH 4.3) is correct!