A spoonful of dark fermented beans can give a stir-fry a deep, savoury character. A large handful can also make the dish much saltier than intended. Treat these beans as a seasoning ingredient before treating them as a vegetable portion.
The Woks of Life’s ingredient guide describes douchi as individual salted fermented beans and notes their saltiness if used without rinsing. They differ from a prepared black bean sauce, which can contain additional ingredients and its own balance of seasoning.
Start by checking what is in the packet
Look for whole or broken fermented beans rather than assuming every product labelled black bean is the same. Read the ingredients and the storage directions. Ordinary canned black beans, fermented beans and a bottled sauce do different jobs in a recipe.
If the recipe calls for rinsing, rinse the measured quantity and drain it before chopping or adding it to the pan. Do not rinse the entire packet and return wet beans to storage. Prepare only what you need and keep the remainder according to its label.
Chopping some of the drained beans distributes their flavour differently from leaving every bean whole. Follow the recipe first, then notice whether you prefer distinct small pieces or a more evenly dispersed seasoning.
Leave room for the rest of the sauce
A dish may also contain soy sauce, stock or another salty ingredient. Measure those components rather than adding each until it tastes strong on its own. Taste the finished sauce when it is safe to do so before deciding whether anything else is needed.
For an initial trial, choose a tested recipe with a modest quantity of beans and a clear method. Record the brand, whether you rinsed them and the amount used. That note is more useful than deciding that all fermented black beans are too salty after one heavily seasoned dinner.
Keep the first experiment simple. Once you know the ingredient, you can judge how it fits vegetables, tofu or a larger recipe without losing its character under a collection of competing sauces.