The Japanese ingredient koji is the fungus that grows on rice, barley, soybeans, or corn after it is inoculated with a fermentation culture called Aspergillus oryzae. It resembles thin rice porridge and is full of enzymes that produce amino acids when they interact with protein. One of those amino acids, glutamate, is responsible for the taste we know as umami, which is present in miso and soy sauce and makes foods especially savory and flavorful. There are two types used for cooking: Ama-koji and Shio-koji. Ama-koji, called for here, has no salt, so you can control the seasoning yourself. In this simple recipe, it's added to salmon fillets before they hit the grill, which lightly cures them and adds an umami kick.