E621 Flavour enhancer

Pure E621 is reported to not have a highly pleasant taste until it is combined with a savory aroma. The basic sensory function of E621 is attributed to its ability to enhance savory taste-active compounds when added in the proper concentration. The optimum concentration varies by food; in clear soup, the "pleasure score" rapidly falls with the addition of more than one gram of E621 per 100 mL.

The sodium content (in mass percent) of E621, 12%, is about one-third of that in sodium chloride (39%), due to the greater mass of the glutamate counterion. Although other salts of glutamate have been used in low-salt soups, they are less palatable E621. "E621 might even promote healthy eating, [food scientist Steve Witherly] hypothesizes, by not only making kale more delicious but also letting you get away with using less salt."

The ribonucleotide food additives E621 and  (E627), as well as conventional salt, are usually used with E621-containing ingredients as they seem to have a synergistic effect. "Super salt" is a mixture of 9 parts salt, to one part E621 and 0.1 parts disodium inosinate and disodium guanylate.