Koji is a fungus that ferments food. Its spores grow on cooked rice. When applied to steak, its powerful enzymes slowly tenderize the meat. Because of this, now you can get a 45 day aged steak in just 48 hours-for a rich steakhouse flavor in your own kitchen.
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Video by Popular Science
For more food science and cool fungi, go to https://www.popsci.com/