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A Study of Mold Growth

Developers:

Benjamin Edoff, Robin Reilly

Philadelphia School District

Dr. Allen Marks, Deborah G. Fradkin

Rohm and Haas Company

Grade Level:

K through 12

Discipline:

Mycology, Hygiene, Food

Goals:

Upon completion of this lesson, the students will:

1. Be introduced to terms associated with fungi.
2. Apply the Scientific Method to problem solving.
3. Recognize the importance of a control in an experiment.
4. Develop data collection methods and observation skills.
5. Discuss and develop safe laboratory procedures.

Objectives:

Upon completion of this lesson, the student will be able to:

1. Understand where fungi and molds come from.
2. Learn how fungi and molds are formed by growing them in the classroom.
3. Determine the percentage of fruit juice in a fruit.
4. Identify approximately 4 types of mold by color and size.
5. Create graphs documenting fungal growth.

Background:

On a warm afternoon just after Spring break, a group of 2nd graders arrived at Science class, and began to settle down for their last class of the day. As they would be dismissed for the day at the end of class, book bags were in tow, and the children were getting their pencils out. As I walked by one group, a boy was retrieving his pencil from deep in his bag, and as he did so, a sandwich in a flip-top baggie tumbled out. Immediately a chorus of "Euuu", "Yuck", "Disgusting", and "Nasty" went up, and all the children gathered around to get a look. Apparently the bologna and cheese sandwich had spent its Spring break in the book bag, and was covered with several kinds of fungi. Immediately, a flood of questions followed, and I shelved the lesson plan for the day and we did a mini-lesson on the event, which led to discussion of germs, hygiene and food spoilage. It was a rich session, and this series of experiments grew from that teachable moment. The objective is to expose common foods which children frequently have for lunch to different conditions to see what happens.

As elementary school teachers we always tell students the importance of washing our hands to prevent transmission of germs and disease. However, the effects of washing to curtail this transmission are never immediately apparent to us, other than our hands looking and smelling clean. Air-born particles and their role are usually not considered.

We live at the bottom of an ocean of air which contains many different particles, most invisible to the naked eye. One common type of air-born particle is called a spore. When spores land on a suitable surface, under the right conditions, they can germinate to produce molds. There are four common types of molds. They are green, blue, black, and water molds. Bread molds are called Rhizopus nigricans.

Molds live on foods and liquids of animals and other living and non-living objects. Most molds produce reproductive spores that are very tiny particles. These spores are in a case or sac called sporangium. Once the spores mature, this enclosed case opens and the spores spread into the environment. These spores produce hyphae that are broken down by the enzymes inside the food or liquid. In effect, the relationship between the food and the mold is mutually beneficial. The food provides nourishment to the mold and the mold breaks down the dead, organic matter present, becoming visible to us as it grows.

The three necessary conditions for successful mold growth are food, water, and moderate temperature. When these three conditions are met, molds will grow. In this experiment the hosts upon which mold will be encouraged to grow are fruit, bread, and other common lunch foods. Molds need a temperature of about 80 degrees Fahrenheit (room temperature) to do best. Adequate moisture must be present for optimum growth. Water is needed so that the parts of the cell can interact freely. Light also affects mold growth. Light is not necessary for mold survival. Plants need light for survival, but unlike plants, molds do not photosynthesize, or make their own food using energy from the sun. In fact, molds tend to grow better in the dark, because strong light can cause them to become dried out, and their spores will not be able to germinate.

Molds are useful since they produce several commonly used drugs. Penicillin is a very famous drug that is derived from molds. A British scientist named Alexander Fleming discovered it in 1928. While working with bacterium he noticed that the bacterium around a mold was dying. This resulted in a new drug called Penicillin that is used to treat bacterial infections. It is a widely used drug because there are few side effects compared to those of other drugs. Molds have produced different forms of penicillin.

Lesson 1 - Handling White Bread under Different Sanitary Conditions and Observing the Effects on Mold Growth

Several options are available to the teacher for this series of experiments, which are explained below. However, the need to address safety with the children must come first.

Under no circumstances should any child taste any of the foods at any time. When handling the samples, gloves must be worn, even though most items are enclosed in bags or foil. While safety glasses are optional, some spores will become air-born during handling, and to reduce the unlikely chance of an eye reaction, glasses should be used if available. Have all students wash their hands at the end of the lesson. When the experiment is over, simple disposal in a plastic bag using a twist tie to close it is adequate because none of the materials or molds are toxic.

Within the classroom setting, teachers can choose to test as many conditions as they wish, for the bread, fruit, etc. In each case however, it is necessary for each group to include a control. For example, a group may have a set of samples handled by unwashed hands and a set of samples handled with hands rinsed in water, or a set of samples handled by unwashed hands and a set handled by hands washed using soap. Teachers and children are invited to come up with additional conditions or items consistent with the procedures described below.

Management of the activities can also be adapted to suit particular needs. It is suggested that cooperative groups of 4 or 5 children be established, with groups testing one, two or more conditions each plus a control, depending on the size and level of the class.

Materials:

Procedure:

Children work in groups of 4 or 5:

In order to determine the effects of washing hands before handling bread, the groups will set up four treatment conditions:

1. handling bread for 15 seconds with unwashed hands (which serves as a control),
2. handling after rinsing with water for 15 seconds,
3. handling after washing with soap for 15 seconds, and
4. handling after washing with antibacterial soap for 15 seconds.

For the unwashed hands condition, each child handles a slice for 15 seconds before passing it on to the next child in the group, who in turn, handles it for 15 seconds before passing it on. Each student handles each of the three slices before wrapping one in aluminum foil, placing one in a flip-top sandwich bag, and one in a sealed Ziploc(tm) bag, together with a damp paper towel in each (Use the same amount of water on each paper towel). These three enclosures are those most commonly used by children who bring their lunches to school.

Next, one or two groups will rinse their hands in water for 15 seconds, dry them, and handle a slice of bread for 15 seconds before placing one in the foil, one in the flip top bag, and one in the Ziploc(tm) bag. The 15-second rinsing or washing is done before handling each of the three slices. The other group members will need to fill the roles of timekeeper and those who assist in labeling and preparing the bags, foil, and paper towels beforehand. It is important that the person washing touches nothing but the bread before placing it in its enclosure and then closing or folding them shut. Furthermore, the other students should NOT touch the bread at all. Students can take turns washing if desired.

Likewise, other groups follow the same procedure for hands washed with the bar soap or with the antibacterial soap. When completed, each group will have between 6 and 12 enclosed slices of bread, depending on the number of conditions tested. For example, if a group tests just one condition plus a control group, they would have 6 test items - 3 slices handled with unwashed hands, placing 1 wrapped in foil, 1 in a flip top bag, and 1 in a Ziploc(tm) bag. Then they would do the same with another three slices after choosing to wash with either water, soap, or antibacterial soap. If two conditions plus a control group are tested, there would be 9 test items.

Then, take all of the slices and put them in a cardboard box (drawer or closet) and close it so that it remains dark inside. Let them sit undisturbed for one week.

Finally, children write up the purpose, generate their own hypotheses, list all materials used, and record the procedure they used.

Counting and Recording Colonies, Wet/Dark Conditions

Before returning the bread to each group, review the purpose of the experiment, hypotheses, and procedures used. It is important at this point to discuss and review safety procedures, and have the class and teacher together create the rules to follow in handling the bread. No one should touch a mold directly. After reviewing the rules, distribute gloves and safety glasses to each child.

Return the bread to each group. Have the children group them by placing the handling conditions horizontally, and the three enclosure conditions vertically as in Table 1. Table 1 also shows the total number of colonies of the 4 different types of mold that we found on each piece of bread. Table 2 is a replicate table with the mold results removed and separate columns for each type of mold. Table 2 can be used as a template for teachers to use when recording results in their classroom.

Table 1
Number of Colonies of Mold For Wet/Dark Condition

Table 3 shows our results for all the mold growth conditions.

The data sheet above can be used for recording the number and color of colonies observed. Up to four different colors of colonies may be present: black, green, yellow/brown, and red. Regardless of size, simply count the number of colonies, one color at a time, and list them on the data sheet for each of the conditions. Do not disturb the contents of the plastic bags - if a mold is growing on the paper towel, include them in your count. In the case of the foil, it is necessary to turn the bread as the count is made. Children should be instructed to wear gloves or use tweezers and only touch the bread at places where no mold is present. After all groups have finished recording their tallies and comparing results, collect the bread and return it to its box.

Conduct a class discussion to share and compare their findings, and ask if there is a better way to view the results. Introduce the use of a bar graph, and demonstrate how to display the data graphically. Use a colored marker or chalk the same color as the mold to make each bar. Review the data and conduct a class discussion, followed by students writing their results and conclusions.

Wet/Light Conditions:

Following the same procedures outlined in Lessons above, classes may conduct this part separately or simultaneously. In this series, all samples are left exposed to ambient light for a week. All other conditions remain the same. Discussion of the effects of light on mold growth can then be made in comparison with the wet/dark data. You may wish to run a series with dry conditions as well.

For the wet/dark conditions, we noted the following by order of most to least colonies counted. The bread in the flip top bags showed the most mold growth followed by the aluminum foil and finally the ziploc bags. The flip top bags had up to four different colors while the foil and the ziploc bags had the least.

We concluded that the ziploc bags were the most effective in preventing mold growth because they did not allow the foods to be exposed to the air. See Table 3 for a summary of all our results on bread. As mentioned above, mold growth requires water. As you can see from Table 3, when we tested bread under dry conditions, very little mold grew. This may be an instructive addition for teachers to include.

Lesson 2 - Determining Mold Amounts on Whole Fruits

Materials for whole fruits:

Students should work in pairs or groups of four for this activity. Teachers may decide to use these activities separately or simultaneously depending on the age level of their students. In addition, teachers should not feel restricted to using the fruits included in our series of experiments.

Activity 1 - Apples

Each group takes one whole apple and places the apple into a one-pint container. There should be 7 containers with 7 apples inside. Fill each container with the following solutions or until the apple floats.

Activity 2 - Pear

Each group takes one whole pear and places the pear into a one-pint container. There should be 7 containers with 7 pears inside. Fill each container with the following solutions in Activity 1 above or until the pear floats.

Activity 3 - Strawberry

Each group takes one whole strawberry and places the strawberry into a 1/4 pint container. There should be 7 containers with 7 strawberries inside. Fill each container with the following solutions in Activity 1 or until the strawberry floats.

Activity 4 - Raisin

Each group takes two whole raisins and places the raisins into a 1/4 pint container. There should be 7 containers with 14 raisins inside. Fill each container with the following solutions in Activity 1 or until the raisin float.

Activity 5 - Lemon

Each group takes one whole lemon and places the lemon into a 1/2 pint container. There should be 7 containers with 7 lemons inside. Fill each container with the following solutions in Activity 1 or until the lemon floats.

Note:

1. For the water condition fill the container enough so that the fruit can float.
2. For the dry condition simply place the fruit inside its respective containers.

Partially cover each fruit in its container with a lid and store all the containers in a five-gallon pail with the lid ajar. Add one pint of water to the bottom of the pail to prevent moisture loss. The lid should be placed on each pail to allow airflow from outside. After one day, open up the containers and pour off the water in the containers. Allow mold to grow for 7 days and then observe the results. Record your results using the following "YUK" factor ratings:

Table 4 shows the results from our evaluation. Table 5 is a blank that can be used as a template. The data in Table 4 is shown graphically as a bar graph generated in an Excel spreadsheet.

Lesson 3 - Determining Mold Amounts on Cut Fruits

Materials for cut fruits:

Repeat this activity using the same procedure as in Lesson 2 except cut each fruit into pieces before putting them into the container. The pieces of the fruit should be about 1/4 the size of a whole apple, 1/4 the size of a whole pear, 1/2 the size of 2 raisins, 1/2 the size of a lemon, and 1/2 the size of a strawberry. Use the "Yuk Factor" Scale and the accompanying data sheet (Table 5) to record your observations. Table 6 shows the results we found for cut fruit. As can be seen by both the whole and cut fruit experiment, certain fruits (like strawberries) are more prone to mold and certain solutions (like fruit punch) will enhance mold growth. Differences between the whole and cut fruit can be attributed to the skin of the fruit. The solutions that enhance mold growth do so because they provide food for the mold. Discuss your findings with your students and try to hypothesize the reason for your results. This may lead to ideas for further independent study or individual science projects for your students.

pH of the Solutions Used in Soaking Foods

Explain the concept of pH (see glossary and other texts for reference materials on pH) to the students. For practice you can measure the pH of the solutions used to soak the fruit.

he following is a list of the pH of the solutions that were used in this experiment.

Sugar Content in Fruits

The following list provides the sugar content¹ (in grams) for each of the fruits used in this experiment. We did not find a correlation between mold growth and sugar content.

¹Data above provided by Lois Wegfahrt, R.D. at PennState College of Agricultural Sciences, Cooperative Extension Bucks County, Neshaminy Manor Center (Doylestown, PA)

Lesson 4 - Determining Mold Amounts on Other Lunch Foods

Materials for other lunch foods:

Using the same seven solutions that we used in Lesson 2 (Determining Mold Amounts on Whole Fruits), we tested cheese, cookies, potato chips, and bologna in the same manner.

Partially cover each whole food in its container with a lid and store all the containers in a five-gallon pail with the lid ajar. Add one pint of water to the bottom of the pail to prevent moisture loss. The lid should be placed on each pail to allow airflow from outside. After one day, open up the containers and pour off the water in the containers. Allow mold to grow for 7 days and then observe the results. Table 7 gives the "YUK" factors for our results. Table 8 gives a blank template.

We concluded that the most mold growth were in the foods that were soaked in coffee and fruit punch. Next came the foods soaked in water and lemon juice. Finally, the foods soaked in soap and the nothing category had the least mold growth.

Bologna proved to be the food which molds like the best. This is probably due to a consistent moisture content and an adequate source of food for the molds to grow. Next were the cookies that the molds liked as well. Potato chips showed some growth but surprisingly, the cheese showed absolutely none. We concluded that this occurred because the cheese had preservatives in it.

Lesson 5 - How much fruit juice is there in a piece of fruit?

Materials:

Procedure for Juicing the Fresh Fruit

For the following fruits (strawberries, pears, lemons, apples, and grapes) we used three methods. We used a hand strainer, garlic press, and a masher to turn each of the whole fruit pieces into juice. This procedure was repeated for each of the above fruits.

1. Weigh the fresh fruit and then cut it into small pieces (as needed to press out the juice).
2. Using a hand strainer, or a garlic press, or a potato masher, press the fruit against the sides of the container until as much of the crushed fruit comes out as possible.
3. Use a cheesecloth to further separate the juice from the fruit.
4. Squeeze out the remaining juice by hand.
5. Weigh the juice.
6. Divide the weight of the juice by the weight of the fruit. Multiply by 100 to get percent.

The following example shows how we determined the weight of the juice in a strawberry.