Rinse the dried shiitake mushrooms with water to remove any dirt.
Put the rinsed shiitake and water in a resealable zipper bag. Close the bag, making sure to remove the air.
Leave the bag in a refrigerator overnight to allow the shiitake to be fully reconstituted. (Reconstituting at a low temperature will enhance their sweetness.)
Strain the soaking liquid in a fine strainer, such as a tea strainer.
Remove the hard stems of the reconstituted shiitake before using. (Stems can be used later in soups, stews, and sauces to add umami.)
Slow rehydration at less than 10°C for 5-24 hours is required to increase the Umami Guanylate.
To reduce soaking time, take out the Shiitake one hour after you started soaking, remove the stem, soak for another hour until soft. Shiitake absorbs water from the cut surface of the center, so it can be quickly and plumply rehydrated.