Teaching Objectives

Beginning concepts

• Bacteria are involved in the making of some of our foods.
• Acid-forming bacteria can live in an acidic environment in which other organisms cannot live.
• The release of a gas is a sign that a chemical reaction is taking place.

Advanced concepts

• Acid-forming bacteria (lactobacilli)thrive in oxygen-lacking environments.
• Fermentation is the breakdown of sugars in the absence of oxygen to release energy, carbon dioxide and alcohol or lactic acid.
• The pH scale is a convenient method of expressing the acidity of a solution. (A pH of less than 7 is acidic. The lower the number, the higher the acidity.)

Time Required for Exercise

• Stage 1: Construct the bottle-1 hour
• Stage 2: Set up the fermentation chamber-1 hour
• Stage 3: Making kimchee-7 to 14 days

Materials Needed

This exercise requires the following:

• two 2-liter soda bottles
• large lid (92 mm diameter) of a plastic petri plate
• pH indicator paper (litmus paper; Hydrion brand, obtainable in small vials from lab suppliers)
• small plastic pipette
• Do not wash vegetables! 1- 1 1/2 kg head of Chinese cabbage (Brassica rapa; also called napa or petsai), leaves cut into 5-7 cm chunks.
• 1 hot red chili pepper, chopped (or hot chili powder)
• 2 cloves garlic, thinly sliced
• 3 tsp. non-iodized (or pickling) salt

Exploratory Questions

Ask your students questions and discuss ideas they might have such as:

• Where does the liquid come from?
• When you push down on the sliding seal, what gas is released?
• Why do you need the sliding seal?
• If cabbage doesn't taste sour, why does the kimchee that's produced taste sour?
• What other foods are made by this process? Why does pickling preserve foods?

Exploration

• Cut two soda bottle as indicated in Figure 1. Be sure to cut the top of the second bottle just below the shoulder. This tip will be used to form the sliding seal.(See Step 4.)
• Alternate layers of cabbage, garlic, pepper and a sprinkling of salt in the soda bottle, pressing each layer down firmly until the bottle is packed full. Notice the aroma of the garlic and pepper. These ingredients flavor the product. Caution: When working with chili pepper, take care not to touch eyes or mouth. Wash hands thoroughly when finished.
• Place petri lid, rim side up, on top of ingredients and press down again (Figure 2). NOTE: Within a few minutes liquid begins to appear in bottom of bottle as salt draws liquid from the cells of the Chinese cabbage.
• Press down occasionally for an hour or two. After that there should be sufficient space to fit the lid cut from the second bottle inside the first bottle, forming a sliding seal (Figure 3.)
• Upon pressing firmly with sliding seal, cabbage juice will rise above the petri plate and air will bubble out around the edge of the petri plate.
• The Chinese cabbage will pack to two-thirds or half the volume of the bottle. Press daily on the sliding seal. Keep the cabbage covered by a layer of juice at all times.
• Notice bubbles of carbon dioxide (CO2) gas escape each day when pressed. The gas is produced as lactic acid bacteria grow on the sugary contents of the Chinese cabbage juice in the salty solution.
• Measure and record the acidity of the fresh juice on top each day with litmus paper. Tape the indicator paper on the bottle and write the pH (acidity level) above it.
• Each day take up a quantity of the juice with a plastic pipette and observe the degree of turbidity (cloudiness) representing the growth of lactic acid bacteria in the fermentation solution.
• Note the increase in turbidity and change in acidity together with the continued production of gas as the pickling proceeds.

After a few days to a week or more (depending on the temperature), the pH will have dropped from 6.5 to about 3.5, and you will have kimchee.

Accompanying Activities

1.A gray-line turbidity strip for measuring the increasing cloudiness of the juice can be drawn on the side of the plastic pipette with a fine-tipped black marker. (See Figure 4.) Hold the pipette with gray line strip away from you. Look through the cabbage juice-filled pipette to the lines on the opposite side. As turbidity increases, the finer, lighter lines will disappear. After some time, the darker lines will become less visible. This provides a quantitative measure of the turbidity.

2.After the kimchee is mature (pH of 3.5 or less), remove the lid of the kimchee bottle. Let the kimchee bottle stand for one hour, then replace the lid. After a few days, check the chamber for evidence of bacteria or mold. Has there been any bacterial or mold growth in the chamber? Why or why not? Do the same with a can of sauerkraut or a jar of dill pickles. Compare what happens with the results obtained with the kimchee chamber.

3.Research the food preservation methods of other cultures. Why and how do various cultures pickle foods?

4.When the kimchee is ready, organize a multicultural lunch or ethnic food fair. Serve the kimchee, along with foods from other cultures.

Background Information for the Teacher

Pickling is one of the most ancient forms of preserving food. It involves the microbial conversion of sugars into lactic acid through the growth and activity of acid-forming bacteria known as lactobacilli. As lactobacilli grow, they convert the natural sugars in plant juices into lactic acid. Under the high acidity (=low pH) created by the lactobacilli other food spoiling organisms cannot grow. Lactobacilli are found almost everywhere in our environment and are known as anaerobes because they grow under conditions in which oxygen is lacking. Many foods can be preserved through natural pickling. Some common ones are sauerkraut, yogurt, dill pickles, and silage for cattle. The ancient Chinese and other cultures learned the value of pickling thousands of years ago. Today a spicy pickled Chinese cabbage product known as kimchee is a major part of the diet of Koreans.

You and your students can make kimchee and study lactic acid fermentation in a 2-liter bottle by using the following recipe and procedure.

References

Chun, J.K., "Chinese Cabbage Utilization in Korea: Kimchee Processing Technology," 1981. in Talekar, N.S. and T.D. Griggs, editors. Chinese Cabbage, Asian Vegetable Research and Development Center, Taiwan, China, 1981. Reprint available from Wisconsin Fast Plants, UW Madison, Department of Plant Pathology, 1630 Linden Drive, Madison WI, 53706.