Does your family have jellied cranberry sauce with Thanksgiving dinner? Jellied cranberries are thick and retain the shape of the mold in which they are placed, which might mean a turkey-shaped mold or even the shape of the can if you use one of the popular canned verities. Taking a bite of wiggly jellied cranberries can be a fun addition to a delicious meal, but cranberries can also be served as a liquidlike sauce. Both versions use the same ingredients, so what makes one turn into a gelatin whereas the other stays more fluid? To find out, try out this holiday-flavored science activity!
Cranberry sauce can be served either as a gooey liquid or as a solid jelly. The jellied version is solid enough to retain the shape of the container in which it's placed whereas the sauce version is much more fluid. The difference between the fluid sauce and the jelly versions comes down to pectin.
Pectin is a natural polymer—a series of molecules that attach to one another to form long chains. It is found between plant cells and within their cell walls. Pectin helps "glue" the plant cells together, keeping their tissues firm. And in cooked cranberries as well as in other fruit jams and jellies this pectin can help stick the cooked fruit together to form a solid jelly. When cranberries are cooked, their pectin polymers tangle and interact, forming a net that traps dissolved sugar molecules so they can't flow. This creates a firm shape. Cranberries naturally contain a lot of pectin, which helps keep the berries nice and firm. This extra pectin gets released when they are cooked. But what determines if the resulting cranberry sauce is liquid or jelly?
• Two 12-ounce bags of fresh or frozen whole cranberries
• Two cups of granulated white sugar
• Measuring cup
• Pot, at least four-quart capacity
• Heat-resistant, long-handled spoon for mixing on the stove
• Candy thermometer (It's highly recommended to use one that comes with a clip to attach it to the side of the pot.)
• Watch or timer
• Four ramekins or other small, heat-resistant containers, such as mugs or a muffin tin (They should each hold at least three quarters of a cup, and be the same size and shape.)
• Butter knife and plates (optional)
• If you are using frozen cranberries, thaw them in advance.
• Pour two packages of cranberries into a colander. Rinse the cranberries; throw away any that are squishy rather than firm; drain thoroughly and set them aside.
• Make sure an adult helps with making the cranberry samples on the stove. When it is on the stove, the cranberry mixture will be very hot and prone to splattering. Be cautious when stirring—if the mixture gets on your skin, it could burn.
• Add two cups of water and two cups of sugar to the pot. If you are using a candy thermometer with a clip, attach the thermometer to the side of the pot. Heat the pot on a stove-top burner, adjusting the burner to medium-high heat. Stir the water and sugar mixture until all the sugar is dissolved.
• Add the cranberries and stir occasionally. After a couple of minutes, the cranberries should start popping open. What does this sound and look like?
• Keep stirring until you can count slowly to five without hearing another cranberry pop open. What is the temperature of the mixture? If the temperature is below about 210 degrees Fahrenheit, continue stirring and monitoring the temperature. You'll need to keep stirring the mixture almost continuously, and you may need to turn the heat up to high to reach the 210-degree F mark. (Note: Much above sea level water boils at lower temperatures, such as at about 203 degrees F at an elevation of one mile. Consequently, if you live at a high elevation, your mixture might not reach 210 degrees F and you should aim for a slightly lower temperature.)
• Once the temperature reaches about 210 degrees F (or the equivalent for your elevation), start timing. How do the cranberries look after three minutes? Are they mostly whole, completely unrecognizable from the rest of the sauce or somewhere in between? At this point, scoop out one ladle full of cranberry mixture and carefully pour it in a ramekin (or other small, heat-resistant container). What is the consistency of the mixture you scooped out?
• Continue stirring, timing and observing the cranberry mixture. Remove another full ladle of cranberry mixture after seven minutes since you started timing. Pour it into a separate ramekin. How do the cranberries look now? What is the mixture's consistency like?
• In this same way remove a full ladle of cranberry mixture at 11 minutes and another at 15 minutes since you started timing, putting each scoop into their own ramekin.
• How do the cranberries look after 15 minutes of cooking compared with three minutes? How did the consistency of the cranberry mixture change over time as it was cooked? How do the colors of the mixtures compare with one another? Do you see a relationship between these observations and how long the cranberries were cooked?
• Extra: If you have time, let the cranberry sauce samples cool at least two hours at room temperature. After they've cooled unmold each sample onto a fresh plate by running a butter knife along the inside edges of the ramekin or container to gently loosen the cranberry mixture, placing a plate facedown on top of the ramekin and then flipping over the ramekin and plate together. What is the color and consistency of the different samples once they've cooled? How do they compare with one another? Do any of them retain the molded shape of the ramekin, which means they're considered to be jellied cranberry sauce?
• Extra: Sugar plays an important part in allowing pectin molecules to find one another when cooking cranberries. To investigate this, try this activity again but this time try increasing and/or decreasing the amount of sugar you use. What happens to the cranberry sauce's ability to solidify as you change the amount of sugar in the recipe? Hint: If you're having trouble interpreting your results, you might also want to think about (or try discovering) what sugar does to the boiling point of water.
• Extra: Try this activity with other fruits. What other fruits have enough natural pectin to create solid gels?
Observations and results
Did the cranberry mixture that cooked the longest make the thickest and firmest jellied cranberry sauce? Did the one that was cooked for the shortest amount of time make a relatively runny, liquidlike cranberry sauce with whole cranberries still visible?
The longer the cranberries were heated in the hot liquid, the more they should have broken apart, which releases more and more pectin polymers into the water. The more the pectin polymers bind to one another, the greater the mixture’s structure and firmness. (Also, water is released as steam while you cook the cranberries, which means that the longer they cook, the less water they hold.) Taken together, this is why the cranberry sauce that was cooked the longest should have been the thickest and had the fewest whole cranberries visible when it was hot and being stirred. It should also have been the firmest, thereby retaining its shape the best once cooled. On the other hand, the cranberry sauce that was cooked for just three minutes should have been the most liquidlike, with whole cranberries visible, when it was being prepared, and it should have been the thinnest sample once cooled. As you should have seen in this activity, jellied cranberry sauce (which retains its shape) can be made by cooking fluid cranberry sauce for a longer amount of time.
You may enjoy a tasty snack of cranberry sauce!
More to explore
On the Jellying of Jelly, from the Alaska Science Forum
What Is Pectin?, from the International Pectin Producers Association
Polymer Basics, from the University of Southern Mississippi
From Sauce to Solid: The Science of Cranberry Condiments, from Science Buddies