Which Fruits Can Ruin Your Gelatin Dessert?

Abstract

Objective

The purpose of this science fair project is to determine if the enzymes in some fruits prevent gelatin from solidifying.

Credits

Andrew Olson, PhD, Science Buddies

Edited by Sandra Slutz, PhD, Science Buddies

Sources

• Shields, R.K. (2003). Jell-o or Gel-no: Which Fruits Contain a Protein-Digesting Enzyme that Prevents Gelatin from Solidifying? California State Science Fair Abstract.

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Introduction

If you like making gelatin for dessert, you might have noticed that the box recommends against adding certain kinds of fruit, such as pineapple, kiwi, mango, ginger root, papaya, figs, or guava. But why? What happens when you add these fruits to your gelatin? Why can you use some fruits, but not others? You’ll find out for yourself in this science fair project, but if you do a bit of background reading first, you’re likely to find that people have a hard time getting the gelatin to solidify when they add certain fruits. To discover why, you first need to know a little about what gelatin is and how it normally sets.

Gelatin is a protein made from collagen. Proteins are a basic type of matter that make up all living things. Your skin, your blood, your hair—all of these are made up of many different types of proteins. One of those proteins is collagen. Collagen is a structural protein found in all animals, meaning that it helps give animals their structure, or shape. Collagen can be found in many parts of your body, including your skin, bones, muscles, and cartilage. Gelatin is a form of collagen that has undergone a chemical reaction to change it slightly and make it able to solidify when you're cooking with it. When you’re making a gelatin dessert, you dissolve the gelatin mix in water. The gelatin proteins are like microscopic (meaning too small to see with just your eyes) strands of spaghetti. These long, thin, flexible gelatin proteins tangle up with one another, the same way strands of cooked spaghetti do when they're all in one pot together. As the gelatin proteins tangle, they form mesh pockets that trap the water, sugar, and other flavoring agents that you’ve added to your dessert. The end result is a wiggly-jiggly solid to enjoy.

Now, back to our original question. What happens when you add fruit to your gelatin? Some fruits, like strawberries, oranges, and apples, are a tasty addition; the gelatin solidifies around the chunks of fruit. But if you add fruits like pineapple, guava, mango, or kiwi, you end up with a runny mess that never solidifies. It turns out that this second group of fruits all contain proteases (pronounced PROH-tee-ay-siz), like papain (pronounced puh-PIE-uhn) and bromelain (pronounced BROH-muh-lin). Proteases are a special class of protein that act like a pair of scissors, cutting other proteins up. Could it be that the papain and bromelain in these fruits are cutting the gelatin protein into such small pieces that they are no longer able to tangle together and create a solid structure? This is exactly the question you’ll tackle in this science fair project.

First you’ll see for yourself whether one of these protease-containing fruits interferes with gelatin’s ability to solidify. If it does, you’ll test whether it is the protease that interferes with solidifying by inactivating the protease (which means to make it stop working) in the fruit and then adding the fruit to the gelatin. You might be wondering how you will possibly inactivate the protease yourself. Remember that proteases, like papain and bromelain, are also proteins themselves. Most proteins can be inactivated using a variety of methods. One such method is called denaturation. Denaturation changes the structure, or shape, of the protein, without changing what it is made up of. Exposure to heat is one method of denaturing proteins. A good example of this process is cooking an egg. When the egg is raw, the egg white—which has lots of proteins, called albumins—is transparent and liquid; but after cooking, it becomes opaque and solid. In the case of eggs (and most proteases), denaturation of the protein causes an irreversible change. The heat will permanently inactivate the fruit’s protease. Does this allow the gelatin to solidify or not? Ready to find out? If so, get out your spoon, because with this science fair project, you'll get to enjoy some of your results as dessert!

Terms and Concepts

• Gelatin
• Protein
• Collagen
• Protease
• Papain
• Protein denaturation
• Baseline
• Negative control
• Positive control
• Experimental condition

Questions

• Of what is gelatin made?
• How does gelatin solidify?
• What do proteases do?
• Which fruits contain papain, bromelain, or another protease?
• Papain and bromelain are often called natural “meat tenderizers.” Why is this? What does it mean? How do they tenderize meat?
• What are commercial meat tenderizers, like the ones in the spice aisle of the grocery store, made out of?

Bibliography

• Howstuffworks.com. (2006). What Exactly Is Jell-O Made From? Retrieved July 11, 2006, from http://home.howstuffworks.com/question557.htm

Materials and Equipment

• Clear plastic cups (18); Note: if you would like to test additional fruits, add 6 cups for each type of fruit.
• Permanent marker
• Chopped fresh fruit (3 cups of each fruit you’d like to test); choose at least one fruit with protease and one fruit without protease from the lists below. Note: Use only fresh fruits, unless you are trying one of the Variations at the end of the experiment.
  • Fruits with protease, such as figs, ginger root, guava, kiwi, mango, papaya, and pineapple
  • Fruits without protease, such as apples, blueberries, oranges, raspberries, and strawberries
• Knife
• Cutting board
• Meat tenderizer (3 teaspoons); available in the spice aisle of grocery stores
• Pot, large enough to hold 18 cups of hot liquid
• Optional: Fruit/vegetable steamer; see the Experimental Procedure for more details.
• Teaspoon measuring spoon
• Dry measuring cup, 1-cup volume
• Small bowl or cup
• Spoons for stirring (6)
• Stovetop
• Oven mitt
• Gelatin mix (enough to make 18 cups of gelatin); any flavor is fine. See package details to determine how many packages you will need to yield 18 cups. Note: each additional fruit you try will require another 6 cups of gelatin.
• Lab notebook

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Experimental Procedure

Caution: Preparing gelatin involves cutting with a knife, and pouring and stirring boiling hot water. Adult supervision or assistance is highly recommended.

Project Overview

1. For this science fair project, you will have several controls and test conditions. For more information about controls, read the following. You might also want to read the Science Buddies guide to Experimental Procedures. Each condition will be made in triplicate, meaning three cups of gelatin for every condition, to ensure that your results are repeatable.
   1. A baseline sample is one to which you can compare all other test cases. In this experiment, the baseline controls are the gelatin samples with no fruit added to them. You will compare the consistency of all other gelatin samples to these baseline controls.
   2. A negative control is a case where you expect to see no change from the baseline. In this science fair project, the negative controls are the gelatin samples with fruit that do not have protease. Both the raw and the cooked samples will be negative controls. What do you expect the consistency of these samples to be compared to the baseline samples?
   3. A positive control is a case where you expect to see a known change from the baseline. In this science fair project, the positive controls are the gelatin samples to which you add meat tenderizer. Meat tenderizers contain proteases, like papain and bromelain; the exact protease depends on the brand. What do you expect the consistency of these samples to be, compared to the baseline samples?
   4. The test or experimental conditions are the ones that you have a hypothesis about that needs testing. In this science fair project, the experimental conditions are the gelatin samples containing either raw or cooked fruit that do have protease.

Testing the Different Gelatin Conditions

1. Label the 18 cups according to their contents. Each gelatin condition should have a #1, a #2, and a #3 cup.
   1. Plain gelatin: Total of three cups
   2. Raw [name of fruit]: Total of six cups—three for the fruit with protease and three for the fruit without protease
   3. Cooked [name of fruit]: Total of six cups—three for the fruit with protease and three for the fruit without protease
   4. Protease: Total of three cups
2. Cut up each of the fruits. Ask for an adult's help with this step. Be sure to wash your cutting board and knife after you cut each fruit.
3. Cook 1½ cups of each type of fruit. Fruit should be either steamed or boiled for 5 minutes. Leave the other 1½ cups raw.
4. Add approximately ½ cup of fruit (cooked or raw) to each of the plastic cups that are labeled as containing fruit.
5. Put 3 teaspoons (tsp.) of meat tenderizer in a small bowl. Add 3 tsp of water and mix until the tenderizer has dissolved. Put 1 tsp. of the meat tenderizer solution in each plastic cup labeled Protease.
6. Make the gelatin according to the package instructions. Add 1 cup of gelatin liquid to each of the plastic cups. Using a spoon, thoroughly stir the contents of each cup. Make sure to use a different spoon for each condition. You’ll need a total of six spoons.
   1. Refrigerate all of the cups, noting the time at which you put them inside the refrigerator in your lab notebook.
   2. Check the consistency of the gelatin in each cup at regular intervals (once or twice an hour). Be consistent with when you check. Examine the gelatin carefully and record your observations in your lab notebook. In which conditions does the gelatin set? In which conditions does the gelatin remain as liquid? Are there any in-between cases?

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Variations

• Does freezing fruit denature protein-digesting enzymes? Design an experiment to compare the enzyme activity of fresh and frozen pineapple.
• Do other methods of fruit processing denature enzymes? How about drying or canning? Design an experiment to find out.
• Design an experiment to test which type of fruit has the most enzyme activity per unit weight.
• Does the meat tenderizer’s effect on the gelatin depend on how much tenderizer is added? Can meat tenderizer be denatured to neutralize the effect?
• Do research on alternative methods of protein denaturation, or other methods for inactivating enzymes. Choose one or more of these methods and see if it works on protein-digesting enzymes in fruit. (Don't eat the results of this experiment!)
• For other Science Buddies projects involving enzymes, see: Liver Stinks!, A Juicy Project: Extracting Apple Juice with Pectinase and Enzyme-Catalyzed Reactions -- What Affects Their Rates?.

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Related Links

• Science Fair Project Guide
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• Cooking & Food Science Project Ideas
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