We prize fungi for their rich, almost meaty flavor and their ability to intensify the flavor of many dishes. These qualities are largely due to a high content of free amino acids, including glutamic acid, which makes mushrooms—like seaweeds—a concentrated natural source of monosodium glutamate. Another taste enhancer that’s synergistic with glutamate, GMP (guano-sine monophosphate), was first discovered in shiitake mushrooms, and contributes to the rich taste.
The characteristic aroma of fresh common mushrooms is mainly due to octenol (an 8-carbon alcohol), which is produced by enzymes from polyunsaturated fats when the tissue is damaged, and which helps deter attack by some snails and insects. More octenol is generated from the gill tissues than from other parts, and this is one reason that common mushrooms with immature, unopened caps are less flavorful than the mature version with prominent gills. Brown and field mushrooms have more flavor than the white mushroom, and the “portobello,” a brown mushroom allowed to mature for an additional five or six days until it’s about 6 in/15 cm across, is especially intense.