Wednesday, January 29, 2014

Sweet, Salt, Bitter, Sour - They Ain't The Half Of It

Biology concepts – umami, taste, flavor, gustation, glutamate, chemoreception, CD36, fat taste, water receptor, calcium

Perhaps I was a little hasty when I said umami wasn’t the name
of a new band. Apparently “Umami” is the name of a band from
Minneapolis. They are described as an electro/psych band,
whatever that is. I like it when obscure science words are used
in culture – makes me think I’m in on some secret. Umami isn’t
that obscure a word, but I used to know a band made up of
statisticians called The Outliers.
Ever heard of umami? It’s not the name of a new band, or even a bad Robin Williams movie. It’s a taste; the fifth taste that humans can sense. Umami is the taste of savory; meats and other high protein foods. And what do we have to thank for umami? Seaweed.

Until 1908, science believed that most flavors were just combinations of the four traditional tastes - sweet, salty, sour, and bitter. But don’t get the idea that taste and flavor are the same - oh no. Taste is our gustatory sense, but that isn’t the same as flavor – flavor is something bigger than taste.

You know how having a head cold makes food bland? Well, that’s because smell is a big part of flavor; you don’t smell your food when you have a cold. Food stimulates all your senses - temperature, touch, smell, what it looks like and even how it sounds as you chew it. All these things add up to flavor.

This is why chefs say you eat with your eyes first, and why they try to incorporate different textures into a single dish. They’re trying to appeal to all your senses. Therefore, eat slowly to enjoy your food more. Give all your senses time to participate. And you might just eat less, since your satisfaction will come from the total experience, not just the craving for a particular taste.

But back to the origins of umami. The Japanese had an idea that there was another taste, mostly since their traditional cuisine used so much seaweed, and this flavor couldn’t be accounted for by the other four tastes.  Chemist Kikunae Ikeda wanted to identify the active molecule in seaweed, the one that gave it taste. He called it umami, from the Japanese words for delicious (umai), and taste (mi).

Ikeda’s biochemical studies led to the identification of glutamates as the molecules to which people reacted. And they aren’t just in seaweed, most living organisms contain truckloads of glutamates. When cooked, all glutamates convert to L-glutamate, the amino acid. Ikeda determined that this is what some of our gustatory (taste) receptors sense. Gustation is from Latin gustare = taste, and this is where the word gusto comes from; to taste life.

It turns out that umami taste is not produced by just L-glutamate.
Glutamate comes mostly from meat, as they are high in proteins,
but the nucleotides inosinate and guanylate also perceived as
savory tastes. Inosinates are also found in meats, but also in
seafood. Vegetables are a major source of guanylates, but so are
mushrooms – and we all know that mushrooms are fungi – right?
When you taste something, a chemical signal in the taste cells on
the tongue is converted to an electrical impulse. This is carried by
either the facial nerve or the glossophayrngeal nerve to the brain.
The most palatable form of glutamate that Ikeda could identify was monosodium glutamate (MSG), so he immediately set out to produce and sell it, starting in 1909. Made a pretty penny, he did. Now MSG is a common flavor enhancer in Japanese cooking, including soy sauce. World-class chefs are designing “U-bombs” (umami-filled dishes) to take advantage of the new official taste.

This is why identifying an umami taste receptor for L-glutamate makes sense. This is nature telling you that you need to eat protein, and by giving it a favorable neural response (it tastes good), it increases the chances that you will seek out protein sources for nutrition.

Glutamate has several functions, even beyond its role as one of twenty protein building blocks. Glutamate is the most common neurotransmitter in the central nervous system, and plays a crucial role in long-term potentiation (LTP) and learning. Glutamate is also an intermediate in synthesizing many of the molecules in glycolysis, gluconeogenesis, and the citric acid cycle. I’ve said it before and I hope it jumped into your mind just now – nature hates a unitasker.

So you sense L-glutamate through a gustatory receptor that is specific for that molecule, and the electrical impulse is converted to a specific taste – we call it savory. The cloning of the taste receptors in the late 1990’s (actually umami was the first) started people thinking about other possible tastes. Could there be a sixth taste sense – how about fats? Do we taste fats?

When in the insula of the brain, the input is sorted with other
input and interpreted as a taste. It is the perception that is
important. For the fatty acid receptors, they are receptors, they
are located in the taste cells, and they do carry information via
the same two nerves to the same part of the brain. But are they
then interpreted as a taste?
Much research has been performed in this area in the past few years and a couple of fatty acid receptors on the tongues of rodents and primates have been identified, specifically, CD36 and GPR120. But does this mean we “taste” fat? We said taste doesn't equal flavor – we should now add that sensation may not be the same as taste. Just because there are specific chemoreceptors on taste cells for different fatty acids doesn’t mean that we perceive the sensation as taste.

It has been shown that fatty acids in the oral cavity do have a threshold level for sensation, and that the fatty acid taste receptors do lead to specific changes in physiology. When subjects were given fatty acids on their tongues, they very quickly showed increased serum triglyceride levels, increased pancreatic hormone release, increased release of GI lipases (enzymes to breakdown fat) and a slowly of the GI tract (it takes more time to digest fats).

What is lacking here is a conscious perception of the discernable nature of the fatty acid (like how sugars are sweet or glutamates are savory). The 2009 studies by Mattes and colleagues controlled for the mouth feel, smell, and so forth of fats, so it was definitely the fatty acid receptor that was stimulating the responses, but no where did it say the subjects tasted something. However, his 2011 paper says that fat may very well be a basic taste.

This gives us a new way thinking about taste receptors. Taste is a type of chemoreception, but perhaps it’s only one subset of oral chemoreception. Gustatory chemoreception is a lot more than just tasting something. However, some researchers challenge this division, saying that participants do have a measurable psychophysical response when fatty acids on the tongue reach a threshold level – they do taste something.

Again I say, “nature hates a unitasker.” CD36 is the fatty receptor in
taste cells, but it also works in macrophage recognition of oxidized
fatty acids and the onset of atherosclerosis, and in the activation of
platelets by fatty acids. You can see in the cartoon that CD36 sticks
into the membrane at two places and loops out of the cell. If you
change the order of its amino acids, it’s shape will change. How well
it binds to fatty acids, or activates all those downstream signals will
also be affected. This is why people with different versions of CD36
may eat different amounts of fat. I wonder if people with poor CD36
versions also have more trouble with CD36 functions in other cells?
The CD36 glutamate receptor has been especially well studied in the past couple of years. It comes in several slightly different forms (polymorphisms – slight differences accounted for by single or few amino acid differences in the sequence of the protein), but these differences have a big effect.

When divided into groups based on which variant of CD36 they possessed, a couple of studies from 2012 (here and here) show that responses to fat and how much the subjects craved fats were different. Those who sensed fats most readily (at lowest concentrations) tended to eat less than those who needed more fat in order to trigger the responses.

The hypothesis of a 2013 review of fat taste and obesity says that those who sense less fat are more likely be tipped toward a hunger stimulating hormonal profile, while those who more readily sense the fatty acids in their food tip toward satiety (fullness). There are many hormones involved here and we could get bogged down very fast, so let’s leave it at that for now; undoubtedly someone is trying to make a diet pill based on it.

So, could there be more oral chemoreception events going on – a seventh taste? An eighth?  Let’s talk very briefly about two possibilities. Maybe we can taste calcium. Yes, you could taste your Tums. A 2008 study indicated that mice can perceive calcium as a specific taste. A single 2012 study extends this to humans as well. Calcium is sensed via a certain receptor (Tas1R3), which works with other proteins to sense sweet and umami. But here, it apparently works on its own (we will talk more about the receptors in the posts to come). I need to see more research before I buy calcium taste completely.

Taste number eight - do you think you can taste water? The common argument is that you can taste what is in the water, not the water itself. How or why would you taste water; you’re 65-70% water all the time! What good is it to taste the main ingredient of life? How about this – do you sense the water by taste receptor, not just by temperature, sound, smell, or mouth feel?

There are voluntary swallows an involuntary swallows. But even
in voluntary swallows there are involuntary parts. You don’t think
about closing off your trachea with your epiglottis, it just happens.
This closing is how you keep food and liquid from ending up in
your lungs. Water in your laryngeal pharynx is one stimulus to get
you to swallow and close off the wind pipe until the possible
problem is gone.
Yep, mammals have receptors in the oral cavity that specifically sense the presence of water. In some mammals, like dogs and rabbits, using salt water inhibits the firing of the laryngeal nerve fibers connected to the water receptors; not so in cats and rats. We’ll get to why in a second.

What is the purpose for water receptors in the oral cavity (really, they are in the entrance to the throat, the laryngeal pharynx)? It may be that this is an evolutionary protection from aspirating (breathing in) liquid to the lungs. Liquid in the lung is a bad idea, since it stops gas transfer and promotes bacterial growth. If acids or other liquids that could damage the lungs or throat get in their somehow, it would definitely be better to swallow them than to breathe them in.

When you were a fetus and a very young infant, stimulation of the water receptors in your throat caused you to swallow immediately. As you aged and gained more muscular control, the reflex was replaced by coughing – this is the hypothesis of a 2001 study on the reflex. But the water receptors are still there and still aid you as a stimulus for voluntary swallowing. Whether we taste water or not, I’m glad I have the chemoreceptors.

So, the dampening of the reflex by Cl- in salt might be helpful keeping you from constantly having the urge to swallow. This leads to another point – the power of suggestion. Can you do anything right now other than think about the saliva in your mouth and whether you should be swallowing? Creepy, isn’t it.

Don’t count out the idea of water as a basic tastant (something you can taste). A 2010 study showed by monitoring brain waves that people respond to water using the same pathways as taste, and the responses look the same. And a 2012 study indicates that rats have distinct portions of the gustatory cortex of the brain for identifying both salt and water. If we can taste umami to make sure we eat enough protein, and sweet to make sure we eat enough carbohydrate, why not water to make sure we keep hydrated?

The idea here is that everything seems better if you are in love.
With love, this is a fantastic summer day in a beautiful place.
Without love, it’s just sand in a whole lot of uncomfortable places.
Same with taste, water is water – unless you're in love.
One final point that reflects just how complex taste is – did you know that being in love makes water taste sweeter? Participants in a December 2013 experiment were asked to think or write about love, hate, or jealousy. Then they were asked to describe the taste of a new product (really just distilled water). Those who wrote or thought about love rated the water to be sweeter than those who contemplated hate or jealousy.

It seems that the brain pathways for rewarding feelings in love and in consuming sweet are the same. You can’t discern between the two, and one can stimulate the other. So when you say you love eating sweets, maybe you really do!

 Next week, we can go further into taste. Do people who are supertasters taste good or taste well?

Newman L, Haryono R, & Keast R (2013). Functionality of fatty acid chemoreception: a potential factor in the development of obesity? Nutrients, 5 (4), 1287-300 PMID: 23595136

Pepino MY, Love-Gregory L, Klein S, & Abumrad NA (2012). The fatty acid translocase gene CD36 and lingual lipase influence oral sensitivity to fat in obese subjects. Journal of lipid research, 53 (3), 561-6 PMID: 22210925

Keller KL, Liang LC, Sakimura J, May D, van Belle C, Breen C, Driggin E, Tepper BJ, Lanzano PC, Deng L, & Chung WK (2012). Common variants in the CD36 gene are associated with oral fat perception, fat preferences, and obesity in African Americans. Obesity (Silver Spring, Md.), 20 (5), 1066-73 PMID: 22240721

Chan KQ, Tong EM, Tan DH, & Koh AH (2013). What do love and jealousy taste like? Emotion (Washington, D.C.), 13 (6), 1142-9 PMID: 24040883

MacDonald CJ, Meck WH, & Simon SA (2012). Distinct neural ensembles in the rat gustatory cortex encode salt and water tastes. The Journal of physiology, 590 (Pt 13), 3169-84 PMID: 22570382

For more information or classroom activities, see:

Umami –

Fat taste –

Water receptor –

Wednesday, January 22, 2014

A Taste Of Things To Come

Biology concepts – gustation, taste papilla, evolution, defense

The miracle berry is the fruit of the Richadella dulcifica bush
from West Africa. The plant also goes by the name Synsepalum
dulcificum. Why it would have two scientific names, I have
no idea. It has been chewed before meals in Africa for
hundreds of years. Now we have flavor-tripping parties
in the US to have fun with its properties.
Miraculin, what a great name for a protein! Of course, with a name like that it better do something pretty special. Miraculin is the active molecule in the Miracle Fruit, the favorite classroom activity of middle school science and high school biology classes everywhere. The Miracle Berry is the common name for the fruit of the West African plant, Richadella dulcifica.

For those of you who haven’t done this in class, here’s what happens. You eat the berry, and then try a slice of lemon. It tastes sweet! But the berry didn’t taste sweet when you ate it. Try a sour patch kid candy – it tastes sweet too! The effect lasts about an hour and it feels weird; your brain expects one thing yet experiences another – it’s like an optical illusion for your mouth. Biologically, this is a lot of chemistry just for taste. You get the sugar, protein, fat, or salt from what you eat whether you taste them or not, so is it important to taste things?

It must be important to taste things, or else we wouldn’t do it. Gustatory sensation is more than just a little complicated at the cellular and molecular levels, so it must play an important role in the survival and evolution of many species, otherwise it wouldn't be worth the costs.

We see things to find food, avoid predators, or find mates. We hear things to localize predators/prey or to find our kin when we can’t see them.  Smell is central for communication amongst species (pheromones) and for sensing danger (like smoke). But these examples describe gaining information at a distance, and are important for communication and safety. Does gustation fit into any of these categories?

Tasting something can’t be done from a distance – humans have to stick the target in their mouths – so what’s the big deal? It’s important because your brain is asking the question, “Should I swallow what’s in my mouth?" "Is this O.K., or is it going to kill me?”

Evolution has honed our brains to crave those things we need and spit out those that will do us harm. Sweet foods translate as energy – your brain says, “Eat this, it has carbohydrates – you need those.” What would be the best way for your brain to convince you to eat what is good for you? It bribes you with a pleasant payoff; we perceive it as tasting good, and we want more.

The olfactory neurons stick through the cribiform plate
in the nasal cavity. These neurons are raw nerve endings,
with receptors for different molecules. As such, they are
the only place where your central nervous system comes
into direct contact with the outside world, and they have
more receptor diversity than any other part of the body.
Just think of how many different smells you can recognize.
There aren’t that many receptors; different combinations
give you different odors.
On the other hand, babies don’t like sour or bitter. In terms of evolution and survival, nature is telling us to stay away from these tastes. Plants that make toxic chemicals are often bitter, so our primitive brain tells us that bitter = poison.

Rotting foods are acidic; acids are often the by-products of contaminating bacteria and fungi. Therefore, our old brain tells us to stay away from sour (acidic) foods. The gustatory sense is definitely protective. As humans, we can use our large brains to evaluate other cues as to food safety, so we can learn to like bitter and sour tastes; most animals just go with what Mother Nature tells them.

Gustation is a direct chemosensory process. Molecules to be tasted must come into direct contact with the sensors (receptors) in the mouth. This is similar to your sense of smell, but with one distinction; the chemicals you smell are volatilized in the air. For example, you don’t smell a rose by sticking it up your nose, the rose scent molecules traveling in the air from the flower to your olfactory receptors high in your nasal cavity. Smell is distance chemo-sensing.

For taste, the target molecules to be sensed are carried in liquid, not air. You take a bite of something, chew it up to release some of the molecules, and they mix with your saliva. Saliva is more than 99% water, and it is the water they delivers the dissolved (water-soluble) molecules to your taste receptors. Our taste receptors respond to things that dissolve in water or fat. Things like vanilla, cinnamon and spices are not soluble in water, but they are in fat. Hurrah for fat!

On the left side are the different kinds of papillae on human tongues.
Foliate papillae are found only around the outside edge of the vallate
papillae. The filiform papillae do not have taste buds associated with
them. Note that how all the taste buds are located on the sides of the
papillae. This is where saliva will pool and be in place to activate the
receptors. On the right is a cat’s tongue. The filiform papillae are
longer and look like a comb, which is how they are used.
Notice that it isn’t our taste buds that sense the molecules. Taste buds don’t sense anything, they are just houses for our taste receptor cells. And the houses are located in neighborhoods called papillae. There are four types of papillae in the mouth; fungiform, foliate, circumvallate, and filiform. However, only the first three have taste buds associated with them.

Papillae are basically mounds of epithelial tissue that stick up from the surface of the tongue (see picture to left). Those with taste buds tend to be round or mushroom shaped, while filiform papillae are cone shaped and tend to point toward the back of the mouth. 

Filiform papillae are the most numerous, but are not directly involved in taste; they increase the tongue’s friction to help move foods toward the throat and to help break up food to release the taste molecules. In different animals they can have different shapes; in cats they are long and spindly, and though they feel like sandpaper, they are useful for grooming.

Taste buds are found only on the sides of the papillae; the food molecules must dissolve and move into the crevices between papillae. The taste buds themselves are small packages of gustatory receptor and supporting cells.

The cartoon shows how taste buds are constructed. Only the microvilli
are in the pore, so they “taste” the chemicals that reach the pore. At the
base of the receptor cells are attached to neurons. When a receptor cell
is depolarized along its membrane, it transfers that depolarization to
the neuron and an action potential is moved to the brain. The right is a
micrograph of actual taste buds. I thought it would be good to see the
real thing and compare it to the cartoon.
The receptor cells are housed below the surface, but have microscopic cytoplasmic projections (microvilli) that stick out of the gustatory pore and sample the chemicals that are washed over them (see picture). The taste receptor molecules are located all over the microvilli.

Each receptor corresponds to one taste sensation, so each receptor cell responds to just a single taste. It is the combination of all the receptors cells activated and the intensity of their activation that leads to complex tastes. We use to think that specific taste receptors were limited to certain areas on the tongue. But now we know that specific taste receptors are more concentrated in certain areas, but are present everywhere. For instance, you can sense sweet everywhere on the tongue, but sweet receptors are most concentrated at the tip.

When the particular tastant (the molecule that activates the receptor cell), fits into the taste receptor on the microvilli, it sends a single to a nerve which is embedded at the base of the cell. The more receptors that are engaged by tastant, the bigger the signal. This signal leads to a neural action potential that travels along the taste neurons to the brain, where they are converted to our sense of taste.

The receptors fit with their ligands (the tastant molecule) in a lock and key arrangement, although often more than one ligand will fit into a receptor. For example, the sweet receptor is a heterodimer (made from two different parts, called T1R2 and T1R3), and sucrose fits well into the receptors and is sensed as sweet. However, lactose (the sugar in milk) doesn’t fit as well, so it is sensed as less sweet.

Fructose is a great fit, so it is sensed as more sweet than sucrose. This is probably why high fructose corn syrup is added to everything today, it satisfies our craving for sweet better than regular sugar does. Artificial sweeteners are thousands of times sweeter than sucrose because they bind to the sweet receptor more tightly. Therefore, you can use a lot less of the sweetener than you would use of the sugar – and no (or few) calories.

This image is from the 2011 PNAS paper on miraculin
function. The two colors of the MCL protein in the top
cartoon represent the two shapes of MCL, one at neutral
pH and one at acidic pH. In the neutral pH conformation
(shape) the receptor is not activated, but it is occupied.
This is why you see in the bottom picture you see that
artificial sweeteners cannot activate the receptor after
the berry is eaten.
So we have two parameters that regulate our sense of taste; 1) how many receptors are activated at one time, and 2) how good a fit the molecule makes with the receptor.  So now that we know about taste receptors and action potentials, how might miraculin make sour things taste sweet?

A 2011 paper from the University of Tokyo has started to let us in on the secret. Remember that miraculin doesn’t make bitter things taste sweet, or salty things taste sweet, only sour – and sour things are acidic. It seems that it’s the acid that makes the difference. By manipulating the pH of the mouth, the researchers showed that miraculin has no flavor at neutral pH, but as the pH of the mouth decreases, the sweet taste increases.

So you eat something sour (acidic) after the miracle berry, and the pH of your mouth drops. Now the acid tastes sweet. The hypothesis is that miraculin binds to the sweet receptor in the lock/key fashion, but the shape of the protein doesn’t activate the receptor. But when the pH drops, the shape of the miraculin protein changes (protein folding is very much affected pH), and it activates the sweet receptor. This sends an action potential to the brain and we perceive sweetness. Anything that lowers the pH of the mouth is perceived as sweet. Sweet!

Gymnemic acid is isolated from another plant, but it suppresses the
sweet receptor action. The powder on the right is the isolated form, from
the plant, although it is not pure gymnemic acid. There are more
active compounds in the plant, but gymnemic acid is the most
potent one. It has been sold as a diet aid and now there is some
evidence that it may be active against diabetes.
There are other molecules that have the opposite effect. Eat some gymnemic acid for instance and then try a piece of chocolate. It won’t taste sweet at all - not a big money maker for the food industry. But there may be a way to use gymnemic acid. A study in India from 2012 says that treating early diabetic rats with gymnemic acid will even out swings in their blood sugar and will prevent early kidney damage. However, this effect may be due to the fact that gymnemic acid is an antioxidant, not to its ability to antagonize sugar receptors. Keep this in mind – we will come back to it in a couple of weeks.

We have more to say about our sense of taste, like what a supertaster is, and how we keep adding new tastes – no longer are we confined to just sweet, salt, sour, and bitter!

Koizumi, A., Tsuchiya, A., Nakajima, K., Ito, K., Terada, T., Shimizu-Ibuka, A., Briand, L., Asakura, T., Misaka, T., & Abe, K. (2011). From the Cover: Human sweet taste receptor mediates acid-induced sweetness of miraculin Proceedings of the National Academy of Sciences, 108 (40), 16819-16824 DOI: 10.1073/pnas.1016644108

Baig, M., Gawali, V., Patil, R., & Naik, S. (2011). Protective effect of herbomineral formulation (Dolabi) on early diabetic nephropathy in streptozotocin-induced diabetic rats Journal of Natural Medicines, 66 (3), 500-509 DOI: 10.1007/s11418-011-0614-y

For more information and classroom activities, see:

Taste sensation –

Miraculin –