Awww, folks, don't WASH kombu, don't even soak it overnight. Just take a
paper towel, moisten it, gently wipe the kombu and then placed on top of the
beans and start cooking. The white stuff on the kombu is the flavor
enhancing material that is naturally in the kombu. It rises to the top in
the drying process.
By the time you get though boiling it, it is going to be sterile, and the
boiling makes it very, very tender. Please, process it minimally. It is a
natural food and it is a good source of minerals.