Key concepts
Taste
Perception
Senses
Food
The brain

Introduction
During the holidays we often find ourselves surrounded by a wide variety of taste sensations. Have you ever wondered how well we sense different tastes? People are generally able to discern five basic tastes: sweet, umami (also known as savory), salty, sour and bitter. Is it easier to detect some of these flavors compared with others? In this science activity, you (and maybe some friends or family) will find out by exploring your taste thresholds for sweetness, saltiness and sourness. Get ready to find out how low you can go!

Background
Our sensory system for taste is remarkably sensitive. Not only can we detect substances at extremely low concentrations, we can also differentiate between molecular compounds that are closely related. For example, we can distinguish between different stereoisomers, which are molecules that are made of exactly the same components, but are mirror images of one another in their structure. The artificial sweetener aspartame is an example of this—it tastes sweet to us, but its stereoisomer (its opposite) does not. 

This amazing sensitivity is made possible by our taste buds. Taste buds, located on small bumps on the tongue called fungiform papillae, are each made up of about 50 to 150 taste receptor cells. On the surface of these cells are receptors that bind to small molecules related to flavor. Through sensory nerves, the receptors relay the taste sensation information to the brain. This process allows us to discern five basic tastes.

Materials
• Measuring spoons
• Water, preferably distilled
• 12 paper or plastic cups
• Permanent marker
• Kitchen scale or measuring spoons
• Granulated sugar or sucrose
• Table salt
• Vinegar
• Spoons
• Cotton swabs
• Paper towels
• Piece of paper and pen or pencil (optional)
• Taste-test volunteers (optional)

Preparation
• Pour 6 tablespoons (tbsp.) of distilled water into a paper or plastic cup. Add 10 grams of sugar (or about 2 1/2teaspoons (tsp.)) and stir until the sugar is dissolved. This gives you a 10 percent sugar solution, approximately. Label the cup.
• Pour 2 tsp. of the 10 percent sugar solution into a new cup. Add 6 tbsp. of water to it and stir. This gives you a 1 percent sugar solution. Label the cup. 
• Repeat this dilution process (diluting 2 tsp. of the previous solution in a new cup with 6 tbsp. of water) to make 0.1 percent and 0.01 percent sugar solutions. These are called serial dilutions. Be sure to label the two new cups. What do you think is the lowest concentration you'll be able to taste the sugar in?
• Repeat these steps (using clean utensils) to create salt solutions that have concentrations of 10 percent, 1 percent, 0.1 percent and 0.01 percent. Label the cups. For 10 grams of salt, you can use 1 3/4 tsp. of salt. What do you think is the lowest concentration you'll taste the salt in?
• Again repeat the steps (using clean utensils) to create vinegar solutions that have concentrations of 10 percent, 1 percent, 0.1 percent and 0.01 percent. Label the cups. Use 2 tsp. of vinegar initially. What is the lowest concentration you think you'll taste the sour vinegar in?

Procedure
• Rinse your mouth with plain water and wipe your tongue dry with a clean paper towel. Dip a clean cotton swab into the 10 percent sugar solution and smear it all around the surface of your tongue. Can you taste the sweetness?
• Repeat the previous step to test the 1 percent, 0.1 percent and 0.01 percent sugar solutions, rinsing your mouth and wiping your tongue before testing each solution. Which solution is the lowest concentration at which you can still taste the sweetness? This is your approximate taste threshold for sugar. You can write this down to remember later.
• Rinse your mouth with plain water and wipe your tongue dry with a clean paper towel. Dip a clean cotton swab into the 10 percent salt solution and smear it all around your tongue. Can you taste the saltiness?
• Repeat the previous step to test the 1 percent, 0.1 percent, and 0.01 percent salt solutions. Which solution is the lowest concentration at which you can still taste the saltiness? This is your approximate taste threshold for salt. You can write this down.
• Rinse your mouth with plain water and wipe your tongue dry with a clean paper towel. Dip a clean cotton swab into the 10 percent vinegar solution and smear it all around your tongue. Can you taste the sourness? Repeat this process to test the 1 percent, 0.1 percent and 0.01 percent vinegar solutions. Which solution is the lowest concentration at which you can still taste the sourness? This is your approximate taste threshold for vinegar. You can write this down.
• Were your taste thresholds (the lowest concentration at which you could still taste the flavor) the same for all three tastes, or did you have lower thresholds for some of them? Did the solutions that were 10-fold more concentrated taste 10 times stronger?  
• Extra: Try repeating this activity using several volunteers. Compare your results. Do some people generally have lower thresholds than other people? Is there a variation in which taste has the lowest threshold for individuals in the group? 
• Extra: Recruit several volunteers in different age groups to take this threshold-of-taste test. Does taste threshold change predictably with age?
• Extra: In this activity you used 10-fold serial dilutions to roughly establish your threshold of taste. Design a test to determine your threshold with higher precision. What exactly is your taste threshold for sugar, salt and vinegar?

Observations and results
Could you taste all of the 10 percent solutions, but none of the 0.01 percent solutions? Did the sugar solutions have the highest threshold, meaning you could only taste it in the more concentrated solutions, compared with the salt and vinegar solutions, which had lower thresholds?

For the sugar, salt and vinegar solutions, the 10 percent solutions should be detectable by nearly everyone who tries the test, whereas almost nobody should be able to detect the 0.01 percent solutions because the concentrations are too low. The basic tastes of sweet, salty and sour have different thresholds, or concentration levels, at which they can be detected. In other words, it is easier to detect some flavors at low concentrations compared with other flavors. Taste thresholds can vary from person to person. You may have seen that the sugar solutions were harder to taste at lower concentrations compared with the salt and vinegar solutions. In other words, the sugar solutions may have had a relatively high taste threshold compared with the salt and vinegar solutions. You may have also seen that the vinegar solutions had a lower threshold compared with the salt solutions (meaning the vinegar was easier to taste at lower concentrations), but this difference can be minor and may require testing by many individuals to see a clear trend.

More to explore
Physiology of Taste, from R. Bowen, Colorado State University
Taste (Gustation), from Tim Jacob, Cardiff University, Wales
Gustatory and Olfactory Senses, from Michael D. Mann
Measuring Your Taste Threshold, from Science Buddies

This activity brought to you in partnership with Science Buddies