all the research i've managed to get my hands on indicates that the only place scientists have isolated umami receptors is in rats' tongues. i haven't been able to find ANY reference to the detection of umami receptors in human tongues.
the original insistence for umami as a fifth flavour is the legacy of kikunae ikeda, who 1908, claimed that as the flavour of glutamic acid (which he found in seaweed broth) couldn't be replicated by combinations of sweet, sour, salty and bitter, it must be new.
it is, of course, impossible to verify the results of combining EVERY possible combination of sweet, sour, salty and bitter, so i guess we should take ikeda's assertion with a pinch of umami. but lo and behold, here in feb 2000, chaudhari and roper at the university of miami CLAIM to have found an umami receptor in the tongue of rats.
does anybody have a reference to a paper which shows the finding of umami receptors on human tongues? why are we so sure that glutamic acid, if detected by humans, has seperate receptors and isn't detected by a comination of sweet, sour, salty and bitter?