Lets say that you have a canning recipe that says to process with the water bath method. Being the creative cook that you are, you want to make some modifications to the recipe. After all, having your own flair is what makes cooking fun. The only question is, " is it still ok to use the water bath method?"
The fact that the recipe says to use the water bath method means that the PH of the recipe is lower than 4.6. Anything with a PH higher than 4.6 must be canned with a pressure canner in order to ensure that any bacteria in the food is destroyed.
Now lets say that you want to add blueberries to your recipe. Fortunately the US Food and Drug Administration has a website that will tell the Approximate PH of Foods. If you look up blueberries, you can see that their PH is below 3.33. Since this is lower than 4.6, you are still safe to use the water bath method.
Assuming that you don't have a PH meter among your kitchen utensils, it is best to take a, "better safe than sorry," approach to recipes of unknown PH. In other words, if you add even one ingredient with a PH above 4.6, then assume that the whole recipe has a PH above 4.6.
Removing an ingredient can also be a food safety issue. For example if you look up the PH of tomatoes, you will see that it is in the range of 4.3 to 4.9. Tomatoes really straddle the line of what is safe to can with a water bath method. For this reason, many recipes for canning tomatoes call for lemon juice in order to add enough acidity to ensure safety. It wouldn't be wise to take the lemon juice out of the recipe. If a recipe calls for lemon juice, vinegar, or ascorbic acid (vitamin C), then do not remove or decrease the quantity of this ingredient.
A key to food safety is just to remember that if the PH is above 4.6 then you must use a pressure canner - not a water bath canner.