Key concepts
Chemistry
Food Science
pH
Acidity

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 individual taste buds! Inside each taste bud are as many as 100 receptor cells. Each receptor is best at sensing a single sensation and sends signals about it 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 tongue’s acid detectors.

What does it mean for something to be acidic or basic? It all has to do with ions. In water (H2O) a small number of the molecules split up to form hydrogen ions (H+) and hydroxide ions (OH−). Pure water and solutions that have an equal number of hydrogen ions and hydroxide ions are neutral—that is, they are neither acidic nor 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. 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 besides measuring pH to find out if a food is acidic or not—by using baking soda. Also known as sodium bicarbonate, it is a base that reacts with acids to generate carbon dioxide gas, a reaction well known from science fair 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
Six cups or glasses
Plate
Vinegar
Soda/soft drink
Lemon juice
Milk
Dish soap
Water
Spoon
Baking soda (It must be fresh; if it is too old, it will not work as well.)
Medicine dropper or pipette
Fruits or fruit juices and other foods or drinks, baking powder (optional)

Preparation

  • Take one cup and fill it with tap water. Then add a squirt of dish soap to make a solution for testing. Note: This is the one solution that you should not taste; the rest will be okay to sample.
  • Pour a little bit of the other test solutions (water, lemon juice, vinegar, milk, soda) each into its own cup.
  • Use the spoon to cover the plate evenly with baking soda.

Procedure

  • Start with the vinegar first. Take the medicine dropper or pipette and suck up a little of the vinegar from the cup.
  • Bring the dropper to the plate with baking soda and slowly squirt one single drop onto the baking soda on the plate. What happens once the vinegar drop hits the baking soda? Can you see some bubbles?
  • Rinse your dropper with some water and then use it again to take a sample of the soft drink. Again, squirt one drop carefully onto the baking soda. Choose a different spot on the plate. Is there a reaction happening? Do you see carbon dioxide generation? How does the reaction compare with the vinegar?
  • Rinse the dropper again and move on to the next sample: Take a little bit of water and put one drop of your sample onto the plate. What happens this time? Is water acidic?
  • With a clean dropper, suck up some lemon juice and squirt a drop onto the baking soda. Choose a fresh spot that does not have any solution on it yet. How many bubbles do you see? Did it react more than the water or less?
  • Repeat the same procedure with the milk. Does the baking soda react with the milk to form bubbles? Do you think milk tastes sour?
  • Test your last sample, which is the dish soap solution. Can you tell from the reaction if it contains a lot of acids? What pH do you think the soap solution has?
  • From all your observations, can you make predictions about which of the liquids contains the most acids or would taste the sourest? You can repeat the baking soda test on all of your samples once more. Which one is the most acidic, which one is the least acidic?
  • Extra: Do some research to find out the pH of all your test solutions. Do your results agree with your findings?
  • Extra: Can you find more liquids to test? You can also use different fruits or vegetables and squeeze them to get some liquid to test if they are acidic or sour. Do a taste test afterward to see if your observations match your taste.
  • Extra: Repeat the same procedure but this time use baking powder instead of baking soda. Do some research to find out what the difference between the two is and compare the reactions with the same test solutions. Do you see more bubbles with the baking soda or with the baking powder for each of the same samples?
  • Extra: Instead of putting a drop of your test solution onto the baking soda, try to do the same test the other way around. Take a little bit of the baking soda and put it into the glass with your test solution. What happens when the baking soda comes in contact with the liquid? Is the reaction the same as when you put the solution onto the baking soda or different?
  • Extra: If you have access to litmus paper (which is used to measure the pH of a solution), measure the pH of each of your test samples. What is the pH of each of them and did all of the acidic samples form bubbles during your baking soda test?

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 vinegar. They both contain lots of acids. The juice of a lemon contains about 5 to 6 percent citric acid, which also gives the lemon its sour taste. Vinegar consists of 5 to 20 percent 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 an acidic reaction is the phosphoric acid that is added to soft drinks for preservation and taste enhancement. This is one reason why it is so bad for your teeth (besides the sugar); this acid can damage the tooth enamel.

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. When you used baking powder instead of baking soda, however, you might have noticed that the reactions for each of your test samples were 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
Pour all your test solutions into the sink drain. You can dispose of the baking soda in your trash or rinse it off with water into the sink.

More to explore
How the Tongue Tastes Sour, from Nature News
Approximate pH of Foods and Food Products, from the U.S. Food and Drug Administration
How Sour or How Sweet Is Your Lemonade?, from Scientific American
Rocketology: Baking Soda + Vinegar = Lift Off!, from Science Buddies
The Difference between Baking Soda and Baking Powder, from Phys.org
Science Activity for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

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