Our taste buds in particular are designed to help us recognize and pursue important nutrients: we have receptors for essential salts, for energy-rich sugars, for amino acids, the building blocks of proteins, for energy-bearing molecules called nucleotides. Raw meat triggers all these tastes, because muscle cells are relatively fragile, and because they’re biochemically very active. the cells in a plant leaf or seed, by contrast, are protected by tough cell walls that prevent much of their contents from being freed by chewing, and their protein and starch are locked up in inert storage granules. Meat is thus mouth-filling in a way that few plant foods are. Its rich aroma which cooked comes from the same biochemical complexity.
Harold McGee, On Food and Cooking, 123.