Abstract

Objective

To determine if people older than age 60 have a higher threshold of sensitivity to detecting salt than people younger than age 30.

Introduction

Has this situation ever happened in your family? You're all seated around the dinner table in front of steaming plates of food. One person tastes his meal and says, "Could you please pass the salt?" While another person says, "Oh, this is so delicious! Perfect!" Everyone has a different ability to detect a salty taste, one of the five tastes that humans can sense. On top of that, everyone prefers a different level of saltiness in foods, often based on past exposures to levels of salt. If you're cooking for a crowd, this can make getting the level of saltiness "just right" tricky, if not impossible!

Taste is a complex experience. Humans can sense fives tastes: salty, sour, sweet, bitter, and savory (or umami). Sensing is done through receptor cells in taste buds on the tongue. There is genetic variability in the number of taste buds, but many people have about 10,000 of them. The receptor cells transmit signals to the brain. The experience of taste comes from these signals, combined with other information about a food's texture, viscosity, mouth feel, appearance, and aroma, as well as a person's appetite, cultural traditions, and past encounters with the food. It's no wonder there are such strong and differing feelings about which foods taste good!

A basic liking for a salty taste seems to be something humans are born with, probably because salt is essential to survival. Humans, like all animals who evolved from creatures that started in the sea, carry a small piece of the ocean inside their bodies. That bit of oceanlike water is located in the fluid in the space around cells and in the plasma, the fluid part of your blood. Plasma is kind of like the broth in a soup. It is the medium that carries proteins, carbon dioxide, clotting factors, hormones, red and white blood cells, and mineral ions all throughout your body. The mineral ion, sodium (Na+), in the plasma comes from the salt (NaCl) that you eat in your food. That sodium is what makes your blood, sweat, and tears taste slightly salty.

Sodium is vital for many body processes, like transmission of nerve impulses, muscle function, and regulation of blood volume. Many animals, including humans, must take in a small amount of sodium each day to replace the sodium that is lost through sweat and urine. The amount of sodium is kept in balance inside your body through your kidneys, which regulate how much sodium is excreted. If you take in too much sodium, for example, then your kidneys will increase the amount of sodium excreted to avoid an excessive increase in blood volume, fluid retention, and swelling (called edema).

Wild herbivores, like deer, who eat a completely plant-based diet, have to supplement their diet with salt to get the sodium they need. These animals find salt in brine springs, or in natural outcroppings of salty rock, called salt licks. Wild carnivores, or animals that eat only meat, do not have to eat salt. Their sodium needs are met through the flesh and blood that they eat. In human history, salt became highly valued as people developed farming and ate more grains and vegetables. Even when diets were supplemented with the meat of domesticated farm animals, the meat was not enough to meet all the sodium needs, so the rock known as salt became like gold! Somehow, people knew that they needed it, and that their domesticated farm animals needed it too (actually, in amounts 5-10 times that of the people). People learned that salt was an amazing preservative for their food, as well as an antibacterial and anti-fungal agent. Many cultures thought it had spiritual or magical powers. It seems strange today, but centuries ago, people were paid in salt, just like it was money, and wars were fought over salt!

Today, the modern salt industry lists about 14,000 uses for salt. In cooking, it is prized for reducing bitterness, enhancing smells, and balancing sweetness. In this science project, you'll explore this extraordinary, edible rock by looking at the threshold of sensitivity to detecting salt. You'll see at what concentration of salt in saltwater people can first identify the taste as "salty," and you'll see if age affects that threshold of sensitivity.

Terms, Concepts, and Questions to Start Background Research

• Detect
• Savory
• Umami
• Receptor cell
• Variability
• Texture
• Viscosity
• Mouth feel
• Aroma
• Plasma
• Excretion
• Retention
• Herbivore
• Carnivore
• Domesticated
• Preservative
• Antibacterial
• Anti-fungal
• Threshold
• Sensitivity
• Concentration
• Control

Questions

• What factors go into the experience of taste?
• Why do humans and some other animals like salt?
• What role does sodium play in the body?
• How has salt been used throughout human history?

Bibliography

• McGee, Harold. On Food and Cooking. New York, NY: Scribner, 2004.
• Kurlansky, Mark. Salt: A World History. New Your, NY: Walker Publishing Company, Inc., 2002.

This source provides an animation that describes how the salt that you eat works inside your body to transmit nerve impulses:

• McKinley, M. and O'Laughlin, V. (2006). How the Sodium-Potassium Pump Works. McGraw-Hill Higher Education. Retrieved September 16, 2008, from http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

This source describes the effect of sodium on the physiology of the human body:

• Merck Research Laboratory Contributors. (2003, February). Sodium. The Merck Manuals Online Medical Library. Retrieved September 16, 2008, from http://www.merck.com/mmhe/sec12/ch155/ch155k.html

For help creating graphs, try this website:

• National Center for Education Statistics (n.d.). Create a Graph. Retrieved May 23, 2008, from http://nces.ed.gov/nceskids/CreateAGraph/default.aspx

Materials and Equipment

• Pitchers (3)
• Plastic wrap, if the pitchers do not have lids
• Masking tape
• Permanent marker
• Measuring spoons
• Kosher salt, non-iodized
• Soup pot, 3-gallon capacity or larger
• Liquid measuring cup, 1-quart (qt.) size
• Large spoon
• Access to a sink
• Dish towel
• Small disposable cups (at least 60)
• Pencil
• Volunteers, older than age 60 years (at least 10)
• Volunteers, younger than age 30 years (at least 10)
• Lab notebook

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Experimental Procedure

Making Your Salt Solutions

To conduct this experiment, you will need to make three batches of saltwater solutions, each with a different concentration of salt. The first batch of saltwater will be the control. It will contain no salt. The second batch will contain 0.05 percent salt. The third batch will contain 0.10 percent salt (double the salt concentration of the first batch).

1. Fill the first pitcher up with tap water and cover it with a lid or with plastic wrap. This will be your first batch of saltwater, your control, containing no salt. Put a small piece of masking tape on the pitcher and label it with the number "1."
2. With your measuring spoons, measure out exactly ½ tsp of salt and put it in the clean and dry soup pot.
3. Add 5 qt. of tap water to the soup pot with the liquid measuring cup.
4. Stir the soup pot for approximately 5 minutes with the big spoon to allow the salt to dissolve.
5. Have an adult help you pour the saltwater solution into the second pitcher. Cover the pitcher with a lid or with plastic wrap. This is your 0.05 percent solution. Put a small piece of masking tape on the pitcher and label it with the number "2."
6. Rinse and dry the soup pot.
7. With the measuring spoons, measure out exactly 1 tsp. of salt and put it in the clean and dry soup pot.
8. Add 5 qt. of tap water to the soup pot with the liquid measuring cup.
9. Stir the soup pot for approximately 5 minutes with the big spoon to allow the salt to dissolve.
10. Have an adult help you pour the saltwater solution into the third pitcher. Cover the pitcher with a lid or plastic wrap. This is your 0.10 percent solution. Put a small piece of masking tape on the pitcher and label it with the number "3."
11. If you are not going to be testing your volunteers all at the same time, or on the same day, then place your pitchers in a refrigerator to keep the solutions fresh. Note: Always allow the pitchers to come to room temperature before testing.

Preparing Your Cups for Testing

1. With the pencil, label:
   1. 20 disposable cups with the number "1."
   2. 20 disposable cups with the number "2."
   3. 20 disposable cups with the number "3."
2. When a volunteer is ready to test, make sure your pitchers are out of the refrigerator and at room temperature. Fill a cup with the number 1 on it half full from the number 1 pitcher. Fill a cup with the number 2 on it half full from the number 2 pitcher. Fill a cup with the number 3 on it half full from the number 3 pitcher. If you are not going to be testing any more volunteers right away, then cover up your pitchers and put them back in the refrigerator.

Testing Your Volunteers

1. Give each volunteer the three cups that you filled in the Preparing Your Cups for Testing section. Ask each volunteer to taste the cups in order of 1, 2, and 3, and to tell you what each one tastes like: water or saltwater. Write down your volunteer's response in one of the data tables appropriate for his or her age, like the ones below, in your lab notebook.

Under Age 30 Data Table

Volunteer	1	2	3	4	5	6	7	8	9	10
Cup 1
Cup 2
Cup 3

Over Age 60 Data Table

Volunteer	1	2	3	4	5	6	7	8	9	10
Cup 1
Cup 2
Cup 3

Analyzing Your Data

1. Count how many volunteers under age 30 said cup 1 tasted like saltwater. Enter your count into a third data table, like the one below. Count how many volunteers under age 30 said cup 2 tasted like saltwater, and record your count. Count how many volunteers under age 30 said cup 3 tasted like saltwater, and record your count.
2. Repeat step 1 of this section for the volunteers over age 60.

   Count of Volunteers Who Tasted Saltwater
   Salt concentration	Under Age 30 Volunteers	Over Age 60 Volunteers
   Cup 1: 0.00% salt
   Cup 2: 0.05% salt
   Cup 3: 0.10% salt

3. Make a bar chart showing salt concentration on the x-axis (0.00%, 0.05%, and 0.10%), and the counts of each group of volunteers who identified the solutions as saltwater on the y-axis. You can make the bar chart by hand or use a website like Create a Graph to make the graph on the computer and print it. Looking at your bar chart, does age affect ability to detect saltiness?

Variations

• Evaluate whether genetics affects salt-detection ability. If someone is a "supertaster," is he or she able to detect salt at weaker concentrations than most people? Does the number of taste buds affect salt-detection ability? See the Science Buddies science fair project, "Do You Love the Taste of Food? Find Out if You're a Supertaster!" for help on evaluating the number of taste buds.
• Do other taste sensations have an age sensitivity component? Modify the experiment to test sweet, bitter, sour, or umami.

• For more science project ideas in this area of science, see Human Biology & Health Project Ideas.

Credits

Kristin Strong, Science Buddies

Last edit date: 2010-11-24 22:00:00