The abundance of online resources that students have access to makes it difficult to determine whether the information they find is accurate. Several online sources have claimed that hamburgers from fast-food chains do not decompose, and we developed two exercises that allow students to (1) test this claim, (2) learn about fungi, and (3) reflect on their eating habits. In both exercises, we inoculated fast-food items with the fungus Rhizopus stolonifer. The first exercise was a controlled experiment with hamburgers, and the second was a screening trial in which all students brought in their own fast-food items. In both exercises, animal-based products and fatty baked products (e.g., biscuits) allowed R. stolonifer to grow, while condiments and bread products tended to inhibit growth. Our students indicated that they would be more critical of online information and that they would eat less fast food. These exercises met our objectives and engaged our students, and we encourage others to develop exercises that examine online claims.

Introduction

Use of online sources can have a substantial influence on student perceptions and knowledge (Metzger et al., 2003; Metzger, 2007). Online sources that propagate misconceptions, whether intentionally or not, can be pernicious. Persistent misconceptions can be deleterious for public health, for instance with nutrition, antibiotics, and vaccination (Wansink, 2006; Barbacariu, 2014; Carter et al., 2016).

We used two types of exercises in an introductory college biology lab to test the veracity of a common online claim that fast food does not decompose while also teaching college biology students about fungi. Several websites have recently claimed that hamburgers from one or more prominent fast-food chains can last a year or longer without decomposing, even remaining “edible” (Little, 2014; Jewell, 2015; LaCapria, 2016). These websites assert or imply that if fast food does not decompose, then fast food is not digestible or a healthy product that their readers should be feeding themselves or their children. The most common explanations for the lack of decomposition refer to these products containing “chemicals” or “fake” ingredients that last forever. These explanations and other news reports may contribute to a public conception that fast food is “fake food” (Stempel, 2011; Andrews, 2014). Our college students were largely familiar with the supposed longevity of fast food (see below), but, like other members of the population (Paeratakul et al., 2003; Bowman & Vinyard, 2004), they still claimed to eat fast food regardless. These websites have drawn the attention of the chains themselves, some of which have offered rebuttals such as the food items being dehydrated to the point that fungi and other decomposers cannot grow (Little, 2014). The claims have also been tested by the Food Lab blogger Kenji López-Alt (López-Alt, 2010, 2016), who concluded that hamburgers (both purchased and homemade) are too dry in an open-air environment to grow mold but can grow mold in a moisture-controlled environment.

We selected fungal growth as a proxy for decomposition because it was an opportunity to show students that fungi are ubiquitous, beneficial, and important but greatly underrepresented in biological education and general public knowledge. Some 100,000 fungus species have been described (Chapman, 2009), and they function as decomposers, symbionts, pest control agents, and food sources in virtually every ecosystem on earth (Ferron, 1978; Gianinazzi et al., 2010; Norris et al., 2011). Fungi benefit humans by providing medicinal compounds (Kück et al., 2014), and many cheeses and breads – and all beers, wines, and spirits – would not exist without fungi. For all their benefits, they can also be detrimental as agents of wildlife, plant, and human diseases (Fisher et al., 2012) and because they can spoil natural products, including food, by decomposing them or producing harmful toxins (U.S. Department of Agriculture, 2013).

Our two exercises included a controlled experiment inoculating hamburgers with mold and an inquiry-based screening exercise inoculating dozens of food items with mold. Similar exercises inoculating bread are used widely in “fungus labs” in biology courses (e.g., Perry et al., 2006); however, to our knowledge, lab manual exercises rarely if ever allow students to inoculate fast food. Moreover, our screening exercise was unique in allowing students to select their own food items and investigate the nutrient makeup of those items before predicting results. These exercises were developed under the assumption that allowing students to work with food items they habitually consumed and that they themselves selected would engage their interest and lead to effective learning. In spring 2018, we used surveys and normal assessments in our course to assess students' content knowledge and attitudes toward (1) Internet stories, (2) fungi, and (3) fast food. We demonstrate the success of these exercises over several semesters and encourage the readers to adopt these exercises and develop similar exercises.

Learning Goals

Upon completion of both exercises, students should be able to

  • evaluate accuracy and reliability of online information,

  • identify dependent and independent variables of an experiment,

  • formulate a hypothesis to answer a question,

  • experimentally test the hypothesis and conclude whether it is rejected, and

  • critically evaluate whether their own nutritional choices are appropriate.

These exercises are appropriate for high school biology students or college students in introductory or nonmajors biology, microbiology, or nutrition courses. More specifically, the exercises apply three-dimensional learning as defined by the Next Generation Science Standards (NGSS) (NGSS Lead States, 2013). After the exercises, high school teachers should be able to assess whether students have met Life Science Core Idea standards and assess their progress in mastering Science and Engineering Practices and Crosscutting Concepts (Table 1).

Table 1.
Alignment of the learning goals for our two exercises with the Next Generation Science Standards (NGSS Lead States 2013).
Science & Engineering PracticeDisciplinary Core IdeaCrosscutting Concept
Analyzing and interpreting data
Mean mycelial diameter is used to determine whether fungal growth is supported or inhibited. 
LSI-C: Organization for matter and energy flow in organisms
Fungi are heterotrophs that break down organic matter to acquire matter and energy. 
Patterns
There are specific food items and their components that tend to support or inhibit fungal growth. 
Constructing explanations
Hypothesize whether fast-food components will support or inhibit fungal growth. 
 Structure and Function
Fungi are composed of filamentous hyphae that maximize surface area:volume ratio. 
Engaging in argument from evidence
Defend or critique the claim that fast food does not decompose. 
  
Obtaining, evaluating, and communicating information
Evaluate the accuracy of the mold myth and similar online claims. 
  
Science & Engineering PracticeDisciplinary Core IdeaCrosscutting Concept
Analyzing and interpreting data
Mean mycelial diameter is used to determine whether fungal growth is supported or inhibited. 
LSI-C: Organization for matter and energy flow in organisms
Fungi are heterotrophs that break down organic matter to acquire matter and energy. 
Patterns
There are specific food items and their components that tend to support or inhibit fungal growth. 
Constructing explanations
Hypothesize whether fast-food components will support or inhibit fungal growth. 
 Structure and Function
Fungi are composed of filamentous hyphae that maximize surface area:volume ratio. 
Engaging in argument from evidence
Defend or critique the claim that fast food does not decompose. 
  
Obtaining, evaluating, and communicating information
Evaluate the accuracy of the mold myth and similar online claims. 
  

Exercise 1: Experiment Procedure & Results

During the spring 2018 semester, our introductory biology class (21 students) was split into two groups, one of which (Group 1, 11 students) inoculated hamburger patties from an international fast-food chain while the other group (Group 2, 10 students) did not. The hamburger patty group was further subdivided into three teams (3–4 students per team), each of which inoculated five patties of the following three types: plain patties, plain patties with cheese, and patties with ketchup and mustard. Patties were cooled to 15–20°C before inoculations. The other group was subdivided into three teams, two of which inoculated five slices of off-brand sliced white bread from a local supermarket and one of which inoculated five 100 mm plates of Sabourad dextrose agar (SDA; Carolina Biology Supply Company, no. 786781). To be consistent, 60 mm disks of each type of food sample were cut and placed in Petri dishes.

We selected Rhizopus stolonifer cultured on potato dextrose agar (Carolina Biological Supply Company, no. 156223) for our exercises because of its ubiquity, availability, and low potential to cause harm to students. Samples were inoculated by punching a 3 mm circular plug of R. stolonifer mycelium from the plate and transferring the plug to the center of the food disk or plate with forceps (Figures 1 and 2). Punches and forceps were sanitized with ethanol between samples. All inoculated samples were sealed with Parafilm and incubated 96 hours at 25°C, the temperature indicated by the vendor. Students recorded the presence or absence of visible growth in sealed plates and used digital calipers to measure the diameter of the mycelium at its widest point. Sealed plates were autoclaved afterward according to standard safety guidelines.

Figure 1.
Students inoculating fast-food samples with Rhizopus stolonifer.
Figure 1.
Students inoculating fast-food samples with Rhizopus stolonifer.
Figure 2.
Example setup of inoculated hamburger samples. Rhizopus stolonifer mycelium is in the center of the samples (red circles). Photographed samples are labeled only with type of sample and date for anonymity, although samples used for classes should include further identifying information such as specific names of the food items and student names.
Figure 2.
Example setup of inoculated hamburger samples. Rhizopus stolonifer mycelium is in the center of the samples (red circles). Photographed samples are labeled only with type of sample and date for anonymity, although samples used for classes should include further identifying information such as specific names of the food items and student names.

In our 2018 trial, R. stolonifer grew on all of the plain patty samples and SDA plates, grew on one patty with cheese, and did not grow on the patties with condiments or on the bread. The mean (± SE) mycelium diameter was 53.6 ± 2.63 mm on the plain patties, and mycelium filled the dish or plates (90 mm) in the rest of the samples. Previous experience in 2015–2016 had indicated that cheeseburger patties typically do support growth of R. stolonifer, while patties with condiments generally do not. Bread from these restaurants generally contains calcium propionate, added to inhibit mold (Sancho-Madriz, 2003), while condiments may be acidic or contain antimicrobial compounds (Silvério & Lopez, 2012).

Exercise 2: Screening Procedure & Results

During four semesters (2015–2018), our college biology students selected and brought their own fast-food samples to inoculate for the presence or absence of visible mold growth. We screened only for presence or absence because the samples varied in size and shape. Inoculations were made using R. stolonifer mycelium as described above, and using a suspension of R. stolonifer spores in distilled water (spore density was not controlled). All plates were sealed using Parafilm and kept sealed until they could be autoclaved.

In spring 2018, our students brought in 20 different items from nine different sources, including a gas station, a grocery store bakery, and five of the most internationally known fast-food franchises. Some items were inoculated intact (e.g., chicken nuggets) while others were cut (e.g., sandwich bread) to fit Petri dishes. Agar plugs were placed in the center of the items to inoculate. Rhizopus stolonifer failed to grow on all nine of the samples made of bread or tortillas, but it grew on all the samples based on animal products or vegetables (including potato products). Other opportunistic fungi grew in some samples, suggesting that spores invaded during the inoculations or were already present (Figure 3). In our experience with >60 items inoculated, bread products by themselves rarely support R. stolonifer growth, while fatty baked products (e.g., biscuits, desserts), animal products, and products with sauces, glazes, or gravy are more likely to support growth. Our results indicate that moisture and preservatives are likely the most important factors regulating mold growth.

Figure 3.
Opportunistic fungi growing on fast-food samples. The presence of these fungi invites discussion about the ubiquity of fungi in the environment. Plate photograph is cropped to remove identifying information in label.
Figure 3.
Opportunistic fungi growing on fast-food samples. The presence of these fungi invites discussion about the ubiquity of fungi in the environment. Plate photograph is cropped to remove identifying information in label.

Student Worksheets

We used standard worksheets and electronic spreadsheets for both exercises. Worksheets prompted students to list the ingredients of their food items to the best of their ability, using online lists of ingredients or food labels. Students would then write a hypothesis explaining why the item would or would not allow visible mold growth, based on the ingredients and condition of the food item (Table 2). To help our students, we explained to them that fat, carbohydrates, and proteins were beneficial nutrients for the growth of all heterotrophs. Antimicrobial and preservative ingredients were also identified. Students were also able to recognize that cooking could sterilize food items or make them too dry to support growth (Table 2). In the screening exercise, students were only prompted to predict the presence or absence of mycelial growth; however, in the experiment exercise, students were prompted to predict which item the class investigated would have the greatest diameter of mycelial growth. This prompt allowed us to correct vague or innacurate wording (e.g., “The [restaurant name] burger … will not grow a lot of mold” or “We believe…”). Each measurement was entered into a shared electronic spreadsheet, and our students used the shared data to compare items, calculate means, and draw conclusions. Students completed the exercises by concluding whether the item was a suitable substrate for fungal growth or not.

Table 2.
Examples of student hypotheses, predictions, and results. For some items, students generated hypotheses for both presence and absence of mold growth.
ItemExample HypothesesPredictionResult
Uncooked pepperoni slice (university cafeteria) Protein and fat facilitate mold growth.
Salt, spices, and bacterial cultures inhibit mold growth. 
Mold will grow. No mold growth. 
Beef patty with cheese (chain restaurant) Proteins and fats in both items facilitate mold growth.
Moisture in cheese makes up for dry patty. 
Mold will grow. Mold growth over entire patty. 
French fry, no ketchup (multiple chain restaurants) Dry, cooked, salty exterior inhibits mold growth. Mold will not grow. Mold visible only at location of inoculation. 
Ketchup-covered French fry (multiple chain restaurants) Sugar and moisture in ketchup facilitate mold growth.
Ketchup's acidity inhibits mold growth. 
Mold will grow. No mold growth on some; mold growth covering <50% of surface on others. 
ItemExample HypothesesPredictionResult
Uncooked pepperoni slice (university cafeteria) Protein and fat facilitate mold growth.
Salt, spices, and bacterial cultures inhibit mold growth. 
Mold will grow. No mold growth. 
Beef patty with cheese (chain restaurant) Proteins and fats in both items facilitate mold growth.
Moisture in cheese makes up for dry patty. 
Mold will grow. Mold growth over entire patty. 
French fry, no ketchup (multiple chain restaurants) Dry, cooked, salty exterior inhibits mold growth. Mold will not grow. Mold visible only at location of inoculation. 
Ketchup-covered French fry (multiple chain restaurants) Sugar and moisture in ketchup facilitate mold growth.
Ketchup's acidity inhibits mold growth. 
Mold will grow. No mold growth on some; mold growth covering <50% of surface on others. 

Student Survey & Results

In spring 2018, we assessed the exercises using a survey to determine attitudes about online information and student attitudes related to the nutritional value of fast food. Half of our students (10/21) reported that they ate fast food at least twice a week, and only two students claimed to eat fast food less than once a week. The majority of our students (14/21, 66%) were familiar with the online stories about fast-food hamburgers, and students from both groups were equally likely to have read them. Students who inoculated hamburgers (Group 1) were more likely to say that hamburgers decomposed after the exercises than students who inoculated other food items (Group 2). The most common reasons students gave for hamburgers not decomposing were “fake meat” and preservatives, and these responses were given after seeing R. stolonifer growth. The “fake meat” response indicated that students did not necessarily connect “fake meat” with inhibition of fungal growth, even if the idea of “fake meat” deterred them from eating fast food. Clearly, the “fake ingredient” claims the students had seen online for years influenced their views. In the future, we recommend further examination of the actual content of hamburgers (Prayson et al., 2008).

Two items on the survey prompted students to rate the nutritional values of fast food overall and of hamburgers in particular, using a 1–10 scale. The exercises did not significantly change how students rated the nutritional value of fast food overall or of hamburgers (Figure 4). Group 1 assigned them lower values of nutrition both before and after the exercises than Group 2, which is an artifact of our sample group. Students did not associate suitability for R. stolonifer growth with a food's being healthy for humans, given that fungal growth is a far simpler variable to measure than human health. Human nutrition is associated with interactions among several complex systems.

Figure 4.
Mean (± SE) student ratings of perceived nutritional value on a 1–10 scale (10 = most nutritious) of fast food overall (A) and of fast-food hamburgers (B). Group 1 inoculated hamburgers, while Group 2 inoculated bread.
Figure 4.
Mean (± SE) student ratings of perceived nutritional value on a 1–10 scale (10 = most nutritious) of fast food overall (A) and of fast-food hamburgers (B). Group 1 inoculated hamburgers, while Group 2 inoculated bread.

Although the exercises did not affect how students viewed the nutritional value of fast food, they may have had an impact on students' nutritional choices. Only three of the 18 students who responded said the exercises had no effect on their interest in eating fast food, while the remaining 15 responded that the exercise made them less interested in eating fast food. We infer that students found the act of disassembling the items and seeing them grow mold objectionable.

Discussion

These exercises were impactful for our students in three ways. First, investigating a popular online claim was an engaging way to practice development and differentiation of hypotheses and predictions and to improve quantitative literacy. Students often confuse hypotheses and predictions (McPherson, 2001). Roughly 75% of the students in our college introductory course began with difficulties in differentiating questions, hypotheses, and predictions and in setting up and completing a spreadsheet. Since these exercises were part of a course that primarily surveyed biodiversity, they were the only opportunities for our students to learn about hypotheses and predictions. The experiment also served as an opportunity to improve skills in experimental design, although we did not measure that explicitly. Our students found that, contrary to their initial conceptions, some fast-food items could grow mold. They were able to see that a familiar online claim was not true and that other information they find online might not be accurate.

Second, the exercises paired with a standard teaching unit effectively assisted students with learning about fungi. Planning to inoculate fast-food items that some frequently consumed gave the students a greater stake in the exercise than they typically would have following a lab manual. The increased interest was evident when we tested our students for content knowledge after the exercises. We were able to assess effectiveness by comparing student performance over multiple semesters with and without the exercises; quiz grades increased by >10% starting in the year (2014) we began using this exercise.

Third, these exercises allowed students to share and consider their nutritional practices, especially their consumption of fast food. Examination of dietary choices is especially important as students begin college, when many are in control of their choices for the first time and may make choices that harm their long-term health (Stockton & Baker, 2013). We gained interesting information about what students ate, and they were able to discuss their choices with their classmates and with us. Many of our students declared interest in a change from potentially harmful eating habits after the exercises.

Our students tested the online claim that fast food does not decompose only by observing and measuring mold growth. They were able to determine what types of fast food allowed mold growth and came up with explanations for their observations. The claim can be further tested; for instance, students could weigh items over time and test for growth of other decomposers besides fungi. Swabbing burgers and attempting to culture and identify decomposing bacteria is an extension that also allows students to discuss food safety, which is currently a critical issue (Centers for Disease Control and Prevention, 2018).

In conclusion, there are essentially unlimited opportunities to examine online claims in lab courses as more online claims are made and students share information they read online. We recommend that high school and college instructors seek out their own online stories and develop ways to test them or seek original information in peer-reviewed scientific literature. Such activities can be effective and engaging learning experiences.

Acknowledgments

We thank Tom Diggs, Swapna Bhat, and anonymous reviewers for constructive comments on the manuscript, and we thank all the students who have participated in this project since 2014.

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