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February 22, 2010 4:16:38 PM PST
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WHICH FRUITS CAN RUIN YOUR GELATIN DESSERT?

Why are some fruits not recommended for adding to gelatin? Here is a basic science fair project for investigating enzymes that prevent gelatin from solidifying. It's an experiment with edible results!

Objective: The purpose of this project is to test the hypothesis that food items containing a protein-digesting enzyme will prevent gelatin from solidifying.

If you like making gelatin for dessert, you may have noticed that the box recommends against adding certain kinds of fresh or frozen fruit, e.g., pineapple, kiwi, mango, ginger root, papaya, figs, or guava. This is because these fruits contain the protein-digesting enzymes papain or bromelain that can prevent the gelatin from setting.

So why would a protein-digesting enzyme interfere with the setting of gelatin? To find out, you should do some background research on the ingredients that go into gelatin, and how it normally sets (see Howstuffworks, 2006, for a good start).

Although papain and bromelain can digest proteins, these enzymes themselves are proteins. Most proteins can be inactivated by denaturation. Denaturation changes the structure of the protein, without breaking the chemical bonds between the amino acids that make up the protein backbone. Exposure to heat is one method of denaturing proteins.

A good example 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 enzymes) denaturation of the protein causes an irreversible change. Most enzymes are inactivated by denaturation.

In this science fair project, you will perform two tests:

  • First, you will choose one or more of the fresh fruits from the list above to see if they prevent gelatin from setting.

  • Second, in order to test if enzyme activity is responsible for the effect, you will also test the fruit(s) after cooking (to denature any enzymes present in the fruit).
With this science fair project, you'll get to enjoy some of your results as dessert!

Materials and Equipment:
  • Clear plastic cups (6 to test one fruit and create a control, and 4 more for each additional fruit you will test)
  • Fresh fruit, such as pineapple, kiwi, mango, ginger root, papaya, figs, or guava
  • Two sauce pans
  • Oven mitt
  • Gelatin mix
  • Lab notebook
  • Optional: You can try papain tablets, available at vitamin/health food stores, as a positive control.
Experimental Procedure:

Safety Note: Preparing gelatin involves pouring and stirring boiling hot water. Adult supervision or assistance is highly recommended.
  • Determine how many different fruits you will be testing, then calculate the number of cups of gelatin to make.

  • For each fruit tested, you should make at least four cups of gelatin: two for uncooked fruit and two for cooked fruit.

  • You will also need two extra cups for plain gelatin (as a negative control, to make sure that the gelatin alone sets properly).

  • If you want to try different amounts of each fruit, you will need 4 cups for each amount tested (again, 2 cups for cooked fruit, and 2 cups for uncooked fruit). You will also need extra cups for ground-up papain tablets, if you are using them.

  • Label two cups Fresh and two cups Cooked, and include the type of fruit and amount of fruit on all four cups.

  • Make the gelatin according to the instructions on the box.

  • For each type and amount of fruit to be tested, add fresh (uncooked) fruit to two cups.

  • For each type and amount of fruit to be tested, add fruit that you have cooked (boiled or steamed for 5 minutes) to two cups. Use the same amount of cooked fruit as you used for the fresh fruit.

  • Optional: grind up two papain tablets and dissolve in a small amount of water. Add equal parts of this solution to two cups of gelatin as a positive control. Mix well.

  • Finally, add gelatin only to two cups. This is your negative control (to make sure that the gelatin alone sets properly).

  • Refrigerate all of the cups, noting the time in your lab notebook.

  • Check the consistency of the gelatin in each cup at regular intervals (once or twice an hour). 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?
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.

  • Papain and bromelain are also the active ingredients in many meat tenderizers. Design an experiment to see if meat tenderizer can prevent gelatin from setting. Does the effect 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:

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