A flavour for taste: Detecting the flavour of food 15/07/07. By Caroline Cross How do we detect the flavour of food, and why do we prefer some foods to others?

If anyone offers you a 'miracle fruit' – a rather nondescript red berry the size of an olive – don't then fall for the 'sweet lime trick'. If you eat the berry, limes or any other sour food will taste deliciously sweet, courtesy of a glycoprotein in the miracle fruit that masks the tongue's sour receptors. Of course, after an hour or so the effect wears off and the expensive sweet limes you've just bought will be revealed as normal sour limes.

The receptors hoaxed by the miracle fruit are part of a remarkably complex and adept system, capable of detecting the components of the cornucopia of foods we encounter. Today, the sense of taste is usually associated with our likes and dislikes, emotional responses to different foods. Yet for most animals – and for humans in our evolutionary past – taste is a survival issue, as it enables key foodstuffs to be identified. In general, sweet foods contain energy-rich sugars, food that tastes salty or savoury (the 'umami' taste of monosodium glutamate) contain salts or amino acids, and bitter or sour tastes often indicate food is poisonous or has become rancid.

In the last few years, rapid advances have been made in our understanding of how these five tastes are detected: most of the taste receptors have been identified, and we are beginning to decipher how the brain processes information from the mouth. Less clear is the wider 'gustatory experience' – the integration of a range of sensory signals, including smell, to produce what we know as flavour. Even so, before long it might be possible to manipulate foods to contain vital nutrients that might once have been rejected because of their bitter taste.

Tickling the taste buds

The tongue, our primary taste organ, is dotted with papillae, tiny structures that contain taste buds. The papillae do not correspond to specific tastes: sensitivity to all tastes is distributed across the whole tongue and other regions of the mouth such as the epiglottis and soft palate, although some areas are more responsive to certain tastes than others. (The 'taste map' of the tongue, still found in many textbooks, is an oversimplification.) Within the papillae are taste buds, clusters of taste receptor cells whose tiny, finger-like microvilli project into a central cavity.

When we eat, food washes into the taste bud cavity, where receptor proteins on the surface of the microvilli lie in wait, ready to detect different components of the food. We now know many of these receptors: those for bitter, sweet, umami and sour tastes have been identified by Professor Charles Zuker (Howard Hughes Medical Institute, University of California, San Diego), Dr Nick Ryba (National Institutes of Health, Bethesda) and colleagues.

The bitter taste receptors were identified in 2000; this family of 30 related proteins, called T2Rs, can distinguish between a variety of bitter compounds. The sweet and umami receptors, found in 2003, are composed of proteins called T1Rs. A combination of T1R2 and T1R3 produces a receptor that responds to natural and artificial sweeteners, while receptors made up of T1R3 alone respond only to high concentrations of sugars. The umami receptor is made up of T1R1 and T1R3 proteins, and is triggered by glutamate and aspartate (common ingredients of savoury snacks).

Having detected a particular food, the bitter, sweet and umami receptors activate their partner G proteins on the inside of the cell. The G proteins – members of a family of proteins that control signalling for many different processes in the cell – spark a cascade of reactions that cause the cell to depolarise, and an electrical signal is sent towards the brain.

In 2006, Professor Zuker, Dr Ryba and colleagues identified a channel protein (a pore through the cell membrane) called PKD2L1 as a candidate sour receptor. This appears to work in a rather more direct manner: sour tastes are acidic and so contain hydrogen ions; the flow of these through the channel would cause the cell to depolarise and, again, send a signal to inform the brain. Although the salt receptor has not yet been identified, it is also thought to be a channel protein, probably responding to sodium and potassium ions arriving at its gates.

The tongue can feel other sensations, such as astringent tannins in tea or unripe fruit. Spearmint and menthol taste 'cool' because they activate the TRP-M8 ion channel on nerve cells that signal cold, while capsaicin – the spicy component of chilli peppers – tastes hot because it activates a nerve cell ion channel called TRP-A1, which is sensitive to hot temperatures.

The identification of the genes that produce taste receptor proteins is allowing researchers to examine whether subtle variants in the genes underlie why we like some foods but not others. For example, people who can taste phenylthiocarbamide (PTC), a synthetic bitter compound, are less likely to eat bitter, cruciferous vegetables such as broccoli and Brussels sprouts. This sensitivity, or lack thereof, is determined by two variants in the TAS2R38 gene, which produces one of the bitter taste receptor proteins.

Similarly, slight genetic differences in receptor proteins might explain why one person needs five spoons of sugar in a coffee and another needs only one or two – because the first person's sweet receptors need more sugar to get the same kick. Cats' well-known indifference to sweets has been found to be due to a deletion in the T1R2 gene.

Tasty questions

At present, scientists only partially understand how taste signals are transmitted from receptor cells to the brain. What is clear is that each taste receptor cell produces only receptor proteins for one type of taste. Activity from single types of taste cell is sufficient to trigger innate behavioural responses in animals. For example, activation of sweet receptors triggers attraction to food sources, while activation of bitter receptors results in strong aversion. The messages from the mouth are transmitted and processed along various neuronal stations of the brain, including the brain's primary taste cortex – which represents what the taste is, independently of how much we actually like the food.

But taste recognition in the mouth is only one ingredient of what we think of as 'taste'. What we often describe as taste is really 'flavour by mouth' and is the overall sensation of eating a food, which is a mixture of taste, smell, texture and temperature. The secondary taste cortex is likely to play a key role here, as it is connected to primary taste cortex and processes other inputs, such as the smell and sight of the food, but we are still only at the beginning of understanding how the full 'gustatory experience' is constructed.

With advances in our understanding of taste receptors and taste perception, it may be possible to alter flavours to make nutritious foods more appetising. There is also a growing market for foods containing so-called 'nutraceuticals', sometimes known as functional foods – such as cholesterol-lowering drugs added to spreads and drinks. And there is potential to manipulate the chemicals involved so they are more palatable. If a food component has a health benefit but makes the food taste bitter, then it might be possible to adjust the molecules so it maintains the health benefits but no longer triggers bitter taste receptors – a development that would be good for manufacturers and consumers alike.

Caroline Cross is a freelance writer based in Reading.

Further reading

• Chandrashekar J et al. The receptors and cells for mammalian taste. Nature 2006;444(7117):288–94. Abstract .
• Huang AL et al. The cells and logic for mammalian sour taste detection. Nature 2006;442(7105):934–8. Abstract .
• Mueller KL et al. The receptors and coding logic for bitter taste. Nature 2005;434(7030):225–9. Abstract .
• Zhao GQ et al. The receptors for mammalian sweet and umami taste. Cell 2003;115(3):255–66. Abstract .