Thursday, March 20 2008
Smell and Taste
I can't believe he had that much wine in his nose
Continuing on our series about sensory evaluation and tasting wine. First we talked about human senses and then about how your brain interprets (or in some cases, misinterprets) those sensations and now today we'll talk about how tastes and smells get from your glass, through your senses to your brain.
Who knew it was so complex?
Gustation: Taste
Gustation is a chemical sense. It involves the detection of stimuli dissolved in saliva by the taste buds. While located primarily on the surface of the tongue, taste buds are also found on the soft palate, tonsils and upper 1/3 of the esophagus.
17th century-Italian scientist Marcello Maphigi recognises papillae on the tongue were organs of taste. In 1867 two German scientists described how tiny structures embedded in and around the papillae on the tongue were the actual mediators of taste.
Say 'aaahh!'
You cannot actually see 'taste buds'. The visible structures of the tongue are called papilla, and resemble round volcano-like structure. Each one contains 250 -270 taste buds, for a total of over 10,000 taste buds in the average human. Taste buds are minute, and at least 60 could fit on the period at the end of this sentence. Since these mounds of overlapping taste cells look like the petals of a flower, they were named 'taste buds'.
Taste buds are in a constant state of flux. Although they seem to us to be of unchanging composition, all 10,000 taste buds are shed and regenerated every 10 days. As amazing as these structures are, they are capable of perceiving only five different things: Sweet, Sour, Salt, Bitter, and Umami. For those surprised by a fifth taste element, umami is the flavour that distinguishes toast from bread or a nice grilled steak from raw meat, and is most easily perceived in pure crystalline monosodium glutamate (MSG). This savoury character in wine is often derived from lees contact in whites, or oak exposure in reds.
To make things even weirder, it's now generally accepted that human beings can also perceive lipids through the gustatory system--we can detect fat on our tongues. This is probably the reason why ice cream, butter and french fries are so delicious. Of course there's no lipids in wine, unless you're trying to ferment bacon grease. Mmmmm!
What we taste varies greatly. Humans detect tastes at the following concentrations:
Sweetness: 1 part in 200
Saltiness: 1 part in 400
Sourness: 1 part in 130,000
Bitterness: 1 part in 2,000,000
This high acuity for bitterness appears to be an evolutionary defence that keeps us from swallowing poisons, as most organic alkaloids are very bitter. It is interesting to note that women often greatly exceed the 1 in 2,000,000 norm for average humans in detecting bitterness, a supposed evolutionary adaptation for the gathering part of 'hunter-gatherer'.
Sensory Threshold
Not only do our tastes have different acuities but also different thresholds. Sensory thresholds are the limits of an individual's sensory capacities. There are four types of sensory thresholds, which if you taste a cup of water with some added substance such as salt or sugar may be described as such:
- Absolute threshold: Recognition that there is some additive.
- Recognition threshold: The additive can be named (sweet/salty, etc.)
- Difference threshold: How much of a stimulus is required thereafter to distinguish a difference between samples.
- Terminal threshold: The level at which the difference threshold is no longer perceived (i.e., the taste sense is overwhelmed).
Human beings have varying levels of taste acuity. The good news is that your palate is like a muscle, and tasting is a skill, like playing the piano or hitting a great serve in tennis: the more you practise, the better you're going to get. So, my advice to novice wine tasters is to begin drinking frequently and heavily. (Kidding! Just drink lightly.)
Olfaction: Use The Force
Smell is incredibly powerful. Have you ever noticed that when you have a cold nothing tastes very good? Without the ability to smell, everything we eat or drink is dull and lifeless. If you hold your nose when sampling wine it's difficult to distinguish the sample from plain water. Smell hits us faster than taste; it takes 25,000 times more molecules of cherry pie to taste the cherry character than it does to smell it.
Where It All Happens: The Olfactory Bulb
A single cell runs from the olfactory mucosa in the nose directly into the olfactory bulb in the brain. It is this organ that receives the chemical compounds that induce the perception of aromas. In humans, it is very small–no larger than a postage stamp–and consists of two lobes of yellowish tissue at the base of the brain. In lower vertebrates, the lobes are much more massive; in sharks they dominate the entire brain.
I wonder where he left his teeth?
Humans possess approximately 10 million olfactory receptor cells. Each receptor end is a swelling, or knob with approximately 5 hairs (cilia). Unlike neurons in the brain, if damaged the receptors will regenerate. All 10 million will regenerate every 4-5 weeks.
The colour of the olfactory bulb is an interesting indicator of its relative acuity in humans it's light yellow, cats mustard brown and a fox reddish brown. The deeper the shade, the more acute the sense of smell. Animals with the keenest sense of smell tend to walk on all fours with heads close to the ground–or cruise the ocean in big toothy packs!
About 95% of the nasal cavity is unrelated to smelling (it's more like a large air conditioner/treatment plant, warming and moistening air destined for the lungs). A very small amount of air actually reaches the olfactory bulb. We have to inhale, and increase the velocity of our air intake to smell things.
Unlike the eyes and ears the nose is not triggered by energy. I sight photons carry energy to the retinas, in hearing sound waves are a form of energy
hitting the eardrum. The nose is triggered by pieces of matter which
have mass. Sniffing increases the air velocity by 3-4 times, which in turn increases the amount of impact the molecules will have on the olfactory bulb. That's why a good sharp sniff is important in wine tasting, and why dogs stick their head out of car windows. It's the canine equivalent of turning up the volume (and perhaps speed-reading as well).
Lighter molecules travel farther. Large sluggish molecules (candle wax) have less smell than lighter molecules (lavender, violet petals). Not everything has a smell. Stone, glass, and steel do not release
molecules that evaporate at room temperature. Only substances volatile
enough to release microscopic particles into the air have aroma.
Heating a substance increases its smell (simmering stew smells better
than cold stew) by driving off more molecules.
Salad, nom nom nom!
It is a common misperception that humans have a poor sense of smell. There are two facets to olfactory sensitivity, acuity and discrimination. It is true that we are abysmal at acuity judged against animals. For example the threshold for acetic acid (vinegar) in humans is 50 billion molecules per millilitre of air. In dogs, this threshold is less than a half-million molecules per millilitre of air. Thus, dogs perceive acetic acid at 100 million times lower concentration–by that measure their sense of smell is 100 million times better than ours.
However, humans are remarkable discriminators. Trained professionals (i.e. wine tasters) can distinguish thousands of odours and retain memory of each from session to session, and can analyse and compare these odours. Thus a palate is born!
So, if we have the senses, the brain to interpret them, and knowledge of how they're put together, what do we do with all that? We drink! Tomorrow, formal wine tasting, the UC Davis scale (how much does UC Davis weigh, I wonder?) and how to score water.
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