A dulcifiers (pronounced "dool-sih-fy-er") or dulcifer is a brew that neutralizes the poisonous salts in fish caught from the vinegar seas and apparently improve their taste as well. The process of making fish safe to eat, which can take a long time to do properly, often a whole day, is called "soaking," and to soak a fish is to sweeten or dulcify it. Consuming an unsweetened fish would sicken a person, and several meals will eventually result in death.
Many naturally-occurring dulcifiers, including plants, barks, and roots, exist, and form the basis of refined seafood recipes. These often include seasonings that mask the bland flavour of dulcified seafood. There are several types of fish that naturally undergo soaking within their own bodies that can be caught, cooked, and eaten without dulcifers, but do not taste very nice.
One dish that might possibly require dulcifiers is that of vinegar pie.