Abstract
"Slurp...plop!" Recognize that sound? You might if your family usually serves jellied cranberries for the holidays. Jellied cranberries are thick, like gelatin, and retain the shape of the mold in which it was placed, which might mean Aunt Sue's turkey mold or even the shape of the can if you buy one of the popular canned versions. Taking a bite of wiggly jellied cranberries can be a fun addition to a delicious meal, but cranberries can also be served as a sauce. Both versions use the same exact ingredients, so what makes one version turn into a gelatin while the other stays saucy? Find out for yourself in this holiday-flavored science fair project!Summary
Sandra Slutz, PhD, Science Buddies
Objective
To determine how cooking time affects the ability of the natural pectin in cranberry sauce to form a solid gelatin.
Introduction
Do you prefer chunky or smooth peanut butter? Orange juice with or without pulp? Have you ever noticed that sometimes when the same food comes in two slightly different styles or textures, people tend to get very opinionated over which is "better"? Cranberries are another example, served either as a gooey sauce or as a solid jelly. The jellied version is solid enough to retain the shape of the container in which it is placed, including the can if you buy a premade canned version (shown in Figure 1, below). But what is the difference between the sauce and the jelly version? Pectin!
Figure 1. Cranberry sauce comes in two styles: a gooey sauce and a solid jelly, shown in this picture. The jellied version can be easily molded and is a popular canned holiday treat.
Pectin is a natural polymer that is found between the cells of plants and within the cell walls. It helps "glue" the plant cells together and keeps the plant tissues firm. And in cooked cranberries, as well as in other fruit jams and jellies, it can help stick the cooked fruit together to form a solid jelly. How does it work? To answer that question, let's start by defining what a polymer is. The basic idea is that a polymer is a series of molecules that attach to each other to form long chains. Molecules are the smallest possible piece of a material and they are so small that they cannot be seen, even with an optical microscope. A great example to explain polymers is a candy necklace. Pretend that the string is one type of molecule and the candy beads are a second type of molecule. The beads attach to the string, creating a necklace, which is just like a long polymer. If you were to jumble a handful of candy necklaces together in a box, they might, under the right conditions, get tangled together and form a larger formation, right? This is exactly what polymers do. As they tangle and interact, they form a rigid structure that traps other molecules around and in between them. In this way, polymers stiffen materials and give them shape and form. So when you are cooking cranberries, the pectin polymers form a net, trapping liquid sugar water molecules so that they cannot flow, and creating a firmer shape.
You can make cranberry sauce or jellied cranberries out of just water, sugar, and cranberries. So where does the pectin come from to make it jellied? The cranberries. Like other fruits and plant parts, cranberries have pectin in them. Cranberries actually have quite a lot of pectin, which helps keep the berries nice and firm before they are cooked. When the cranberries are heated on the stovetop in water, they pop open. The longer the berries are in the hot liquid, the more they break apart, which releases more and more pectin into the water. As they pop open, the pectin polymers float around in the big pot of liquid. It turns out that pectin is very attracted to water molecules, much like two magnets are attracted to each other, so rather than attaching to each other and tangling together, the pectin molecules bind to the water molecules. This results in a very runny, pink cranberry liquid, which is neither a thick, gooey sauce, nor a nice solid jelly. Not exactly what you want for part of your holiday dinner! To make the pectin molecules attract each other, you need to make the water molecules less attractive. But how? That's where sugar comes in. Sugar molecules are also very attracted to water molecules. So if you make the cranberry sauce by first dissolving sugar in the water and then adding the cranberries, most of the water molecules will already be attached to sugar molecules, leaving no room for the pectin polymers to bind with the water molecules. Instead, the pectin polymers will bind to each other, giving the cranberry mixture more and more structure. The sweet sugar-water molecules get trapped in between the pectin chains, resulting in a tasty cranberry sauce!
Let's get back to how to please everyone at your holiday dinner by serving both gooey cranberry sauce and jellied cranberries. How much cooking is required to release enough pectin to create a nice cranberry sauce? How about to make jellied cranberries, which can be placed in a mold? Head into the kitchen and find out for yourself!
Terms and Concepts
- Pectin
- Polymer
- Cell
- Molecule
Questions
- From where does pectin come?
- How does pectin help form a solid jelly?
- What role do sugar and water play in making cranberry sauce?
- What happens to the amount of pectin in the sauce the longer you heat the cranberries? Why?
- To heat the cranberries enough to pop them open, you need to bring the sugar-water mixture to a boil. During boiling, steam is released. Steam is made of water molecules. Do you think this has any effect on the gelling of the cranberry sauce? Why or why not?
Bibliography
This website offers more information about polymers:
- The University of Southern Mississippi. (n.d.). Polymer Basics. Department of Polymer Science. Retrieved November 4, 2013.
For more information about pectin, try these resources:
- McGee, H. (2004). On Food and Cooking: The Science and Lore of the Kitchen. New York: Scribner, pp. 296-297.
- Bowling, S. (1988, August 22). On the Jelling of Jelly. Retrieved November 19, 2009.
- International Pectin Producers Association. (2001). Facts About Pectin. Retrieved November 19, 2009.
Materials and Equipment
- Adult helper
- Fresh or frozen whole cranberries, 12-oz bags (6); if you're using frozen cranberries, they will need to be thawed first.
- Colander
- Granulated white sugar (6 cups)
- Water
- Liquid measuring cup
- Dry measuring cup
- Pot, minimum of 4-qt. capacity
- Spoon, heat-resistant and long-handled for mixing on the stove
- Stove
- Candy thermometer; it is highly recommended to use one that comes with a clip to attach it to the side of the pot; available from your local home goods store
- Watch or timer
- Ladle
- Ramekins or other small, heat-resistant containers, such as mugs (5)
- Should hold at least 6 oz. (3/4 cup) of liquid
- All five should be the same size and shape
- Optional: Camera
- Butter knife
- Small dessert plates (5)
- Masking tape
- Pen
- Lab notebook
Experimental Procedure
Caution: Have an adult help you make the cranberry sauce samples. Making the samples will require boiling a mixture of cranberries, water, and sugar. This mixture will be very hot and prone to splattering. Be cautious when stirring; if the mixture gets on your skin, it might burn.
Making the Cranberry Sauce Samples
- Using the masking tape and pen, label the ramekins (or other small, heat-resistant containers) as follows: 3 minutes, 6 minutes, 9 minutes, 12 minutes, and 15 minutes.
- Pour two packages (6 cups) of cranberries (already thawed, if you are using frozen cranberries) into the colander. Rinse the cranberries, throw away any that are squishy rather than firm, drain thoroughly, and set them aside.
- Add 2 cups of water and 2 cups of sugar to the pot.
- If the candy thermometer has a clip, attach the thermometer to the side of the pot. If not, set aside the thermometer until step 8.
- Have the adult helper place the pot of water and sugar on a stovetop burner. Adjust the burner to medium-high heat. Note: From this point on, we recommend that the adult helper stirs the mixture in the pot while you supervise, keep track of the time, and record observations in your lab notebook.
- Stir the water and sugar mixture until all the sugar is dissolved.
- Add the cranberries, as shown in Figure 2, below, and stir occasionally. After a couple of minutes, you will hear and see the cranberries popping open.
Figure 2. The photo above illustrates the stovetop setup for making cranberry sauce.
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Keep stirring until you can count slowly to five without hearing another cranberry pop open. Use the candy thermometer to determine the temperature of the cranberry mixture. If the temperature is below 100°C, continue stirring and monitoring the temperature. From this point on, the cranberry mixture will need to be stirred almost continuously.
- Note: Depending on your stove, you may need to turn the heat up to high in order to reach the 100°C mark.
- Note: At sea level, the boiling point of water is 100°C, but if you live at a higher elevation, water boils at a lower temperature. For example, in Denver, Colorado, the elevation is about one mile and water boils at about 95°C. For this reason, if you live at a high elevation your cranberry mixture might not reach 100°C, and you should aim for a slightly lower temperature, such as 95°C.
- Once the temperature of the cranberry mixture reaches 100°C (or a slightly lower temperature if you live at a high elevation), start timing. After 3 minutes, scoop out one ladle full of cranberry mixture and carefully pour it in a ramekin, as shown in Figure 3, below. Specifically pour the mixture into the ramekin labeled 3 minutes. Write down your observations of the consistency of the sauce in a data table, like Table 1, below, in your lab notebook. Also note the appearance of the cranberries; are they mostly whole, completely unrecognizable from the rest of the sauce, or somewhere in between? You might want to take photos of each sample to put on your Science Fair Project Display Boards.
| Sample | Consistency While Hot | Appearance of Cranberries | Color | Does It Hold a Shape? | Consistency When Cooled |
| 3 minutes | |||||
| 6 minutes | |||||
| 9 minutes | |||||
| 12 minutes | |||||
| 15 minutes |
Figure 3. Each cranberry sauce sample should be ladled into a labeled ramekin and set aside to cool, undisturbed.
- Continue stirring and timing the cranberry mixture, removing a ladle full of the sauce every 3 minutes and placing it in the corresponding ramekin. The last sample should be removed at the 15-minute mark. Remember to write down your observations about the consistency of the sauce and the appearance of the cranberries after each ramekin is filled.
Testing the Cranberry Sauce Samples
- Compare the color of all five cranberry samples. Write your observations down in your data table.
- Let the samples cool completely, at least 2 hours, at room temperature.
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After they have cooled, unmold each cranberry sauce sample onto a fresh plate, starting with the 3-minute sample, as follows:
- Gently loosen the cranberry sauce sample from the ramekin by running a butter knife along the inside edges of the ramekin.
- Put the plate face-down on top of the ramekin and then simultaneously flip over the ramekin and the plate. The plate should now be right-side-up, and the ramekin should be upside-down. The cranberry sauce should easily slip out of the ramekin, as shown in Figure 4, below. If it does not, try carefully loosening the sample more using the butter knife.
Figure 4. Unmold each cranberry sauce sample onto a fresh plate. Samples, like the one shown above, which retain the shape of the ramekin, are considered to be jellied cranberry sauce samples.
- Mark down in your data table which cranberry sauce samples retain the molded shape of the ramekin. Also observe how firm the cranberry sauce is. Is there a relationship between the amount of time the cranberries spent cooking at 100°C and the cranberry sauce's ability to create a stable gel?
- Compare the consistency of the five cooled cranberry sauce samples. Are the samples that can not hold a shape all identical in their consistency? How about the ones that can hold a shape?
- To be sure that your results are reproducible, repeat this entire experiment two more times with fresh ingredients and clean materials.
Ask an Expert
Variations
- Devise a more exact method of comparing the consistencies of the cranberry sauce samples, both while hot and once they're cooled. Try consulting the Science Buddies Egg Substitutes, Experiment with Making Gels, and Making Jell-o Stronger science fair projects for ideas on how to do this.
- As outlined in the Introduction, sugar plays an important part in allowing the pectin molecules to find each other. What happens to the cranberry sauce's ability to solidify as you change the amount of sugar in the recipe? Try both increasing and decreasing the sugar. Hint: If you are having trouble interpreting your results, you might also want to think about what sugar does to the boiling point of water.
- Which other fruits have enough natural pectin to create solid gels? Design an experiment to find out.
- You can purchase "no-sugar pectin" for making jam. How do these no-sugar pectins work? Design tests to find out.
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