Activity Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Exploring Acid−Base Chemistry by Making and Monitoring RedCabbage Sauerkraut: A Fresh Twist on the Classic Cabbage-Indicator Experiment Jacqueline L. Linder,† Sumeja Aljic,† Hamzah M. Shroof,† Zachary B. Di Giusto,† James M. Franklin,† Shane Keaney,† Christopher P. Le,† Olivia K. George,† Andrew M. Castaneda,† Lloyd S. Fisher,† Virginia A. Young,‡ and Adam M. Kiefer*,† Downloaded via TULANE UNIV on December 20, 2018 at 14:57:14 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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Department of Chemistry, Mercer University, Macon, Georgia 31207-0003, United States Department of Biology, Mercer University, Macon, Georgia 31207-0003, United States
‡
S Supporting Information *
ABSTRACT: Naturally occurring anthocyanin indicators have been used for many years to introduce science students to acid−base concepts. A classroom activity has been developed that uses naturally occurring anthocyanins to monitor the pH changes associated with the fermentation of red cabbage to sauerkraut by lactic acid bacteria. Through the activity and related discussion, students are introduced to the scientific method, basic laboratory practices, acid−base chemistry, chemical and physical changes, fermentation, and microbiology. Although this activity and accompanying assignments are designed for students ages 6−12, the activity is adaptable to students of all ages and backgrounds.
KEYWORDS: General Public, Elementary/Middle School Science, Public Understanding/Outreach, Interdisciplinary/Multidisciplinary, Hands-On Learning/Manipulatives, Acids/Bases, pH, Physical Properties
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to a way to monitor this pH change in situ with no need for additional supplies. Chemistry majors enrolled in a research-based, junior-level capstone laboratory course at Mercer University were tasked with developing a procedure to ferment red cabbage and monitor the pH change throughout the process. Within a few days, the purple-colored cabbage turned fuchsia, concomitant with the decrease in pH associated with fermentation. The students recognized that this simple, colorimetric change bridged numerous chemical concepts such as acid−base chemistry, physical versus chemical changes, and biological fermentation. Over a five-week period, the chemistry majors developed an activity highlighting the chemistry of the fermentation of red cabbage. They worked with 12 local children ages 6−12 who are homeschooled, meaning they receive their education outside of a traditional public or private school. The activity presented here is the result of this assignment. In addition, this activity has been modified and implemented as a college-level microbiology laboratory and is easily modified for use in middle- and high-school science classes and outreach activities in the local community.
he aqueous extraction of anthocyanins from red cabbage to produce an acid−base indicator solution is a standard demonstration in high school and undergraduate chemistry classrooms.1,2 The vibrant color changes of the deep purple solution to fuchsia under acidic conditions and blue under basic conditions provide the audience with a visual representation of acid−base properties. The acid−baseindicating ability of anthocyanins has also been used to great effect in the bridging of art and science,3,4 in instruction related to the chemistry of natural dyes,5 and in food-chemistry experiments related to homemade sports drinks.6 The fermentation of cabbage to produce sauerkraut7,8 and kimchi9 has also been used in food-science demonstrations and laboratories. During the fermentation process, lactic acid bacteria (LAB) produce lactic acid from glucose, decreasing the pH of the ferment. This prevents the growth of dangerous bacteria, preserving the food against spoilage. While this decrease in pH can be monitored using pH meters, the colorimetric assessment of pH using test strips can be complicated by the natural color of the ferment and added spices. This makes the inexpensive option of using test strips difficult for any ferment with color, limiting the usefulness of fermentation for teaching acid−base chemistry. It follows that the fermentation of foods containing anthocyanins might lead © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: September 20, 2018 Revised: December 2, 2018
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DOI: 10.1021/acs.jchemed.8b00767 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Activity
Table 1. Timeline of Sauerkraut-Activity Components Day
Activity Component
0 or 1 1 1 1 1 1 1 1 2−5 5
Preparation of cabbage; cleaning and sterilization of food-safe containers Washing of hands; discussion of safe handling of food Weighing of salt and cabbage; initial mixing of salt and cabbage Discussion I: chemical vs physical changes; importance of observations Second mixing of salt and cabbage Discussion II: observations, discussion of osmosis, introduction to fermented foods Canning of sauerkraut, cleanup Discussion of fermented foods Independent student monitoring, observations, agitation of cabbage Group discussion and taste test
Participants Instructor Instructor Students Instructor Students Instructor Students Instructor Students Instructor
and students and students and students and students and students
Estimated Time, min ≥60a 5 5−10 10 3−5 10−15 5 30 5 30
a
The amount of time spent varies, depending on the amount of cabbage, the facilities, etc.
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MATERIALS Materials were scaled on the basis of the number of students participating in the activity. Below are the materials required for each student in a group of three to make their own 1 pint glass canning jar of sauerkraut. All chopping of cabbage using a knife and a food processor was done prior to the activity. To prepare the activity, instructors required these materials: • one food-grade scale • chef’s knife, cutting board, and food processor • large, food-safe container (5 gallon capacity, ∼20 L) • 70% ethanol solution for sterilization of glass canning jars To complete the activity, a group of three students required these materials: • 1.4 kg of red cabbage, chopped • 25 g of coarse kosher salt • large mixing bowls, a colander, and utensils • three glass canning jars with lids (1 pint, ∼0.5 L)
crossword puzzle. The cabbage was mixed for an additional 5 min, and squeezed over the colander to remove excess juice. The cabbage rested again in the colander, and students were asked to compare the shredded cabbage before and after the mixing, particularly concentrating on the differences in taste, color, consistency, and texture. During this time, the group defined and discussed the importance of scientific observation. In addition, students were shown a whole head of cabbage and asked to explain the physical changes that occur during the slicing and mixing processes. After the discussion, students rewashed their hands, squeezed the cabbage firmly over a colander to release as much liquid as possible, and used the colander to separate the cabbage from the liquid. Students once again discussed their observations of the process. At this stage, much liquid had been released from the cabbage and the consistency of the cabbage changed drastically. An undergraduate student demonstrated that the weight of the bowl, cabbage, and water was approximately the same before and after the mixing of the cabbage and salt. Students concluded that water in the bowl must come from the cabbage, and the instructor discussed the conservation of mass and the differences between observations and inferences. The instructor and undergraduate students led a brief discussion of osmosis and students were asked whether their observations were in line with what they expect would happen during osmosis. The instructor then directed the discussion to food safety and introduced the concept of fermentation. Students filled their clean glass canning jars two-thirds of the way to the top with the cabbage. The cabbage was compressed, and the container filled to the threads of the jar with conserved cabbage juice. The cabbage and juice were stirred, and the cabbage compressed to the bottom of the container. The jars were labeled with the students’ names and date of bottling. During the canning process, the instructor and undergraduates collected the excess aqueous extract, freezing it for demonstrations on day 5. After canning, the instructor and undergraduate students led a final discussion of fermentation and food preparation focusing on: (i) the historical necessity of fermented foods, (ii) a simplified overview of bacterial competition during fermentation, and (iii) fermented foods that the students have eaten. Students left with a set of instructions on how to care for their sauerkraut and were charged with making daily observations of the sauerkraut over the next four mornings. Each morning, students slowly opened the container, releasing gas produced during fermentation. The students and parents were instructed not to consume any of the cabbage during this time. All students recorded a color change from purple to
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OVERVIEW OF THE ACTIVITY Cabbage has been safely fermented for human consumption for millennia. Although numerous recipes exist, the preparation guide in the Supporting Information has been tested and produces edible sauerkraut, provides an excess of cabbage-juice extract for additional testing and demonstrations, and allows for completion of the activity in a set period of time (Table 1). Beginning the activity on a Monday allows for the activity to be completed on the following Friday. Immediately prior to the activity, the instructor and two undergraduate students cleaned the cabbage with cool water, removing any dead or wilting outer leaves during the process. The base of the cabbage was removed with a knife, and the remaining cabbage was cut into cubes and passed through a food processer using a chop attachment. The chopped cabbage was placed in a food-safe container and stored in the refrigerator until use. Preparation can occur up to 18 h prior to making the sauerkraut. The students were instructed to thoroughly wash their hands, and the instructor led a class discussion of appropriate hygiene in food preparation. Students were assigned to groups of three, and 1.4 kg of shredded red cabbage was weighed into a food-safe bowl. Kosher salt (25 g) was weighed and sprinkled over the cabbage. Students then took turns mixing the salt into the cabbage over 5 min, taking care to squeeze the cabbage tightly to ensure the release of water from the cabbage. The cabbage rested for 5 min while the students worked on the B
DOI: 10.1021/acs.jchemed.8b00767 J. Chem. Educ. XXXX, XXX, XXX−XXX
Journal of Chemical Education
Activity
of the undergraduate helper served as a means of assessing student learning. The activity can easily be modified for a variety of other classroom scenarios. The procedure has also been adapted and incorporated into a nonmajor nursing microbiology course. In this case, the activity was accompanied by additional instruction and assignments to ensure that the material was relevant to the course. Additional modifications can be made to ensure the activity is applicable to different classroom and outreach scenarios. For example, larger classes could ferment the cabbage as a class demonstration rather than individually, and fermentation time can be decreased to 36 h if the cabbage is finely shredded.
fuchsia on day 3 or 4. The group reconvened on day 5 to compare their observations, conduct a taste test of their sauerkraut, and explore the implications of pH change during the fermentation process. The thawed excess cabbage extract collected on day 1 was used to demonstrate the color changes associated with an increase in pH (addition of sodium bicarbonate) and a decrease in pH (addition of lime juice). Students discussed why a decrease in pH leads to food preservation.
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HAZARDS The fermentation of food relies upon encouraging the selective growth of LAB, found naturally on the cabbage, to generate the lactic acid that inhibits the growth of unwanted pathogenic bacteria. In order to limit potential student exposure to pathogenic bacteria, all cutting implements and containers were washed thoroughly with soap and water and sterilized with a 70% ethanol solution (diluted grain alcohol). All students washed their hands thoroughly in front of the instructors. Ingestion of raw cabbage and the sauerkraut produced is at the discretion of the instructor in consultation with parents or guardians. The instructors sterilized the glassware and chopped the cabbage prior to student participation, so students did not handle cutting implements or ethanol. Depending on the policies of the instructor’s institution, appropriate releases and food-sensitivity and -allergy issues should be addressed with parents or guardians. Cabbage juice can discolor skin and clothing, so students were instructed to wear old clothing. During fermentation, the jar must be opened every 24 h to release accumulated pressure. Appropriate venting prevents the fermentation vessel from potentially cracking or shattering.
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CONCLUSION The fermentation of red cabbage is inexpensive, uses readily available materials, demonstrates numerous concepts central to chemistry, and is easily employed in the classroom or in the home laboratory. This activity has been conducted with homeschooled students aged 6−12, but can be altered to teach students of different ages and backgrounds. Similarly, an instructor can choose to use this activity as a class demonstration. This activity modifies the classic anthocyaninindicator demonstration and experiment to incorporate concepts from food science and fermentation in a safe and edible manner.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available on the ACS Publications website at DOI: 10.1021/acs.jchemed.8b00767. Faculty instructions for preparing and executing the activity (PDF, DOCX) Student instructions (PDF, DOCX) Background handouts for students (PDF, DOCX) Crossword puzzle worksheet (PDF, DOCX)
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DISCUSSION The fermentation of red cabbage demonstrates numerous concepts central to chemistry; when combined with instruction and discussion, it can be used to teach scientific principles to students of a variety of ages and backgrounds (Table 2). These principles are reinforced through in-class discussions, student homework, and additional worksheets.
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*E-mail:
[email protected].
Table 2. Scientific Principles Highlighted during the Activity Science Concepts
Corresponding Examples from the Sauerkraut Activity
Scientific method
Observations vs inference, data vs interpretation, using experimentation to draw inferences from collected data Indicator color change with citrus and baking soda, comparison to color change during fermentation Weighing cabbage, taring containers, maintaining a laboratory notebook for observations LAB fermentation, sanitation, hygiene Chopping cabbage vs fermenting cabbage
Acid−base chemistry Basic laboratory skills Microbiology Chemical vs physical changes
AUTHOR INFORMATION
Corresponding Author ORCID
Adam M. Kiefer: 0000-0002-3277-7135 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Garland Crawford and Sylvia S. Bridges for helpful discussions related to the content of the paper and Caryn Seney for helping organize teaching sessions with homeschooled students.
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This activity is particularly accessible to homeschooled students, who may not have access to conventional chemicals and glassware needed for chemical demonstrations. Because of the wide variety of ages of the students present during the initial session, undergraduate helpers were necessary for engaging the students at the levels of their individual abilities. Completion of the written assignments with or without the aid
REFERENCES
(1) Shakhashiri, B. Chemical Demonstrations: A Handbook for Teachers of Chemistry; University of Wisconsin Press: Madison, WI, 1989; Vol. 3. (2) Fortman, J. J.; Stubbs, K. M. Demonstrations with Red Cabbage Indicator. J. Chem. Educ. 1992, 69 (1), 66. (3) Suzuki, C. Making Colorful Patterns on Paper Dyed with Red Cabbage Juice. J. Chem. Educ. 1991, 68 (7), 588. C
DOI: 10.1021/acs.jchemed.8b00767 J. Chem. Educ. XXXX, XXX, XXX−XXX
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(4) Lech, J.; Dounin, V. JCE Classroom Activity #110: Artistic Anthocyanins and Acid−Base Chemistry. J. Chem. Educ. 2011, 88 (12), 1684−1686. (5) Fanis, L. N. A Colorful Connection for Your Classroom. J. Chem. Educ. 2008, 85 (9), 1172. (6) Stoddard, R. L.; McIndoe, J. S. The Color-Changing Sports Drink: An Ingestible Demonstration. J. Chem. Educ. 2013, 90 (8), 1032−1034. (7) JCE staff. Cabbage Patch Chemistry. J. Chem. Educ. 2000, 77 (11), 1432A. (8) Oberg, C. J.; Brown, R. J. Preservation by Fermentation: Focusing on the Chemistry and Microbiology of Vegetables. J. Chem. Educ. 1993, 70 (8), 653. (9) Kiefer, A. M.; Young, V. A. Kimchi: Spicy Science for the Undergraduate Microbiology Laboratory. J. Microbiol. Biol. Educ. 2014, 15 (2), 297−298.
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DOI: 10.1021/acs.jchemed.8b00767 J. Chem. Educ. XXXX, XXX, XXX−XXX
