An enhancer is a substance that increases the flavour of food — or more precisely, that amplifies the flavour already present rather than adding its own. The classic flavour enhancers are the umami agents: monosodium glutamate, disodium inosinate, disodium guanylate, and yeast extracts, which multiply savoury depth, especially in combination. They work on the taste receptors, making the existing flavours of meat, cheese, tomato, and mushroom taste more like themselves. The category is regulated as food additives with E numbers and permitted levels, and products must declare them. Controversy has followed MSG in particular, though the scientific consensus finds normal culinary use safe. Used well, enhancers are invisible — the soup tastes richer, the snack moreish — and the diner never knows why.