I just noticed this proposal that 'starchiness' should be recognised as the sixth primary taste (after sweetness, sourness, saltiness, bitterness, and umami). I've never really been clear on the distinction between a 'primary' taste and any other sort of taste - is this purely about whether the taste is detectable by receptors specifically on the taste buds? I have heard this as an argument against the recognition of 'pungency' ('spiciness') as a taste, since the mechanism of sensing pungency is different, but I believe some cultures do also recognise pungency as a primary taste - so is this a cultural definition, a scientific definition, or neither?
Unless they found a taste receptor, it likely is a part of the texture (including heat conductivity, solubility etc.) part of flavor. Spicy heat actually is related to temperature and texture (triggers heat - as in literal heat - receptors, does not trigger injury receptors and/or temperature receptors deeper in tissue, so we will not mistake spicy for actually dangerously hot).
The five/six basic taste system (the sixth being fat, currently being scientifically debated) does not describe flavor completely: It completely ignores the factors of aroma, texture (including how non-flavor ingredients will just mechanically keep most of some flavor compounds from ever getting near your tongue), temperature and their interactions. It assumes that bitterness is one dimensional, which is also debated.
Adstringency is considered a separate taste in some cultures, one could also consider it a textural factor (excites tissue in a certain way, just as capsaicin does).
I am currently doing some research on the topic an came across these criteria proposed to constitute a "primary taste" by two research groups:
Kurihara & Kashiwayanagi (1998) 1) A primary taste is a characteristic taste which is evidently different from any other primary taste 2) A primary taste is not reproduced by mixing other primary taste stimuli 3) A primary taste is a universal taste induced by components of many foods. If a taste is not often encountered, we do not call it a primary taste, even if it is a unique taste. 4) There are intrinsic taste receptors for a primary taste and single taste neurons specific to the primary taste
Running, Craig & Mattes (2015) 1) has ecological consequence 2) is elicited by a distinctive class of chemicals, 3) stems from activation of specialized receptors, 4) is detected through gustatory nerves and is processed in taste centers, 5) has a quality non over-lapping with other primary qualities, evokes a behavioral and/or physiological response
Sources: Kurihara, K., & Kashiwayanagi, M. (1998). Introductory remarks on umami taste. Annals of the New York Academy of Sciences, 855(1), 393-397.
Running, C. A., Craig, B. A., & Mattes, R. D. (2015). Oleogustus: the unique taste of fat. Chemical senses, 40(7), 507-516.