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Our incredible sense of taste

Publication Date : 24-08-2014

 

There is a forensics lab in our heads that analyses what we eat all the time

 

Watching an episode of a popular TV drama about a criminal forensic laboratory got me thinking about the cool equipment and gadgets they were using.

As a food lover, it would simply be great to be able to analyse the exact combinations of compounds that make up a good wine or a good dish so that it can be replicated again and again.

And then it occurred to me that we actually do have our own portable CSI laboratory in our heads – when I did some research, it turns out that we actually know very little about how this lab works and most of the discoveries about it were made only in the last 15 years or so.

By this lab, I mean of course, the taste buds on our tongues.

We take it for granted that we can taste the splendour and variety of the various types of food we eat.

However, there are very good reasons why we have evolved the sense of taste – and it’s not just so that we can enjoy a plate of foie gras on port sauce, soothed with a chilled Sauternes.

It is not known when the Homo genus evolved the sense of taste but it must have been immensely useful for omnivorous scavengers to be able to tell the difference between edible and poisonous or rotten food.

They were not always right – the sense of taste is not perfect at analysing toxicity – but generally, Palaeolithic man evolved to dislike bitter and sour substances (which tend to be poisonous) and prefer foods which are sweet, salty and umami.

And as we evolved over the last few hundred thousand years to prefer cooked food, it would appear that the tongue has also matched our dietary changes and has elaborated on the tastes we can perceive.

There is now some convincing evidence that we also have a taste sense called kokumi and yet another one for fat, particularly cooking oils.

The taste of kokumi apparently explains why slow-roasted meats or aged cheeses taste so much better than food prepared more quickly.

The fat taste receptors might explain why some people may or may not have a tendency to obesity.

If you are a curious person, and I assume you are one to have read this far, it may be interesting to know how the taste (gustatory) receptors in the tongue work. They don’t work the same way for all the tastes either.

Apart from sensing tastes, the gustatory receptors also perceive the depth or intensity of flavours – it is not like an on-off taste switch but more like an artist’s colour palette.

It takes various combinations of tastes to make up the complex herbal, floral and savoury flavours that we enjoy.

If one assumes (and one may be wrong) that there are seven basic tastes, each with only 10 levels of amplitude or intensity, then we can taste a combination of 107 = 10,000,000 flavours.

No wonder we love food so much – eating is not unlike experiencing a breathtaking work of art with millions of colours and nuances.

As a little digression, the household cat (and many other carnivores) cannot taste sweet.

Their sense of taste evolved in a completely different way from humans and is the result of not including fruits or other sweet foods in their dietary repertoire.

Herbivores, on the other hand, are very fond of sugary fruit – which is why a horse appreciates an offer of an apple whereas your cat would look at you as if you were mad.

Salty

One taste that all mammals share is the taste of salt. The mammalian body requires sodium ions to maintain blood function and is necessary for the transmission of information between nerves and muscles. It also helps digestion.

Hence, there is always a primal need for mammals to ingest salt and the taste receptor for salt on our tongues is also one of the simplest (or primitive).

How it works is pretty straightforward – there is an ion channel in the taste receptor which allows sodium ions to enter. This causes the cell to depolarise, which triggers the release of calcium ions. The sudden influx of calcium ions sets off a neurotransmitter which tells the brain that you have just tasted salt. And that’s it.

Sour

Sour is the next more complicated taste. Although it works like salt, with sour, there are three taste receptors at work.

The first receptor is an ion channel which allows through the positive hydrogen ions in the food (any solution with H+ ions is acidic). This receptor actually is the same receptor as the one for salt which is why sour foods can cut the taste of salt and vice versa.

The second receptor cell after the first are channels specifically for H+ ions, which are now trapped inside the cell. The H+ ions then block potassium ions in the cell.

The third receptor then opens up to sodium ions which bind to the H+ ions and together they depolarise, release shots of sodium and potassium which trigger a neurotransmitter which then tells the brain that you have just sucked on a lemon and that it’s probably not good to do that too often.

Sourness is associated with decaying foods and unripe fruits and is usually a warning taste.

Bitter

The tongue is rather sophisticated when it comes to detecting the many classes of compounds which are bitter. It is also our most sensitive sense of taste.

The simple reason why bitter is important is because many species evolved to be poisonous and to taste bitter as this is an effective way to warn off predators – this warning defence mechanism is adopted by many plants and creatures.

Basically, in nature, anything that tastes bitter is probably toxic and knowing this well in advance of taking a big bite is very critical to survival.

The method by which bitter is detected is rather like how a CSI laboratory would do it, by using very specialised detectors. Simplified, there are G protein-coupled receptors (GPCR) based around a protein called gustducin which react when encountering various bitter compounds.

There are between 35 to 43 types of bitter-sensing GPCRs, although they can sense rather more than 43 bitter compounds when working in combinations. When a bitter compound is detected, the affected GPCRs release chemicals like inositol triphosphate and diglyceride.

How the bitter taste is communicated to the brain is still a little mystery due to the complexity and numbers of GPCRs involved. One theory is that the receptors charge up with calcium ions which then discharge to send a neurotransmitter.

Another theory is that there are special ion channels (as for salt and sour) which are sensitive to bitter molecules which charge up and then discharge to send a neurotransmitter to tell the brain that you have made the coffee too strong or have seriously overcooked the meat.

Sweet

The sweet taste works rather like the bitter taste, except that it involves different GPCRs. The mechanism is similar except that we think that there are probably only two different types of GPCRs for sweet.

These GCPRs release different chemicals for charging up the receptors before sending neurotransmitters to the brain to inform it that you have been eating milk chocolate. Again.

Knowing what is sweet is very important to Palaeolithic man as sweet foods tend to be able to provide a lot of energy. There was always a premium on sweet fruits and foods – people would always prefer to eat sweet food in the past and that is still true today.

Umami

This is an intriguing taste, but it is probably not unique to humans. One would probably already know that it is the taste stimulated by the presence of glutamate, particularly monosodium glutamate (MSG) in food – but actually, umami is also triggered by inosinate (a compound found in meat) and guanylate (found in mushrooms).

The umami taste was discovered by Kikunae Ikeda in 1908, who also isolated MSG from seaweed, patented it and founded Aji-No-Moto – MSG is now manufactured in unimaginably huge quantities from sugar-rich plants.

Curiously, inosinate was discovered by another Japanese, Shintaro Kodama in 1913 and guanylate was identified by Akira Kuninaka in 1957 – so it does seem that the Japanese really are masters of umami.

How the tongue detects umami is fascinating too. The umami receptor is a large folded protein and when it detects a molecule of glutamate, the protein changes its shape and reaches out to grab the glutamate molecule. This change in shape informs the brain that you are really enjoying your soup.

Where inosinate and guanylate come into the picture is that they bind to different parts of the umami receptor – and if the umami receptor is already grasping a glutamate molecule, then the strength of the umami taste is increased by up to 15 times in the presence of inosinate or guanylate.

Hence, it appears that the secret to making a very good natural broth is boiling bones with meat, mushrooms and seaweed, which is exactly what some Japanese cooks do.

Carnivores are probably even more sensitive to inosinate and umami than humans, hence it is always a bad idea to play with carnivores when they are eating.

I used to have a Doberman and he was great fun except when he was chewing on a bone, in which case, he became really rather possessive and dangerous.

Even with humans, the lust for a good dose of umami is rather overpowering at times which is why an overwhelming percentage of processed and cooked foods contain compounds which stimulate umami – such foods are simply almost irresistible to humans.

Even vegetarians love the umami taste as a lot of meat-free food contains natural glutamate or guanylate, particularly soy- or mushroom-based products.

 

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