Why Do Recipes Tell Me Not To Double Batch?
The reason is because of the cook time. For a single batch recipe, you generally call for a 1 minute boil. If you double that recipe, and still only cook it one minute, you will not have sufficient water evaporation. Condense the recipe to the correct brix to make the pectin work or react. But, you don’t mathematically increase the cook time the same as you would your recipe. The general rule of thumb is, if you’re going to use a double batch recipe and the recipe requires a one minute cook time, I would increase it by 50%. So, instead of cooking one minute, cook for 1 minute 30; that gets us in the ballpark. Other than that, the only other way to really determine what the proper cook time would be is to use the refractometer which we will cover later on in a future question.