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Using Food to Aid Understanding of Enzyme Properties

Marian Cothran
Lexington Community College, Lexington, KY
marian.cothran@kctcs.edu

I am constantly challenged, as an instructor of a stand-alone general biology lab for non-majors, to construct laboratory experiences that illustrate abstract concepts such as enzymes and enzyme activity. Since I am a big fan of food and cooking, I decided to build on the everyday experiences of my students in an investigation of determinants of enzyme activity. Thus, the week before our students will do the standard laboratory experiment involving determination of the effect of temperature and pH on the oxidation of catechol to the yellow-brown product benzoquinone with the help of a potato enzyme extract, I hand out a pre-lab investigation to complete. I explain that the problem of fruit darkening, which has irritated cooks and food producers for centuries, involves the same enzyme and similar substrates as the ones that will be used in the lab. Here is the handout.

ENZYMES AND FRUIT DARKENING

Perhaps you have noticed the way apples, bananas, pears, eggplants, avocados, potatoes and some other fruit turn brown or gray when they are sliced or bruised. This is due to the action of an enzyme, polyphenoloxidase, which oxidizes phenolic compounds in the tissues of the fruits or vegetables and causes those compounds to change color as they condense into polymers (large molecules with repeating units). Other fruit, such as oranges and tomatoes, that do not bruise or discolor easily do not have the enzyme. They will eventually brown when exposed to oxygen, but it will be much slower without the enzyme. In intact cells the enzyme and phenolic compounds are in different compartments of the cell; they only interact when there is tissue damage allowing them to come together.

Before you start today's lab, please list situations you have observed that prevent browning of fruits and vegetables as they are prepared for cooking or eating by humans. This will be easy for cooks! For those that are not cooks, talk with your cooking friends or buy some of the fruits and vegetables that are listed above and try to keep them from browning. What works for you?

1.

2.

3.

4.

5.

Try to relate the experiments in next week’s lab with your observations. You can then apply what you learn in biology lab to serve colorful and appealing fresh fruits and vegetables at home. And you will understand the basis for your actions!

This assignment is easier for my female students in general than traditional males, who do not spend much time in food preparation. However, the males usually have female friends that are happy to share their cooking knowledge. My point is that one may have knowledge from experience in a field, or one my need to consult references or experts to obtain the knowledge—and sometimes that expert in the field may be your Auntie Louise!

When students enter the laboratory, they are instructed to get in their lab groups (of 4) and discuss their findings with their labmates for 5 to 10 minutes. Then I call upon groups one by one to reveal one way in which their group found that enzyme action, or browning, could be prevented. I write these on the board, and here are the most common responses:

1. squeeze lemon juice (or lime, orange, or pineapple) on the fruit
2. cover the fruit with plastic wrap or put it in a Tupperware container
3. put the fruit under water (often mentioned in connection with making French fries)
4. put vegetables or fruit in the refrigerator
5. sprinkle Fresh Fruit® over the fruit

I then ask my students why they think these methods are effective, and these are the usual responses I get:

1. Lemon juice is an acid, right? Maybe acids interfere with the chemical reaction
2. The wrap and containers keep air away—isn’t oxygen involved in the darkening reaction?
3. Gee, I don’t know. Is water like plastic wrap?
4. Cold temperatures probably slow down the reaction.
5. What’s in that Fresh Fruit® stuff? (Answer: ascorbic acid)

Next I help the students to understand how their answers connect to the underlying chemistry. Finally I ask them to predict what their results will be before they conduct the experiment. In other words, acidic conditions should prevent or limit the reaction rate of catechol oxidation and both cold and hot temperature should also prevent the accumulation of the colored product.

Note: Two less common responses and student explanations are:

1. cooking—the heat kills the enzyme
2. salting (mostly in conjunction with potatoes)—salt must interfere with the chemical reaction.

Another method used commercially to prevent fruit darkening is exposure of the fruit to sulfur compounds (most students are not aware of this). I explain these compounds bind with phenolic substrates and prevent the polyphenoloxidase action. I usually pass around a bag of dried apricots for students to read the label, which says something like “sulfur dioxide used to preserve color.”
The actual experiment takes about an hour, but with this pre-lab discussion about fruit darkening and evaluation of their experimental results, it fills up a two-hour lab session. Students are interested, active, and focused in this lab. Before they go I remind them that the next week’s lab is on diffusion and osmosis. In that lab they quantify diffusion rate at different temperatures by measuring the concentration of the tea with a spectrophotometer. Here is the information they receive in order to connect this lab with their next one.

Next week you will observe the diffusion of the pigmented tannin molecules in tea leaves as they move from the leaves into different temperature water. These tannin compounds that make the tea brown or black are a result of the same enzymatic chemical reaction that you observe in class today. After tea leaves are harvested they are "fermented" by being crushed or rolled so that oxygen, polyphenoloxidase, and phenols combine to brown the leaves. The resulting compounds give the tea its color and astringency. Astringency is that puckery feeling one gets drinking strong tea or biting into unripe fruit (I remember discovering this as a child harvesting persimmons from a tree in our yard!). The tannins actually combine with the surface proteins in the lining of your mouth and saliva to produce the astringent sensations. Plants use these compounds as chemical defense against herbivores that might eat the plants.

Reference:
McGee, Harold. 1984. On Food and Cooking. Schribner & Sons. (Although I haven’t seen it, I’ve heard that a 2nd edition of this classic book has just been published.)

I welcome any comments or suggestions.

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