Key concepts
Food science
Have you ever tasted delicious grilled chicken and wondered how it got so much flavor? Maybe you have heard your family talk about marinating foods before cooking or grilling them. A marinade is a mixture of seasonings used to flavor or tenderize food. Some cooks have strong opinions about the best way to marinate their favorite food, be it a steak, tofu steak, chicken breast or veggie kebobs. One of the keys to a good marinade is that it stays on the food you are planning to cook. In this activity you will do a test to see what factors might be most important in making a marinade stick to the surface of food. After doing this activity you could use your findings to help you make tastier food for a summer BBQ!
Every culture has its own unique way of preparing food. But whether a recipe is Chinese or American, Italian or Indian, some of the main dishes usually call for a marinade. Some of the original marinades from several centuries ago were briny (very salty) liquids, such as seawater, which helped preserve foods before refrigeration was available. Whatever they are made of, marinades are usually meant to preserve, tenderize and flavor foods.
In this activity you'll test how various ingredients affect the adsorption (yes, with a "d") of a marinade ingredient onto the surface of a food. (The word "adsorb" is used to describe the process by which a substance adheres to the surface of an object, as opposed to being absorbed into it.) The ingredients you'll test are salt, vinegar and sugar. Salt obviously makes foods salty and sugar makes them sweet, but how well do these ingredients work for making a marinade be adsorbed to food? Vinegar is an acid—other acids you might find around the kitchen include lemon juice and orange juice. Does something acidic make a good marinade? Along with these ingredients, instead of actual seasoned marinades you'll use a food dye so you can see the level of adsorption on the food.

  • Knife
  • Cutting board or dinner plate
  • Tofu, extra-firm (You can substitute meat if you choose, but if so, use an adult helper and be sure to wash any surfaces—including your hands—that the raw meat comes into contact with. The dye works well with chicken breast.)
  • Four plastic cups, each at least nine ounces in size
  • Masking tape or painter's tape and a pen or permanent marker (This is so you can label the cups. If you are using disposable cups, you can just use a permanent marker.)
  • Food dye (A dark color, such as blue or green, should be used.)
  • Salt
  • Distilled white vinegar
  • Sugar
  • Tap water
  • Clean spoons or other utensils for stirring
  • Measuring cups
  • Measuring tablespoon
  • Sheet of white paper
  • Clear plastic wrap
  • A fork or a slotted spoon
  • A timer or clock
  • Cut the tofu block into eight cubes, where each cube is about one half an inch on each side.
  • Label four plastic cups as follows: "water," "salt," "vinegar" and "sugar."
  • To the cup labeled "water" add one cup (C) of water. To the cup labeled "salt" add one tablespoon (tbsp.) of salt and one C of water. To the cup labeled "vinegar" add one-quarter C of vinegar and three-quarter C of water. To the cup labeled "sugar" add one tbsp. of sugar and one C of water. How well do you think the different ingredients will stick to the tofu cube's surface?
  • Add eight drops of food dye to each of the cups.
  • Using a clean spoon or other utensil for each cup, stir the solutions so that the dye gets mixed in and the salt and sugar are completely dissolved in their cups.
  • Carefully add two tofu cubes to each of the four cups. Let the tofu cubes marinate in the cups for 45 minutes at room temperature. (Do not worry if the cubes in the salty water float.)
  • While the cubes are marinating, lay a white sheet of paper on a nearby surface and cover it with clear plastic wrap.
  • When the cubes are done marinating, use a fork or slotted spoon to remove the cubes from each cup. Place the cubes on the sheet of paper, leaving the cups that held the cubes nearby so you know which cup each cube came from.
  • Examine the cubes. (If not all of the sides on a cube are equally dyed, examine an average-looking side on the cube.) Which cubes became the most dyed? Which were the least dyed? How do they compare with the cubes from the cup that only had water and food dye? What does this tell you about how well the different ingredients (sugar, salt and vinegar) stick to the surface of the food?
  • Extra: There are many variables that you could try changing in this activity, such as what type of food you test, the amount of time the tofu cubes are marinated, the temperature they're marinated at and the concentration of the ingredients tested. (If you use chicken breast or other meat, be sure to wash everything that comes into contact with the raw meat—for instance, raw chicken might carry salmonella bacteria.) When exploring other variables, be sure to only change one at a time, keeping everything else the same. How well do the tofu cubes become dyed when marinated for a longer amount of time, at a different temperature or with a different concentration of ingredients?
  • Extra: Some marinades have vegetable or olive oil in them. What happens if you mix oil in with the marinade?
  • Extra: You could cut the tofu cubes with a knife after they are done marinating to look at the inside of the cubes. How far has the dye been absorbed into the cubes from the different cups?

Observations and results
Did the tofu cubes in the cup with the vinegar become the most dyed? Were the other tofu cubes less dyed, but all dyed similarly to each other?
You should have seen the tofu cubes that were marinated in the cup with the vinegar/food dye/water solution clearly become the most dyed—or had adsorbed the dye best—compared with any of the other tofu cubes. Vinegar is an acid, like lemon juice and orange juice. Acids can cause foods such as meats to be broken down, or tenderized. (You may have heard people warn not to use too much vinegar in a marinade. This is because the acid can tenderize the food so much that it becomes mushy!) When vinegar is used in a marinade, it breaks down the food’s surface  and lets the marinade be adsorbed there. Salt works well in marinades for meat, too, because it helps break open the cells, allowing the marinade to penetrate into the tissue. Sugar in marinades helps the food taste sweet, but does not help marinades be adsorbed into food as much.
It is not recommended to eat the tofu cubes used in this activity. You could compost them or throw them in the trash. If you used chicken breast or other meat, discard it in the garbage and be sure to wash everything that came in contact with the raw meat, especially if you used raw chicken, which might have salmonella.

More to explore
Marinade Science—How Marinades Work, from Home Cooking
The Secrets and Myths of Marinades, Brinerades and How Gashing Can Make Them Work Better, from
Fun, Science Activities for You and Your Family, from Science Buddies
Flavor That Food! Exploring the Science of Marinades, from Science Buddies

This activity brought to you in partnership with Science Buddies