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Solidifying Science: Why Can Certain Fruits Ruin Your Gelatin Dessert?

An enzyme-based exercise from Science Buddies
jello fruit dessert



George Retseck

Key concepts
Food science
Chemistry
Gelatin
Fruits
Enzymes
Heat

Introduction
Have you ever noticed that if you're making a gelatin dessert, such as JELL-O, it's not recommended to use certain fruits, like pineapple? Why is this? These fruits may prevent the gelatin from solidifying. In this activity you'll get to determine if certain enzymes in some fruits can keep the gelatin from gelling—and whether there's a way to still include these fruits without ruining your gelatin dessert!

Background
If you like making gelatin for dessert, the box often recommends not adding certain kinds of fruit, including pineapple, kiwi, mango, ginger root, papaya, figs or guava. People have a hard time getting the gelatin to solidify when they add these fruits. Gelatin is made from collagen, which is a structural protein found in all animals. Collagen is found in many parts of the body and helps give animals their structure, or shape. Gelatin, which is a mixture of collagen proteins, solidifies when you cook it because its proteins form tangled mesh pockets that trap the water and other ingredients. After the gelatin cools, the proteins remained tangled. This results in your wiggly-jiggly gelatin dessert.

The fruits listed above contain proteases, which are enzymes. Enzymes help make certain chemical reactions happen. Proteases specifically act like a pair of scissors, helping reactions take place that cut other proteins up. In this activity you'll explore whether these protease enzymes are preventing the gelatin from solidifying (by cutting the gelatin's collagen proteins into such small pieces that they are no longer able to tangle together and create a semisolid structure). To do this you'll inactivate these proteases by using heat.

Materials
• One cup of one of the following types of fruit, which should contain proteases: figs, ginger root, guava, kiwi fruit, mango, papaya or pineapple. Make sure the fruit is fresh.
• Knife
• Cutting board
• Measuring cup
• Water
• Stove top
• Fruit/vegetable steamer (optional)
• Pot, large enough to hold three cups of liquid
• Clock
• Three plastic cups or drinking glasses, each at least 12 ounces in size
• Tape and permanent marker or pen (optional)
• Gelatin mix (such as JELL-O), enough to make three cups of gelatin
• Three utensils for stirring, such as spoons or forks
• Refrigerator

Preparation
• You may want to have an adult help cut up the fruit and use the stove.
• Carefully cut up one cup of the fresh fruit.
• Cook one half cup of the cut fruit. Do this by either steaming or boiling the fruit (with about one quarter cup of water) for five minutes. How does the cooked fruit look?
• Add the raw fruit to one plastic cup or drinking glass and the cooked fruit to a different plastic cup. If it's difficult to tell the difference between the raw and cooked fruit by looking at them, you may want to label the cups (with tape and a permanent marker or pen).

Procedure
• Make the gelatin dessert according to the package instructions. You will want to prepare at least three cups of liquid gelatin.
• Add one cup of gelatin liquid to each of the cups with fruit, and add the third cup portion to an empty cup. You should now have three cups with gelatin liquid in them.
• Thoroughly stir the contents of each cup. Use a different, clean utensil to stir each cup.
• Refrigerate all three cups, noting the time at which you put them inside the refrigerator.
• An hour after you put the cups in the refrigerator, check the consistency of the gelatin in each cup. Continue checking their consistency once an hour until the gelatin in the cup without fruit solidifies. (This will probably take about four hours.) In which condition(s) does the gelatin set? In which condition(s) does the gelatin remain a liquid? Are there any in-between cases?
What do your results tell you about how the proteases affect the gelatin solidification process and how heat affects the proteases?
Extra: In this activity you explored fruits that contain proteases, but many fruits do not contain proteases. You could repeat this activity using apples, blueberries, oranges, raspberries and strawberries—all of which do not have proteases. How well does the gelatin solidify when using fruits that do not contain proteases?
Extra: Meat tenderizer contains some of the same proteases that are found in the fruits explored in this activity. Try making a gelatin dessert with meat tenderizer (by dissolving one teaspoon [tsp.] of meat tenderizer in one tsp. of water and adding this to the one cup of gelatin liquid). Can gelatin solidify when it is made with meat tenderizer? If a solution of meat tenderizer is heated, is the enzyme deactivated?
Extra: You used heat in this activity to inactivate the proteases in fruit, but other temperatures and conditions may inactivate the proteases as well. Does freezing the fruit inactivate the proteases? Do other processes, such as drying or canning, inactivate the proteases? 

Observations and results
Did the cup with the raw fruit remain a liquid? Did the cups with the cooked fruit and no fruit added solidify like normal?

Normally the collagen proteins in gelatin form a tangled mesh that traps water and other ingredients in it, giving the gelatin its semisolid form when it cools. Proteases can cut up the proteins so that the gelatin cannot solidify. There are several different kinds of proteases in the fruits recommended for this activity, and using any of these fresh fruits should result in gelatin that does not solidify well, if at all. Heating the fruit (through boiling or steaming), however, should inactivate the proteases, and the resulting gelatin mixture should solidify like normal (or nearly normal—if the fruit was hot when the gelatin was added, the solidified gelatin may have been slightly less firm than that in the cup without fruit). The proteases bromelain and papain (which come from pineapples and papayas, respectively) are often used in meat tenderizers. There are several other fruit proteases, however, such as actinidin (from kiwi fruit), ficin (figs) and zingibain (ginger).

Cleanup
You may enjoy a tasty fruit and gelatin dessert. Be sure to store it in the refrigerator until it is consumed.

More to explore
What Exactly Is JELL-O Made from? , from Discovery Communications, LLC
Science of fruit jellies , from The Naked Scientists: Kitchen Science
Enzymes Make the World Go 'Round , from Rader's Chem4Kids.com
Which Fruits Can Ruin Your Gelatin Dessert? , from Science Buddies


This activity brought to you in partnership with Science Buddies
ScienceBuddies

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