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Have you been curious about something you observed and want to know why? Ask JM! In this series we answer readers' questions. So do write in to us at jantarmantar.why@gmail.com. Be sure to write your name, class and address so we can publish it along with your question.
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I was late for school. I just brushed my teeth and decided to eat an orange before rushing to catch the bus. But the orange was very bitter, so I only ate a piece. When I came home and complained to my mother, she told me that she had eaten the rest of the orange and it was fine. How could that be?
Answer: The cause of the problem, the culprit, is a chemical called sodium lauryl sulphate (SLS), and it is found in many many home-care products. For example, you’ll find sodium lauryl sulphate in foamy, bubbly and soapy products, including: Facial cleansers, Body washes and shower gels, Shampoos, Hand soaps, Detergents, Bubble bath, Shaving cream, and Cleaning products for your home. Apart from toothpastes, of course!
One of the reasons you’ll find it in so many different products is because it’s cheap to produce, abundant, and it works well to create products that foam, lather and bubble.
SLS is what’s known as a “surfactant.” This means it lowers the surface tension between two liquids, or between water and fat. This is why it’s used as a cleansing and foaming agent. Surface tension is the ability to resist an external force due to the cohesive structure of molecules. Water molecules are bound together by strong electrostatic hydrogen bonds (the hydrogen in H2O bond with each other). This allows the surface of water to behave like a cohesive whole---like a cloth---so that for example insects can walk on it. This also makes it difficult to get rid of oily dirt on clothes since water is not able to enter the pores in the cloth and shake the dirt out. 
So a surfactant lowers this surface tension and allows two particles that usually come together to be separated; it also helps create bubbles. That's why you will find sodium lauryl sulphate in washing soap, shampoos as well as toothpaste.
Why does this make oranges taste bitter? Our taste buds can taste five different kinds of taste. The obvious ones are sweet, bitter, salt and sour. (Actually the last two are not so much tasted by the taste buds but the tongue has ion pathways that help detect them). The last taste is called umami, not so common in Indian cooking, but made popular by the monosodium glutamate (MSG) in instant noodles. 
Surfactants do two things to the taste receptors in our tongues. It makes it difficult to taste sweet things, and makes it easier to taste bitter things. The mystery is almost solved: after brushing your teeth, and especially if you have not washed out your mouth well because you are in a hurry, some toothpaste is left in the mouth. The sodium lauryl sulphate molecules stuff themselves into the sweet receptors, so if you eat something sweet, the surfactant blocks them and you cannot taste sweet.
At the same time, sodium lauryl sulphate interacts with compounds called phospho-lipids on the tongue and this dampens down the bitterness receptors. Lipid is a fat and surfactants are designed to break down fats. So the bitter taste of an orange (prominent in varieties like Narthangai) gets enhanced, and so you will find the orange not only not-sweet, but also bitter!
The lesson is to wait for a while after brushing your teeth before eating an orange, or to find a toothpaste that does not contain sodium lauryl sulphate!
Source: https://www.sheffield.ac.uk/chemistry/schools/edible-experiments