Keeping a starter going can produce large quantities of waste, and given the current difficulties in obtaining flour, it seems wrong to simply throw away your discard. I prefer to use mine in recipes, as it adds a little extra flavour to simple bakes such as pancakes, waffles, crumpets, and so on. See my discard recipes here.
However, there is a simple rule you can follow in order to use your discard in other baking recipes. Provided your starter is equal parts flour and water (this is known as 100% hydration), your discard is also half flour, half water, so 100g discard = 50g flour + 50g water. So, you simply need to deduct these quantities from the original recipe.
I have a cake recipe that uses 200g flour and 100g water. I’d like to add 100g discard, meaning I’d be adding an extra 50g flour and 50g water. To keep the consistency of the cake the same, I simply take these quantities off the original recipe. So, instead the recipe comprises 150g flour, 50g water, and 100g discard.
For simplicity, I use water in the calculation above. However, most liquids in baking recipes will be those such as milk or yogurt, and sourdough discard is a great alternative. Some recipes won’t be quite so clear cut in terms of the quantity of liquid, relying of ingredients such as butter for the liquid instead. For a useful guide in adapting more complex recipes, The Boy Who Bakes has a good explanation here.
Effectively, your discard is half flour, half water. Therefore, whatever quantity of discard you use in your recipe, simply halve this and remove the equivalent quantity of flour and liquid from the recipe.
Tips on storing your discard:
Your discard will keep quite happily in a container for around two weeks with nothing adding to it. Just keep adding in your discard each time and save it for when you will use it in a recipe. You’ll notice after a few days it will begin to go flat – this is absolutely fine, as you are no longer using it as a leavening agent, rather a flavour boost in your other bakes.