Many people, when washing shiitake mushrooms, prefer to scrub them by hand. While this may clean the surface, the sand and silt in the gills (also called lamellae) are not removed. In fact, due to the scrubbing, some sand can become more firmly lodged in the gills. When eaten, regardless of the cooking method, this will result in a gritty texture. Furthermore, repeated scrubbing not only damages the nutrients but can also ruin the appearance of the mushrooms.
So, how can you effectively wash the sand out of shiitake mushrooms? To wash them, use a few chopsticks or your hand to stir the water in one direction. This will cause the sand on the surface and in the gills to fall away. Repeat this stirring several times to thoroughly remove all the sand. However, it is important to note that in addition to avoiding scrubbing by hand, you should also not stir back and forth in the opposite direction. If you do, the sand particles will not only fail to fall out, but those that have already settled will be re-entrained into the gills by the reversing current.