Umami, the fifth basic flavor, is a Japanese word meaning "savory". With an improved grasp of this basic flavor you can amplify flavors in any dish – not least vegetarian ones.
Umami is a newcomer among the basic flavors, which might be because it is tricky to isolate and to describe. The japanese word means something to the effect of "savory". This says something about why this flavor exists at all and why it can be crucial to your cooking:
Umami signals protein. The "savory" flavor is there to encourage a hungry human being to eat her fill of food rich in building blocks that the body cannot do without.
Umami is associated with the taste of meat and animal protein, but ingredients such as these are not necessary to put this basic flavor to work in your cooking. In Asian kitchens there has long been an awareness that umami is a necessary dimension for bringing more punch to vegetarian dishes. This has resulted in an expert use of, among others, soy (the beans, but not least the sauce), mushrooms, fermented greens, seaweed, etc...
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