Sensory scientists refer to a phenomenon called “sour-bitter confusion”. This mainly affects English-speaking subjects, who have a tendency to misidentify sour tastes as “bitter”. Sour tastes are associated with acid such as lemon juice, vinegar or cooking apples. Bitter flavours are typically associated with alkaline components in foods such as strong coffee, dark chocolate, beer, some salad leaves (radicchio) or green peppers.
Reliable reporting is of concern if for example you are a food scientist testing new flavours or carrying out market research; you want to be confident that you know what a tester means when they say “bitter”.
The sour-bitter confusion was first identified in a 1967 paper by Meiselman and Dzendolet at the University of Massachussets. When asked to taste samples representing the 4 “basic flavours”, 28% of their male subjects, and 10% of the females, described the sour stimulus (HCl) as “bitter”. This tendency persisted even when the subjects were corrected. Amusingly, the researchers concluded that the sour-bitter confusion may be “the gustatory analogue of abnormal colour vision”. In other words, that those subjects had a taste equivalent of colour-blindness.
Follow-up studies such as O’Mahony et al. (1991) also highlighted a tendency to describe the sour stimulus (in this case, citric acid) as “bitter”, and offered a number of suggestions as to why this might be the case.
The most parsimonious explanation seems to be a linguistic one: that many English speakers simply regard “sour” as a subset of “bitter”. And this may not be so unreasonable. Consider the universal and involuntary reaction to either a strong sour or a strong bitter flavour; your mouth puckers up, saliva is produced, your eyes water and narrow or even twitch. (This is an avoidance reaction to a potential poison.)
Consider also how we apply the term “hot” or “piquant” to a wide variety of foods, from horseradish and mustard to ginger, from garlic to black pepper and chillis, chemically dissimilar but provoking a similar subjective response.
Another consideration is that, as children, we have very limited exposure to bitter foods. We quickly understand “sweet”, “salt” and “sour” because we encounter these flavours from early childhood, while we are learning our native language. We also learn the word “bitter” but, in the absence of bitter foods in many childhood diets, it is not surprising that many of us fail to associate the word with its correct referent. (There are good adaptive reasons for us to avoid and dislike bitter flavours, especially in childhood, as bitterness is a hallmark of highly poisonous alkaloids found in plants such as nightshade or hemlock.)
One other factor that I think is important in creating the sour-bitter confusion: in some English-speaking countries there is a popular drink called “bitter lemon”. This may be one of the most common colocations of the word in the English language (along with “bitter pill” and “bitter medicine”). While the name of the drink in fact refers to the combination of quinine (bitter) and lemon, it creates a strong association in the minds of English speakers between bitterness and the flavour of lemon juice.
Later, as we encounter (and perhaps acquire a taste for) bitter foods such as coffee and beer, the scope of the word expands to include actual bitterness as well as sourness.
It is as if we had learned colour words in an environment with limited examples of the colour “green”, but there was a well-known product in our culture called “green sky”. We would have a good understanding of the meanings of “red”, “black” and so on in our culture, but may grow up thinking that “green” was basically another word for “blue”.
And of course, in other languages, “green” may well be another word for “blue”; the semantic boundaries of colour words are not determined by any physical reality but are entirely culture-dependent. (Interestingly, Berlin and Kay’s famous monograph on basic colour terms was published in 1969, 2 years after Meiselman and Dzendolet identified the sour-bitter confusion). Latin and Homeric Greek had no word for “blue”; Russian has two words to distinguish different types of blue, but no word that covers all the shades we would call “blue”; Japanese may call “blue” some shades that English-speakers refer to as “green”; in old English the word “red” would include colours that we now think of as orange, not red (what colour is red hair?); Finnish has no native word for purple, and so on.
So why should we expect flavour words to have consistent boundaries across different cultures? Perhaps because the basic flavours correspond to a physical reality in a way that basic colour words do not? Are there not 4 different kinds of taste receptors for the detection of salt, sweet, sour and bitter, each arrayed in specific zones of the tongue?
Apparently not. The familiar “tongue map” is a complete myth. And in a 1996 paper entitled “Are there basic tastes?” J Delwiche argues that the idea that there are 4 (or 5—see footnote below) kinds of taste receptors, each corresponding to a “basic taste” is equally unfounded and lacks scientific value. It has been unwittingly perpetuated over the years by researchers using pure samples of each of the 4 putative basic flavours as a fundamental tool of their trade. The unquestioned assumption that there were exactly 4 basic flavours led researchers to use a limited set of stimuli, and to require their test subjects to report their subjective experiences in terms of salty, sweet, sour and bitter.
Finally, there is some etymological overlap; the English word “acrid” (bitter) is derived from Latin acer (“sharp”); the same word acer is also the origin of the French word aigre meaning “sour”, which in turn gives us the English “vinegar”.
So, in conclusion, if some people use the word “bitter” to describe the flavour of lemon juice or vinegar, who is to say they are wrong?
Footnote: 4 basic flavours or 5? Or more?
In the mid-1980s it was announced to the world that a fifth basic flavour had been identified, and that it would be known by the Japanese term umami. The unfamiliar name and the tone of the reporting made it seem like something exotic or obscure, something that perhaps would only be found in Oriental cuisines and might be difficult for western palates even to distinguish. Nothing could be further from the truth. This “new” flavour is nothing more than the characteristic savoury flavour of meat broth, mushrooms or parmesan cheese, familiar to people in every culture literally from our first taste of mother’s milk. It was like being informed of the discovery of the colour yellow.
Can we expect further announcements of additional basic tastes that are familiar to most of us, such as the harsh taste of strong tea or rhubarb? The “cool” taste of menthol? The Japanese wikipedia article on taste has a short section on 6番目の味覚—”the 6th taste”; a flavour receptor for calcium was identified in mice in 2008 (but not yet demonstrated in humans). Or is the whole concept of “basic flavours”, one corresponding to each type of receptor, an oversimplification with little relevance to how we actually experience food and drink?