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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.
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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.
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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.
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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?
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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?
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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.
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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?
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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?
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?
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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.
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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?
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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.
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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?
For
more information or classroom activities, see:
Umami
–
Fat
taste –
Water
receptor –
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