Chemosenses

From Experiment Central, published by U·X·L

People depend on taste and smell to recognize a delicious meal, but these senses also play a key part in survival helping keep us alive. Both senses can warn us of trouble and both are linked to what we eat. Pleasant tastes and smells ensure that a person or animal continues to eat and acquire energy from foods. Unpleasant tastes and smells are one way to ensure a person does not ingest poisons or other materials that can harm him or her.

People get information about the world around them through their senses of hearing, touch, sight, taste, and smell. Each of these five senses is tuned to a specific sensation. You are always using at least one of your senses. The senses send messages to the brain, which processes the information. Taste and smell belong to the chemical-sensing system group, known as chemosenses, which means that the sense is stimulated by specific chemicals. These chemicals trigger a nerve signal to the brain that then "reads" the signal.

How taste works

When people say something tastes good, they are usually referring to the flavor of the food or drink. Flavor is a combination of taste, smell, texture, and other characteristics of the food itself, such as temperature. The sense of taste is complex because it is so intricately linked with flavor and weaves in many of the other senses, especially the sense of smell. There are five basic tastes: sweet, sour, salty, bitter, and umami (pronounced oo-MAM-ee). Umami was described in the early 1900s, but only in the late 1990s did food researchers officially recognize it as a distinct taste. Umami is the taste that occurs when foods with the protein glutamate are eaten. Glutamate is found in meat, fish, and the flavor-enhancing chemical monosodium glutamate, or MSG.

Humans get the sensation of taste through their taste cells, which lie within the taste bud. The average person has about 10,000 taste buds. People regenerate new taste buds every three to ten days. As people grow older their taste buds regenerate at a slower rate, causing their sense of taste to lessen. An elderly person may have only 5,000 taste buds.

Taste buds are onion-shaped structures located primarily on a person's tongue. The majority of buds on the tongue are scattered on the papillae (pronounced pah-PILL-ee), the small projections that give your tongue its rough appearance. Taste buds are also located on the throat, roof of the mouth, and pharynx, but the buds on your tongue provide most of your taste experience. Each taste bud is made up of about 50 to 150 taste cells. Every cell has a fingerlike extension called a microvilli that connects with an opening at the top of the taste bud, called the taste pore.

For food to have taste, its chemicals need to reach your taste cells. The instant you take a bite of food, saliva or spit in the mouth starts breaking down the food's chemical components. These components, or molecules, travel through the pores in the papillae to bind to specific taste cells. The chemicals cause a change in the taste cell, sending a signal via nerves to the brain, which processes the signals.

The chemical reaction in the taste cells varies depending on the taste group involved. For example, salty foods trigger a change in taste cells when enough sodium (the main component of salt) molecules enter the cells through the microvilli. Each taste cell has the ability to recognize different taste groups, yet taste cells specialize in processing one particular group. Researchers have found that taste buds with common taste perceptions may be bunched together on the papillae. Many of the taste buds more sensitive to bitterness, for example, are located on the back of the tongue. This can cause an automatic gag-reflex to help prevent poisoning if something too bitter is ingested.

Smells at Work: Lime or Lemon?

It is the olfactory sense, or sense of smell, that plays a key role in determining your perception of how tasty something is, or its flavor. Flavor is so strongly linked to the olfactory sense that researchers estimate 70 to 75 percent of what humans perceive as taste actually comes from the sense of smell.

Special olfactory cells, located inside the uppermost part of the nose, recognize specific odors. These odors, or chemical molecules, enter the nose and rise upward until they reach the olfactory epithelium , a postage-stamp-size area that contains olfactory receptor cells. Olfactory receptor cells are nerve cells, and each cell lasts about four to five weeks before it is replaced with a new one. These cells have hairlike projections called cilia that are sensitive to odor molecules. A specific odor molecule dissolves in the mucus of the nose. Mucus is a slippery substance that protects and moistens. The odor molecule binds to specific receptors on the cilia, which trigger a chemical signal in the receptor cell. The cell then sends its signal to the olfactory bulb of the brain, and then on to other areas of the brain that recognize it as a specific odor. There can be hundreds of receptors that take part in recognizing one smell.

Olfactory cells can recognize thousands of different odors. The chemical molecules reach the cells through the air you breathe and the food you eat. When you put food in your mouth, chemicals are released while you are chewing. Molecules from the food travel through the passage between your nose and mouth to the olfactory epithelium.

If a person's nose is congested, mucus in the nasal passages can block the odor molecules from reaching the olfactory cells. This will block surrounding smells, and food will lose much of its flavor.

All Senses are not Created Equal

Because the chemosenses are complex mechanisms, there are several reasons why people have varying preferences for smells and tastes. A person's genetics (physiological makeup), upbringing, and familiarity with specific smells and foods can influence his or her likes and dislikes. Odor molecules transmit their signals to areas of the brain that are involved with emotional behavior and memory. When a person smells something, it often brings back memories associated with the object, and those memories can help shape a person's perception of that smell.

Genetics is also a factor in tasting ability. In the early 1930s researchers discovered an inherited trait that determined people's sensitivity to a bitter taste. They classified people as "tasters" or "nontasters" based on whether they were able to detect a specific chemical, which tastes bitter to some people and tasteless to others. Later research found that some people are especially sensitive to this bitter taste. These people are born with more than the average number of taste buds and, as a result, perceive tastes more intensely than the average person. For these supertasters bitter tastes more bitter, sweet tastes sweeter, and salt tastes saltier. Researchers theorize that about 25 percent of the people in the United States are supertasters, 25 percent are nontasters, and the remaining 50 percent are regular tasters.

In the two experiments that follow, you will use the scientific method to examine if genetics affects the sense of taste and how closely linked these two senses are.

Supertasters: Is there a correlation between the number of taste buds and taste perception?

Purpose/Hypothesis

In this experiment, you will test varying concentrations of three tastes on people to predict whether they fall into the category of nontaster, taster, or supertaster. Then you will test your hypothesis by counting the number of papillae of each person to estimate the number of taste buds each person has. If a person has more than twenty-five in a punch-hole-size area, then he/she is classified as a supertaster, five or less is considered a nontaster, and anywhere in between is an average taster.

Before you begin, make an educated guess about the outcome of this experiment based on your knowledge of the sense of taste. This educated guess, or prediction, is your hypothesis. A hypothesis should explain these things:

• the topic of the experiment
• the variable you will change
• the variable you will measure
• what you expect to happen

A hypothesis should be brief, specific, and measurable. It must be something you can test through further investigation. Your experiment will prove or disprove your hypothesis. Here is one possible hypothesis for this experiment: "People who are more sensitive to tastes will have a greater number of taste buds."

Variables are anything you can change in an experiment. In this case, the variable you will change will be the concentration of the solutions. The variable you will measure will be the number of taste buds.

Setting up a control experiment will help you isolate each variable and measure the changes in the dependent variable. Only one variable will change between the control and the experimental setup, and that is the concentration of the solution. For the control in this experiment you will use a cup of plain water (tasteless). For your experiment, you will determine sensitivity to three tastes: bitter, salty, and sweet.

You will first make a 10 percent solution for each substance, then dilute the solutions. Sugar and salt are solids and you will make a 10 percent weight/weight (gram/gram) solution. For liquids you will make a 10 percent volume/volume (milliliter/milliliter) solution. One gram of water equals 1 ml of water.

You will rate people's sensitivity to varying concentrations of grapefruit juice (bitter), sugary water, and salty water. Then you will use blue dye to color each person's tongue's papillae. Because you are relying on human subjectivity, the more people you test, the more accurate your results.

What Are the Variables?

Variables are anything that might affect the results of an experiment. Here are the main variables in this experiment:

• The participants in the experiment
• The cleanliness of the person's palette before the experiment
• The size of the paper hole
• The concentration of the taste
• The substance people are tasting
• In other words, the variables in this experiment are everything that might affect the relationship between a person's sensitivity to taste and the number of his or her taste buds. If you change more than one variable at the same time, you will not be able to tell which variable had the most effect on taste.

Level of Difficulty:

Easy to Moderate.

Materials Needed

• grapefruit juice
• sugar
• salt
• water
• measuring spoons
• gram scale
• three to four helpers
• sixteen small disposable paper cups
• light-colored pen
• blue food coloring
• cotton swabs
• piece of paper
• hole punch (standard 1/4-inch size)
• mirror
• magnifying glass
• Approximate Budget:
• $5.

Timetable

1 hour.

How to Experiment Safely

Check with an adult before you or your helpers taste any of the foods to make sure none of you has any allergies to the foods, or other dietary restrictions.

Use each cotton swab only once, one per person. Tasters should also use a fresh cup for their water. You might want to wear an old shirt in case any dye should spill.

Step-by-Step Instructions

1. Measure out 10 Tablespoons (150 milliliters of water) and pour into a cup.
2. Add 4 teaspoons (15 grams) of sugar for a total volume of 150 ml and stir until all the sugar is dissolved.
3. Write on the cup: "10% sugar."
4. Repeat this process for the salt, labeling the cup: "10% salt."
5. Measure out 9 Tablespoons (135 ml) water and pour into a cup.
6. Add 1 Tablespoon (15 ml) grapefruit juice for a total volume of 150 ml and stir thoroughly.
7. Label the cup: "10% grapefruit."
8. Dilute each solution by 10 percent.
9. From the sugar solution measure out 1 Tablespoon (15 ml) and pour into a clean cup.
10. Add 9 Tablespoons (135 ml) of water and stir until all sugar is dissolved.
11. Label the cup: "1% sugar."
12. To make a 0.1% solution: From the 1% sugar solution measure out 1 Tablespoon (15 ml) and pour into a clean cup.
13. Add 9 Tablespoons (135 ml) of water and stir until all sugar is dissolved.
14. Label the cup: "0.1% sugar."
15. To make a 0.01% solution: From the 0.1% sugar solution measure out 1 Tablespoon (15 ml) and pour into a clean cup.
16. Add 9 Tablespoons (135 ml) of water and stir until all sugar is dissolved.
17. Label the cup: "0.01% sugar."
18. To make a 0.001% solution: From the 0.01% sugar solution measure out 1 Tablespoon (15 ml) and pour into a clean cup.
19. Add 9 Tablespoons (135 ml) of water and stir until all sugar is dissolved.
20. Label the cup: "0.001% sugar."
21. Repeat this process for the salt solution and the grapefruit juice.
22. Place plain water in a cup for the control solution.
23. Create a chart that lists the concentrations and the control on the left, and the three tastes across the top.

Have the taster rinse out his or her mouth with water and make sure the mouth is relatively dry before beginning.

Start with one taste. Switch the five cups around, including the cup of water, not allowing the taster to see the labels. Have the taster dip a clean cotton swab into the solution, smear it over his/her tongue, and wait a few moments. Ask the taster if he/she can identify a taste. If the taster can identify a taste, make a "✓" sign in the box; if not, make a "x" in the box.

Have the taster rinse out his/her mouth with water and repeat the process for all the dilutions, including the control. Once the taster has completed one taste, repeat the process with another taste.

When one taster has finished sampling the three sets of tastes, repeat the process with another helper. Have a helper mix the samples so that you can also sample the dilutions yourself.

Punch a hole in a piece of paper for each taster.

Dip a cotton swab in the blue food coloring and have the tasters wipe the blue swab on the tip of their tongues.

Place the paper hole on the blue area of each tongue.

Using a magnifying glass, look at each tongue and count the round structures, the papillae, that are visible in the paper hole. Look in the mirror to count your own papillae. Write down the results for each taster.

Summary of Results

Compare the results of each person's data chart with the number of his or her taste buds. Did your results support your hypothesis? Did the people who were more sensitive to tastes have a greater number of taste buds? Could the people in the nontaster category only taste the higher concentrations, and the supertasters taste the lower concentrations? Share your results and discuss if the tasters with the greater number of taste buds have a higher sensitivity to tastes in general. If there are any supertasters, do they have a strong dislike for broccoli, cabbage, and cauliflower (bitter tastes) and for strong sweet tastes such as frosting.

Troubleshooter's Guide

Below are some problems that may occur during this experiment, some possible causes, and some ways to remedy the problems.

Problem: A person's responses were inconsistent, sometimes saying he or she could taste the higher concentration and lower concentrated solution, but not the in-between solutions.

Possible causes: The person may have been mixing up tastes. Try repeating the test with that person, making sure the taster cleans his/her mouth with water carefully every time.

Problem: There was no correlation between number of taste buds and perceived taste.

Possible causes: Human error. Examine the taster's reaction to the control solution to ensure that he/she is not mistakenly identifying tastes where there is none. If the taste of water has a "checkmark" then try repeating the experiment with that person, or with someone else. The more people you test, the less chance human error will have a statistical impact on your results.

Change the Variables

Try repeating the experiment (with new helpers) using different concentrations of the solutions, both higher and lower, to get an increased number of data points. You can also change the type of bitter solution you use (for example, a beverage with caffeine in it or tonic water). Another variable you can change is to replace one of the tastes with the sour taste (lemon juice). Always check with an adult before you or anyone else tastes any of the solutions to make sure there are no dietary restrictions.