If you’ve been canning for any length of time, one of the warnings you’ve probably heard is that you shouldn’t double jam recipes. However, no one ever really explains why it’s not a good idea. Let’s change that, shall we?
First off, it’s not recommended practice because if you double the amount of jam in the pot, it just won’t cook as well or effectively. Most jam recipes already call for you to use the widest pot you have, for maximum surface area. This large surface area leads to faster evaporation of water. Fast cooking leads to the freshest tasting, best textured jam.
However, if you double the amount of jam in your pot, you greatly increase the cooking time, because there’s so much more product in the pot that needs to be cooked down. This can lead to rubbery batches, burning and jam that doesn’t set. It can also as much as double the amount of time you spend cooking the jam.
It’s far better to make two smaller batches than it would be to try and double a recipe (unless you have an industrial stove and an absolutely massive pot).
Big thanks to Susie for her email, which made me realize I’d never addressed this issue.