Science Makes Sense-Week 27: Esters, pheromones, artifical flavorings.

I remember that corner store near our house in New Delhi, India. There was a glass bottle with colored sweets: the one shaped like an orange tasted like an orange, the one looking like a banana tasted almost like eating a banana. I loved staring at those candies and once in a while my mother would relent and get me some!
Years later I learnt about artificial flavorings and how fast-food companies spend millions on researching different chemicals to enhance flavor and entice us all to buy them. Those candies in that shop were just sugar, water and artificial flavoring.
Most of the chemicals that enhance flavor are esters. An ester is nothing more than an organic molecule created by a reaction between an alcohol and an organic acid of general formula RCOOR’. Just like in inorganic chemistry when an acid reacts with a base you get a salt and water, when an organic acid, say like acetic acid reacts with an alcohol like methyl alcohol you get methyl acetate and water. The methyl acetate, or correctly called methyl ethanoate,(Ref.1) is the ester, akin to the ‘salt’ in the field of inorganic chemistry. The IUPAC name for acetic acid is ethanoic acid so the ester is methyl ethanoate. Of course the methyl part comes from the alcohol. Low molecular weight esters are soluble in water, but as the molecular weight becomes bigger, as in the case of fats, they are insoluble in water.
There are many common esters that are used for artificial flavoring, but methyl ethanoate is not one of them. Even though it has a fruity odor, it is a colorless solvent, used in lacquers and paint thinners and in pharmaceuticals.(Ref.3) The common esters that are used for artificial flavoring include the following. I am not using the IUPAC names, but the familiar names:

Ester Flavor
Ethyl formate Rum
N-amyl acetate Pears, bananas
N-octyl acetate Oranges
Methyl butrate Apples
Ethyl butrate Pineapples
N-amyl butrate Apricots
Methyl salicylate Oil of wintergreen
Linalyl acetate Lavender, sage(Ref.4)

The food industry is so beholden to esters and strange, unknown flavors are being discovered by playing around creating new esters. Our attraction and desire to eat inexpensive fast-food is enhanced by the flavors they have and some food chemists are focused in the world of esters.
Some esters may be found in pheromones. What are pheromones? They are chemicals that could be released by insects, bacteria or animals to elicit a response from another of its species. It is a means of communication and the response could be territorial,cooperative or sexual in nature.(Ref.5) Amyl acetate, an ester that can mimic the flavor of bananas or pears is also produced as an alarm pheromone in honeybees.(Ref.6)
Esters are also big in the polymer industry. A few weeks back we looked at polymers and polyester, the fabric that does not need to be ironed is here to stay. This is a polymer of an ester. Dacron, another material used to make clothing is made from esters.(Ref.6)
When some esters have the element nitrogen, they form explosives, while phospho-esters are the building blocks for DNA. Some large chain esters are also fats.
Truly esters play a significant role in our lives from creating flavor to causing love!(Ref.6)
Activities for Middle School Teachers
Using model kits for making organic molecules, create simple ester structures starting with acids like formic acid and different molecules of alcohols like methyl, ethyl, propyl alcohols. Introduce isomers wherever possible. How many isomers can you obtain with ROH (general formula of an alcohol) where R is a carbon chain from 1 to 10? How many esters have different flavors when formic acid is replaced by, say, acetic or higher carbon chain acids?
Take a field trip to the supermarket and focus on processed foods. How many of these foods contain flavorings, etc in their ingredients? How many can the students recognize as esters? Look at the perfume/ cosmetics section as well for esters.
Learn the difference between natural and synthetic perfumes. There are perfumes like oil of sandalwood using sandalwood obtained from sandalwood trees in South India; at the same time there are many perfumes synthesized in the laboratory by reacting an organic acid with an alcohol.
Nuggets of Information
Pineapple flavoring contains 20 ingredients that cause that flavor along with ethyl butrate, an ester.(Ref.7)
More about pheromones and esters:(Z)-6-dodecen-4-olide, a circular(12-carbon with a double bond) ester, is associated with the black-tailed deer and is called a “social scent”. Circular esters (called lactones) are also found in the oily poisonous secretion of termites..(Ref.7)
Actually making an ester is not as easy as making a salt. You need a catalyst like sulfuric acid to complete the reaction.(Ref.8) Sometimes, even with the catalyst, the products formed may be 70%. In order to make it 100%, it is advisable to keep removing the other product formed which is water.(Ref.9)
It is interesting to note that while esters are fragrant and responsible for many flavors, the alcohols and carboxylic acids that are the starting components may not be so desirable. For example, methyl butrate gives apples that familiar taste, but methanol is poisonous, and butyric acid has a smell that gives rancid butter its odor – just goes to show how a chemical reaction can effectively change chemical properties! (Ref.4)
Many natural flavors are quite complex. For example, when you are eating a chocolate cake made with real chocolate or eating a sweet flavored with hazel nuts, your sense of smell and taste are hit by several chemicals that combine to produce that amazing, almost sensual feeling inside you as you slowly savor and relish the food! In the case of fruity odors, this can be replicated somewhat using some esters. The trick is in the exact amounts added to not overwhelm but suggest those smells and taste. (Taste and smell are closely linked; when you have a cold, nothing tastes good.) Also remember that for the smell it is important that the ester is volatile so the molecules can enter your olfactory organs.(Ref.10)
Most perfumes produced in the laboratory use esters of low molecular weight.

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