Why flavorings smell and taste the way they do
By Mark Sabini, Justin Yu, and James Lee
Imagine a cave of darkness lined with white pearly stalactites and stalagmites. On the ground is an organic carpet of living red, and there is an uncomfortable dampness in the air. Liquid flows all over the ground, making navigation difficult. Now imagine moving to another location with two doorways, side by side, carrying breezes of air in and out. They both look the same, mirror images of each other. Peering inside, you see what seems to be a grass of some sort, except that it grows all around the surface of the doorway, not just on the ground. You step inside, into a tunnel of blackness, and then whoosh! A strong breeze sucks you in, into oblivion. Now reflect on where you have just been: a human mouth and a human nose. Working together, these organs make up 40% of the five senses, and give flavor to food and aroma to flowers. How exactly do these marvels of nature function?
Taste and Smell
Taste relies on the transformation of chemicals from food (tastants) into nerve signals. The tongue’s mucous membrane is lined with tiny bumps called taste papillae. These little sensors responsible for taste perception consist of taste buds with sensory cells. The sensory cells form a little “cup” (see picture), which contains taste hairs. The proteins on the surface of the taste bud bind the chemicals from the food to the sensory cells for tasting. Then, the sensory cells transform the chemical signals into electrical signals that travel along nerves to the brain. Contrary to popular belief, all parts of the tongue are equally sensitive to the five tastes, sweetness, sourness, umami, bitterness, and saltiness, except the back of the tongue. The cells there are more sensitive to bitterness, most likely as a natural defense against swallowing bitter plants containing poisons.
|Figure 1: Diagram of a typical taste bud|
|Figure 2: Video about taste|
Smell works slightly differently from taste, but still employs the same basic principle: the transformation of chemical signals to electrical signals. At the top of every person’s nasal passage is a patch of neurons that is exposed to air. The neurons have small hairlike projections called cilia, to which volatile food chemicals (odorants) bind and cause transmission of electrical signals along the neurons to the olfactory bulbs on the underside of the frontal lobe. As a result of this mechanism, only volatile compounds will have smells. While they might seem unrelated, taste and smell work hand in hand. The data from the two senses is merged in the insula, a part of the brain, to put together what we know as flavor. This is why you cannot sense flavor when your nose is blocked – the smell component of flavor is missing.
|Figure 3: Diagram of the Olfactory System. The patch of neurons located at the top of the nose feed into the olfactory bulb which sends signals directly to the brain.|
The Chemistry of Flavorings
Esters are a class of organic compounds that have distinctive tastes and smells, and are used widely in the food industry.The ester octyl acetate, for example, is responsible for the characteristic citrus smell and taste of oranges. Esters can be prepared through Fischer-Speier esterification, in which a carboxylic acid and alcohol are reacted in the presence of an acid catalyst. The strong smell of esters is attributed to their volatility and the functional groups attached to the ester group. A list of commonly-used esters and their tastes and smells is shown below.
|Figure 4: Fischer Esterification||Figure 5: Chemical structures of some common esters|
Other flavoring ingredients come from more natural sources, albeit quite strange ones. One example is castoreum, a brown, slimy secretion from the castor sacs located near a beaver’s anal glands. It has a vanilla taste after some processing, as it is originally mixed in with beaver urine and other impurities from the animal. Castoreum contains chemicals such as phenols and ketones, all contributory towards the smell and taste. According to MSU flavor chemist Susie Bautista, castoreum is not used much (since it is not kosher), but greatly enhances vanilla flavor. A similar chemical to castoreum is vanillin, a phenolic aldehyde. Vanillin is a component in both natural and artificial vanilla flavoring. Occurring in natural vanilla pods, vanillin can be extracted from the natural source. Otherwise, it can be synthesized from a chemical called guaiacol and glyoxylic acid. Chemical synthesis accounts for the vast majority of vanillin production in the world.
|Figure 6: Left: Castor sacs from a beaver and castoreum. Right: Vanilla Pod|
|Figure 7:Vanillin and4-Ethylphenol, 1,2-dihydroxybenzene, and 3-hydroxyacetophenone: three components of castoreum; there are clear similarities between the chemical structures which accounts for their similarity in taste/odor|
|Figure 8: Chemical synthesis of vanillin from guaiacol|
Another chemical that has gained much publicity is monosodium glutamate, MSG for short. MSG contains around 80% glutamic acid and contributes to the taste ‘umami,’ which is a taste found in cured meats, mushrooms, and cheeses. MSG is used as a flavor enhancer to greatly improve the flavor of many foods. It does so by providing glutamic acid to bind to the glutamate receptors on the tongue, allowing food to taste “meatier”, and therefore tastier. However, its wondrous properties do not come without negative effects. MSG can lead to eye damage, depression, and a whole host of other problems, as well as cause “Chinese Restaurant Syndrome,” which affects MSG over consumers with headache and asthma. This chemical is one example of why are chemical food flavorings require careful scrutiny. The benefits of taste may be outweighed by the health related downsides.
|Figure 9: Structure of glutamic acid, a major component of MSG.|
Smells and tastes are caused only by the chemical binding of compounds to receptors in the tongue and nose. In order to be smelled, a chemical must be volatile, or no odor components will enter the nose. Many chemicals play important roles in tastes and smell. For example, esters are a class of organic compounds with strong smells commonly found in fruits such as bananas and apples. Other flavor compounds include vanillin and castoreum, which are both used as vanilla flavorings. While both compounds emulate the same flavor, vanillin, a phenolic aldehyde, is found naturally in vanilla pods, while castoreum, a mixture of phenol and ketones, is produced in beaver castor glands. These compounds easily highlight the ability to “fool” the taste buds. Finally, monosodium glutamate, MSG, is a chemical responsible for the umami taste in foods. Its numerous adverse effects show the potential dangers of using flavorings and the need for in-depth analysis of the effects of flavor compounds on humans. The complexity of flavors highlights the need for humans to scrutinize the additives in their food. Who knows – the next vanilla iced coffee you drink might not actually have vanilla.