A Simple Sourness Detector

Introduction

Have you ever wondered why some foods or drinks taste sour, whereas others do not? You might know that your tongue is picking up all kinds of flavors, including salty, sweet, bitter, and sour. But what makes something taste sour? The sour taste is actually influenced by the pH and acids present in foods. In this activity, you will find out how sour different foods are by testing foods and drinks for the presence of acids with baking soda. What do you think is the sourest food you can find in your kitchen?

Background

Taste is detected by taste buds that line the tongue and other parts of the mouth. The human tongue has an average of 10,000 taste buds! Inside each taste bud are up to 100 receptor cells. Each receptor is best at sensing a single sensation and sends signals to the brain, which then identifies sweet, salty, bitter, sour, or umami. Different receptors are triggered by different chemical compounds (or molecules) present in the food. The sour taste receptors are triggered by acids, or more specifically, hydrogen ions (H⁺), and therefore, are the acid detectors of the tongue.

What does it mean for something to be acidic or basic? It all has to do with hydrogen ions. In water (H₂O), a small number of the molecules split up to form hydrogen ions (H⁺) and hydroxide ions (OH⁻). Pure water, or solutions that have an equal number of hydrogen ions and hydroxide ions, are called neutral and thus are neither acidic or basic. An acid is a substance that donates hydrogen ions. Because of this, in an acidic solution there are more hydrogen ions than hydroxide ions. This means the more acidic a food is, the more hydrogen ions are available to trigger the sour taste receptors. A base is a substance that accepts hydrogen ions. When a base is dissolved in water, the balance between hydrogen ions and hydroxide ions shifts the opposite way so the hydroxide ions dominate. The acidity of a solution is measured by the pH scale. By definition, a pH of 7 is neutral. Everything with a pH below 7 is acidic, whereas substances with a pH above 7 are basic or alkaline.

But there is a simpler way to find out if a food is acidic or not, besides measuring its pH: using baking soda. Baking soda is a base (sodium bicarbonate) that reacts with acids to generate carbon dioxide gas, a reaction well-known from chemical volcanos or bottle rockets. The escaping gas can be easily spotted as it forms bubbles once the acid comes in contact with the baking soda. This makes the “sour test” simple: the more gas bubbles you see, the more acidic your food is and the sourer it should taste. Test it out for yourself!

Materials

Preparation

Procedure

Observations and Results

Did you get some nice bubbles for some of your test solutions? You should have seen a vigorous bubble formation with lemon juice, as well as with the vinegar. They both contain lots of acids. The juice of a lemon contains about 5-6% citric acid, which also gives the lemon its sour taste. Vinegar consists of 5-20% acetic acid, which is the acid that reacts with baking soda to form carbon dioxide. With the soft drink, you also should have seen some bubble formation, although not as much as with lemon juice and vinegar. The reason for this is the phosphoric acid that is added to soft drinks for preservation and taste enhancement.  This is also why it is so bad for your teeth (besides the sugar), as this acid can damage the tooth enamel, the material of which your tooth is made.

Milk is a pretty neutral solution—similar to pure water—and should not have reacted with the baking soda. Soap, on the other hand, is a basic solution, which means its pH is greater than 7. It contains no acids that can react with the baking soda and should not have resulted in carbon dioxide formation. If you use baking powder instead of baking soda, you might have noticed that the reactions for each of your test samples was more pronounced. You probably even saw some tiny bubbles with the solutions that did not react with the baking soda. This is due to the fact that baking powder is a mixture of baking soda and weak acids. When the baking powder gets wet, these acids can react with the baking soda and form carbon dioxide.

When doing a taste test, you might have noticed that some foods or drinks did not taste sour at all - even if they contain lots of acids. The trick is that the sour taste is often masked with the addition of lots of sugar. One good example is the soda drink that you tested. Look at its ingredient list – how much sugar does it have? Do you think you would still like it without the sugar?

Cleanup

More to Explore

• How the tongue tastes sour, from Nature News
• The difference between baking soda and baking powder, from Phys.org
• Rocketology: Baking Soda + Vinegar = Lift Off! From Science Buddies
• Science Activity for All Ages!, from Science Buddies

Credits