Save Those Spoiling Strawberries!
AbstractFresh strawberries and summer just go together. Walking through the local farmers' market on a warm day, the bright, red strawberries call out to you, beckoning you to buy them and take them home. The next day, as you get ready to savor the delicious berries, you notice that yesterday's juicy, red strawberries are now covered in...eewwww, mold! In this cooking and food science fair project, you will investigate thermotherapy and whether this technique can preserve strawberries and prevent mold and decay.
Michelle Maranowski, PhD, Science Buddies
To investigate thermotherapy using strawberries, and to determine the right combination of temperature and time to prevent the strawberries from molding.
Strawberries are used in many delicious recipes—strawberry ice cream, strawberry pancakes, strawberry shortcake. The list of ways to enjoy strawberries goes on and on. One of the best ways to enjoy a strawberry is to pick it from the plant, wash it, and pop it straight into your mouth. The sweetness of the berry tickles the taste buds. However, once in a while, the tasty treat turns into something gross—a moldy and decomposing berry. Besides making you sick, moldy berries are a waste of money. For some, wasting money on berries that mold so quickly can practically make them sick!
Is there a way to prevent strawberries (and other fruit) from molding too quickly? Something that you can do easily at home? Pomologists are scientists who study the development, cultivation, and physiology of fruit plants, and they have long been aware of the benefits of thermotherapy treatments. In the thermotherapy process, fruit is exposed to hot water (thermos is Greek for "hot") for a set amount of time. The hot water kills mold spores and slows the spoilage of berries and other fresh fruit. Simply dip your strawberries in hot water for a few seconds to minutes, remove the berries from the hot water, and spread them out on a clean kitchen towel to dry off completely and cool down. Then, repack the strawberries in a basket; it is that simple! But how long will the therapy last? And another key question is whether thermotherapy treatments destroy the flavor of the fruit.
In this cooking and food science fair project, you will investigate the effectiveness of thermotherapy treatments on slowing spoilage of strawberries. Your task will be to find out the right combination of water temperature and treatment time. One benefit of this cooking and food science fair project is that if you figure it out, you may never have to see another moldy strawberry again!
Figure 1. Strawberries can go from succulent and sweet (1.a.) to spoiled and scary (1.b.) in just one day! (1.a.:United States Department of Agriculture, 2006; 1.b.:Flickr, 2007.)
Terms and Concepts
- Control, as in an experiment
- What kinds of mold will grow on a strawberry that is spoiling?
- From your research, can you describe how thermotherapy is used to treat other types of fruits and fruit trees?
- What is the difference between pomology and horticulture?
- McGee, H. (2009, August 26). The Curious Cook: Prolonging the Life of Berries. The New York Times, page D5. Retrieved October 1, 2009.
For help creating graphs, try this website:
- National Center for Education Statistics. (n.d.). Create a Graph. Retrieved October 2, 2009.
Materials and Equipment
- Pot with lid, large enough to hold a strawberry basket
- Strawberries (at least 18 pints of fresh strawberries)
- Baskets, plastic, pint-sized (18). You can find these baskets at your grocery store. Just ask the produce department to give you a few.
- Thermometer. A partial immersion thermometer or a digital probe thermometer suitable for this project are available online from Carolina Biological Supply Company.
- Digital stopwatch
- Slotted spoon
- Kitchen towels, clean (at least 3)
- Paper towels (1 roll)
- Plastic wrap (1 roll)
- Sticky notes
- Masking tape (1 roll)
- Disposable gloves (1 box). Can be purchased at a local drug store or pharmacy, or through an online supplier like Carolina Biological Supply Company. If you are allergic to latex, use vinyl or polyethylene gloves.
- Lab notebook
- Graph paper
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Note: Try to perform all trials of your experiment on the same day so the strawberries are all approximately the same freshness. You will perform three trials of each thermotherapy treatment, further explained below.
Performing the Experiment
- Fill the large pot with water and place it on a stovetop burner on medium heat. The pot should have enough water in order to cover a basket of strawberries, but not so much that the water spills over the side of the pot when you immerse the strawberries.
Take a pint of strawberries and separate them into two groups of about equal numbers of strawberries. Put each group into a new plastic basket. Assign a number to each basket and remember which is which. Record the basket number, the number of berries in each basket, and the date in your lab notebook in a data table like the one shown below. You will expose one of these baskets to thermotherapy and leave the other basket alone for comparison.
Basket Number of Berries Start Date Thermotherapy Conditions (Water Temperature & Immersion Time) Initial Observations Observations After One Day Observations After Two Days Number of Moldy Berries: Number of Moldy Berries: Number of Healthy Berries: Number of Healthy Berries: Appearance of Berries: Appearance of Berries:
- Before placing a basket inside the pot, bring the water to 125°F. Use the thermometer to monitor the water temperature. Record the water temperature in your lab notebook. Spread out a clean kitchen towel.
- Once the water comes to 125°F, turn off the burner and immerse one basket of strawberries into the hot water. Immerse this basket for 45 seconds (sec.). Use the timer to keep track and use the slotted spoon to keep the strawberries submerged.
- After 45 sec. have elapsed, quickly remove the strawberries and the basket from the water and spread them out on the clean kitchen towel to dry and cool off. Use caution when working with the hot water. Use both the tongs and the slotted spoon to remove the strawberries from the water, and the paper towels to gently blot the strawberries off. Do not expose the second basket to thermotherapy.
- When the strawberries are completely cool and dry, put them back into their basket. Note the condition of the strawberries. Do they still look red, or did the hot water affect the color and condition of the berries? Record your observations and the time that you immersed the strawberries in the hot water in your lab notebook.
- Now wrap each of the baskets, both the treated strawberries (those that were heated) and the untreated strawberries (those that were unheated), in two layers of plastic wrap. This will create a nice humid environment for the berries.
- Write the basket number, the conditions of the treatment (treated or untreated), the time of immersion, the date, and the trial number on sticky notes and label the baskets with the notes. Use masking tape to secure the sticky note the plastic wrap. Make sure to record this data in your lab notebook, too. The untreated baskets are control baskets. The purpose of a control is to act as a constant and to highlight any effects the variables in an experiment (like heat or no heat) may have on the experimental group. To learn more about variables and controls, read the Science Buddies page Variables in Your Science Fair Project for a more in-depth discussion.
- Now place the wrapped baskets in a location that will not be disturbed on top of a few pages of newspaper. The newspaper will absorb any excess liquid from the strawberries. Keep the baskets at room temperature and not in the refrigerator.
- Repeat steps 1–9, except this time, immerse the strawberries for 3 minutes in 125°F hot water.
- Repeat steps 1–9, except this time, increase the temperature of the water to 140°F and immerse the strawberries for 45 sec.
- Repeat steps 1–11 two more times. It is important to perform at least three trials of your experiment so that you can be sure that your data is reliable and repeatable. The temperature and time conditions are shown in the table below:
Analyzing Your Data
Leave all three trials of the strawberries undisturbed for one whole day. After a day has gone by, unwrap the baskets of strawberries and make observations about the conditions of the strawberries.
- Record the date in your lab notebook.
- Put on a pair of disposable gloves.
- Note down in your lab notebook how the surfaces of the berries look, if the color has changed, how the berries smell, if the berries are becoming watery. Observe one basket at a time.
Count the number of moldy berries and the number of berries that do not have mold. Record the numbers in your data tables in your lab notebook. Make sure that you keep track of the treatment and the trial for each basket.
- Important: Be sure to change your gloves each time you handle berries from different baskets. This will help avoid any cross-contamination of mold spores from one basket to another during handling.
- Rewrap the berries in the plastic wrap. Let the berries sit undisturbed for another full day.
Repeat step 1 of this section over and over again until all the berries have molded. Once all of the berries in a basket have mold on them, you can throw that basket away, but remember to record in your lab notebook that you have done so and on what date.
- Change the newspaper layer with fresh newspaper as needed.
- Plot the data that you collected on a graph. You can make your graphs by hand or you can make your graphs online at https://nces.ed.gov/nceskids/CreateAGraph/default.aspx. Choose a bar chart to display the data. Label the x-axis Treatment and the y-axis Days Without Mold. On the Treatment axis, group each control and its companion treatment together. Do the treated berries last longer than the untreated berries? Which treatment results in berries that last the longest?
Ask an Expert
- Calculate and plot the rate at which the strawberries get moldy for each treatment.
- Repeat the experiment with blueberries or different kinds of berries.
- Will refrigerating the treated berries make them last longer? Compare how long refrigerated, untreated berries last with refrigerated, treated berries.
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