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What is "sweetness?" How do you define it? A fruit's sweetness varies not only with its age (ripe vs. unripe) but also with its type, or cultivar. A cultivar is a type of plant with desired characteristics that growers have created by breeding and then maintained by cultivation. So, cultivars of the same plant have different characteristics; using apples as an example, Red Delicious are sweeter than Granny Smiths or Lady Apples.
This lesson has your students create a scale that will allow them to measure and compare the sweetness of several types of apples, a non-sweet plant (potato) and a popular, sugar-sweetened soft drink.
Students will:
Science National Content Standard 1: Science as Inquiry
As a result of activities in grades 9-12, all students should develop:
Science National Content Standard 2: Physical Science
As a result of their activities in grades 9-12, all students should develop an understanding of:
Science National Content Standard 5: Science and Technology
As a result of activities in grades 9-12, all students should develop:
Science National Content Standard 7: History and Nature of Science
As a result of activities in grades 5-8, all students should develop understanding of:
PROCEDURE:
1. With a clean 1000 mL graduated cylinder, measure out 900 mL of water and pour it into a clean beaker. Add 100 g of sugar. Stir to mix well and add additional water to bring the final total volume to 1000 mL. This will give you a 10% sucrose solution.
2. Repeat the steps above, making four additional solutions, adding:
3. Pour solutions from each graduated cylinder into individual labeled cups (about 5 ml per cup). Each student should have seven cups total: one cup with plain water (cup 1), five cups containing the ascending sucrose concentrations (1, 4, 6, 8 and 10% sucrose; cups 2 through 6), and one cup of sugar sweetened soft drink (cup 7).
60 minutes
The apple has long been
a popular fruit around the world. A surprising fact about apples is
that most taste bitter when picked from a tree grown from apple seeds
rather than from grafts. Eighteenth and nineteenth century Americans
grew bitter apples in abundance and used them to make one of the most
popular beverages of the day: alcoholic or ("hard") cider. The fabled
Johnny Appleseed helped to propagate the hard-cider apples by planting
many apple seedlings. But, although they occur rarely in nature, sweet-tasting
apples have become the ones most favored by humans, sometimes at the
expense of varieties that aren't as sweet. Sweetness is a biological
desire that's inborn in all human beings, so sweeter apples appeal
more broadly than less sweet apples.
The perception of sweetness starts on our tongues. If you look on your tongue, you will see pink bumps called taste papillae. Inside the papillae are taste buds, which are onion-shaped structures that contain taste cells. The taste cells on our tongues help us gauge the saltiness, sourness, sweetness, acidity, and savoriness of foods and drinks. When sweet molecules from foods and beverages bind to receptors on the tongue, they fit together like a lock and key. Once the sweet receptor is stimulated, a series of biochemical steps take place in the taste cell that results in an electrical signal. Nerves convey this signal to the brain where it is interpreted as sweet. Sugars are not the only compounds to stimulate the sweet receptor. For instance, man-made compounds like saccharin also stimulate the sweet receptor. Babies are born liking sweet; this innate preference is thought to have evolved because sugars are a simple and safe source of calories. Sweetness is not an all-or-nothing experience; rather, it is a matter of degree. Apples or drinks can have a range of sweetness.
SWEETNESS DISCUSSION:
As a class, view Clip 1, Clip 2, and Clip 3 from The Botany of Desire and briefly discuss the concepts explored in the clips. What is the purpose of sweetness? How has the successful dissemination of apples throughout the world been related to sweetness? Is there such thing as too sweet?
Have your students try to express to each other how much sweeter soda is than an apple. Ask them if they think their taste preferences have changed as they have grown older, and if so, to speculate why that has occurred.
Before tasting begins, have your students
first drink plain water to cleanse their palates.
Create a baseline for sweetness by
having students taste the 10% sucrose solution (cup 6) and assign this
level of sweetness a score of 100 on a sweetness scale. Have the
students taste plain water (cup 1) and assign it a score of 0 on a sweetness
scale.
Have students taste the 1, 4, 6, 8%
sucrose solutions and using numbers from 0 to 100, rate the sweetness
of each solution. They should record their ratings on their data collection
sheet. Then provide students with slices or chunks from at least
four different varieties of apples, a slice of potato, and 5 ml of a
sugar-sweetened soft drink to taste (cup 7) and have students rate according
to their sweetness. Your students should rate each item in relation
to the sucrose solutions by assigning it a number. They may re-taste
the sugar solutions to refresh their memories at any time. Some students
may rate the sugar-sweetened soda as higher than 100.
See the example scale of sweetness
ratings (Reproducible 2 - Download PDF) obtained from a class of 10th-grade
students below.
With students' individual assessments
complete, prompt them to think about why we have so few cultivars of
apples in our grocery stores, as well as how our choices of apples in
the grocery store relate to their levels of sweetness.
Encourage students to share their observations
on the experience. They are likely to point out differences in the sweetness
of natural foods and the soda, and to talk about how sweetness is influenced
by context, i.e., sweet seems different in an apple than in a sugar
water solution. How does the color of the apple influence their
perception of sweetness? Ask students to speculate on the growing expectation
and tolerance for sweeter tastes such as sugar-sweetened soda.
Ask students to discuss the role that
sweetness has played in the popularity and distribution of the apple
across North America. Provide the following prompts:
Discussion Questions:
Ask the following questions of your students and discuss their answers:
Concept development: Topics Education
Testing methodology: Scott Stein, Head of Science, Springside School, Philadelphia
Scientific advisor: Danielle R. Reed,
Monell Chemical Senses Center, Philadelphia
Special thanks to Gary K. Beauchamp
and Leslie J. Stein, Monell Chemical Senses Center, for advice and assistance
in developing this material. www.monell.org
