Solidifying Science: Which Fruits Can Ruin Your Gelatin Dessert?

Summary

Key Concepts

Chemistry, gelatin, fruits, enzymes, heat

Credits

Teisha Rowland, PhD, Science Buddies

Introduction

Have you ever noticed that if you’re making a gelatin dessert, like JELL-O, it’s not recommended to use certain fruits, like pineapple? Why is this? It’s because these fruits may prevent the gelatin from solidifying. In this activity you’ll get to determine if certain enzymes in some fruits are preventing the gelatin from solidifying, and whether there’s a way to still include these fruits without ruining your gelatin dessert!

This activity is not recommended for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

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. 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 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. When 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 proteases are preventing the gelatin from solidifying (by cutting the gelatin into such small pieces that they are no longer able to tangle together and create a semi-solid structure). To do this you’ll inactivate these proteases by using heat.

Materials

1 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
Stovetop
Fruit/vegetable steamer (optional)
Pot, large enough to hold 3 cups of liquid
Clock
3 plastic cups or drinking glasses, each at least 12 oz. in size
Tape and permanent marker or pen (optional)
Gelatin mix (such as JELL-O), enough to make 3 cups of gelatin
3 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 1 cup of the fresh fruit.
Cook ½ cup of the cut up fruit. Do this by either steaming or boiling the fruit (with about ¼ cup of water) for 5 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).

Instructions

Make the gelatin dessert according to the package instructions. You will want to prepare at least 3 cups of liquid gelatin.
Add 1 cup of gelatin liquid to each of the cups with fruit, and add a third 1 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 solidifies in the cup without fruit (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 fruits that do not have proteases, such as apples, blueberries, oranges, raspberries, and strawberries. 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 1 teaspoon [tsp.] of meat tenderizer in 1 tsp. of water and adding this to the 1 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? Does changing the pH of the gelatin and fruit mixture affect how well it solidifies?

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 semi-solid form when it cools. However, 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. However, heating the fruit (through boiling or steaming) should inactivate the proteases, and the resultant gelatin mixture should solidify like normal (or nearly normal; because the fruit may have been hot when the gelatin was added, the gelatin may have been slightly less firm than in the cup without fruit). The proteases bromelian and papain (which come from pineapples and papaya, respectively) are often used in meat tenderizers. However, there are several other fruit proteases, such as actinidin (from kiwi fruit), ficin (from figs), and zingibain (from ginger).