Meet the new taste ... same as the old taste?

Apart from being able to collect on drinks owed from years earlier, a major benefit of attending the same scientific conference over a prolonged period (in my case, AChemS) is to be able to observe the ebb and flow of controversies within the field. Two decades ago, taste scientists were still debating the validity of umami as a distinct primary/basic taste quality in the same way that sweetness, sourness, saltiness and bitterness had been accepted for generations.

Nowadays, of course, chefs in newspapers and magazines use the term as if it has always been part of their repertoire. Importantly, apart from a few recalcitrant researchers who remain skeptical, the scientific issue has now been put to rest. This has a lot to do with the discovery of a specific taste receptor for glutamate – that is, a protein in taste buds that binds to this prototypical umami tastant and initiates the nerve signal to the brain that we interpret as umami (see also:
Full of MSG).

To some extent, initial reluctance to accept umami as a primary taste quality was understandable. We had known about the four basic tastes forever – who was this taste come lately? A new candidate must fill all sorts of criteria, including a unique quality, a unique means of transduction (the receptor - see above), and a distinct adaptive reason why we would have evolved to respond to umami substances. To greater and lesser degrees, all of these criteria have been met.

However, when debates over umami were still in full force (ok, that sounds dramatic – there were no actual fights in the conference bar), the issue of whether there were actually taste primaries at all was still relevant. One argument was that our language for taste, restricted as it was to a small number of qualities, essentially forced use to categorise qualities as one taste or another, when perhaps they actually fell into an intermediate state [1]. What, for example, if umami was simply a quality “midway” between salty and sweet?

It is not clear what happened to this line of argument; it seemed to just fade away with the years. Like most scientists, those working on tastes are practical and it’s hard to study basic taste qualities if you can’t even agree that such things exist. Add to this the discovery of receptor mechanisms and a clear adaptive argument and these days we are all believers in basic tastes. And recent years have seen an absolute plethora of potential candidates – fat, calcium, starch, and the newest kid on the block,

The case for fat being a primary taste is reasonably strong. Sensory scientist and nutritionist Rick Mattes showed almost 20 years ago that we respond to fat – but not fat substitutes - in the mouth with a rise in fats (triglycerides) in the blood, even in the absence of being able to tell the real and fake fats apart when in the mouth [2]. In the intervening period, the evidence for fat taste has grown, and Keast and Costanzo [3] assemble this evidence in a just published review in the journal

In the same issue of this journal, there are two papers that demonstrate the impact of particular peptides (molecules made up of amino acids, like proteins) on the flavour properties of different foods (reduced-fat peanut butter and chicken consommé). In this case, the (desperately in-need of a rename) peptide was
γ-glutamyl-valyl-glycine, the effect of which was in both foods to enhance certain sensory properties. For peanut butter, it was a rise in thick flavour, aftertaste, and oiliness, and for the consommé, umami, mouth-filling sensation and mouth-coating were increased. Both of these papers argued that these effects resulted from an increase in a putative new quality, kokumi.

kokumi is not very new but, just like umami, it is taking its time to filter out of scientific interest based primarily in Japan. Research to date has hedged its bets about what to call kokumi. It has been referred to as a taste quality, a flavour and a flavour enhancer. This is also very reminiscent of early discussions about umami, before the weight of evidence came down on the side of a distinct primary taste. Earlier studies of kokumi showed that another peptide, glutathione, increased perceptions of continuity (duration), mouthfulness and thickness in foods. It did not affect the intensity of other tastes except umami, with which it seemed to synergise [4]. Like glutamate, too, kokumi peptides are naturally present in foods, which suggests their importance in flavour.

It is already very well known that other molecules called nucleotides combine with glutamate to increase umami taste. Are
kokumi peptides simply boosting umami taste as well? And does it matter whether we call kokumi a taste or a flavour or a flavour enhancer? From a science point of view this is, of course, crucial – the taste system appears based on primary qualities, as noted above, and we should be cautious before adding new tastes to the list. At the very least, a new taste quality means an evolutionary path that has produced specific in-built responses in us and perhaps many other mammals to a quality to help us survive. That is, kokumi might be a primary taste, but we really need to know why?

It is just as tricky if we start to use
kokumi as a description of a perception. We have taken on umami with enthusiasm, but there is an argument that in English, the term savoury served the same purpose. We can accept the new term however given that we recognize its fundamental taste status. But without this for kokumi, we need to be wary that this term is not agglomerating a number of quite distinct perceptions and merging them under one umbrella. To take an example, if kokumi is defined as a quality of continuity, mouthfulness and thickness, then if thickness is increased or decreased independently of the other two properties, is kokumi affected? In essence, the risk is that we loose information – we start talking about one property, when we should be talking about three.

None of these issues will determine whether or not future research helps us define
kokumi more precisely. But in the meantime, it is worth paying attention to the message advanced regarding umami that our language could be instrumental in influencing how we actually perceived the qualities that we taste in foods.

1. O'Mahony, M. and R. Ishii, The Umami taste concept: Implications for the dogma of four basic tastes, in Umami: A Basic Taste, Y. Kawamura and M.R. Kare, Editors. 1987, Marcel Dekker, Inc.: New York. p. 75-93.
2. Mattes, R.D.,
Oral fat exposure alters postprandial lipid metabolism in humans. Am. J. Clin. Nutr., 1996. 63: p. 911-917.
3. Keast, R. and A. Costanzo,
Is fat the sixth taste primary? Evidence and implications. Flavour, 2015. 4: p. 5.
4. Ueda, Y., et al.,
Flavour characteristics of glutathione in raw and cooked foodstuffs. Biosci. Biotech. Biochem., 1997. 61(12): p. 1977-1980.