David sniffs out the chemistry of garlic.
Segment length: 7:37
- What happens when you crush a garlic clove? What molecules are responsible for garlic
Garlic is often called "the stinking rose." That's because it contains molecules
composed of the same atoms that lurk in burnt matches and rotten eggs.
But if you put a whole clove of garlic right up to your nose, you won't smell much.
The molecules that create the garlic smell are not actually present in natural garlic.
They are synthesized in a reaction that occurs when garlic is cut or crushed. When a
knife slices through garlic, cell membranes rupture, releasing an enzyme called
allinase. Allinase can chemically change a tiny, odorless molecule called alliin into
allicin. Allicin is the pungent, sulfur-containing molecule that can alienate friends
who get too close and add zest to bland food.
During any chemical reaction, atoms rearrange themselves into new substances. But all
chemical reactions do not proceed in the same way. Some transformations occur
spontaneously and explosively while others are hard to initiate and proceed as slowly
as a rusting fence. The ease and speed with which molecules change or undergo chemical
reaction depend on how often they collide and the energy needed to get the reaction
Molecules are surrounded by negatively charged electrons which repel the electrons in
other molecules. For each reaction, molecules must move fast enough to overcome these
repulsive forces. This energy barrier is like crossing a mountain. Just as an ocean
wave may have enough energy to knock you off your feet, a molecular collision must
have enough energy to break chemical bonds. If the reactant molecules have enough
energy, they can climb the energy mountain, react, and fall down the other side as
Usually you can give molecules extra energy and make them collide more often by
heating them up. But alliin and allinase only need to meet at room temperature to
The secret is in the structure of the allinase enzyme. One section of the molecule is
called the active site. In a typical enzyme, the active site looks like a dent or
crevice in the side of the molecule. The shape of the crevice in allinase exactly
matches the molecular shape of alliin. The enzyme and substrate fit together like a
key fits a lock. At the active site, alliin is stretched and twisted until chemical
bonds holding it together snap and allicin is formed. Allicin, which no longer fits
the enzyme, drifts away, leaving allinase unchanged and the whole process starts again.
The enzyme facilitates the reaction by reducing the energy needed to break chemical
bonds. Alliin and allinase have found a low-energy tunnel through the energy mountain.
After ingestion, the odoriferous sulfur molecules circulate in the bloodstream and
escape from your body through exhaled air and perspiration--as any nose will tell you.
- Why don't some people like the smell of garlic? What smells are unpleasant to you?
- How does garlic get into your breath? What steps can you take to minimize garlic
active site the location on the enzyme at which substances attach
atom smallest particle of a chemical element
electron a subatomic particle that carries a negative charge
enzyme a huge protein that facilitates and accelerates chemical reactions but itself remains unchanged
molecule group of atoms held together by sharing pairs of electrons
odoriferous having or giving off an odor
product a new substance produced in a chemical change
pungent a sharp, acrid taste or smell
reactant a starting material in a chemical change
substratesubstrate a substance that reacts with an enzyme to give a new substance
- Block, E. (1985, Mar) The chemistry of garlic and onions. Scientific American,
- Bonar, A. (1985) The Macmillan treasury of herbs. New York: Macmillan.
- Doolittle, R. (1985, Oct) Proteins. Scientific American, pp. 88-99.
- Grosser, A. (1981) The cookbook decoder. New York: Beaufort Books.
- Harris, L. (1975) The book of garlic. New York: Holt Rinehart and Winston.
- Hausman, P. (1989) The healing foods. Emmaus, PA: Rodale Press.
- Kowalchik, C. (1987) Rodale's illustrated encyclopedia of herbs. Emmaus, PA: Rodale
- Scott, D. (1994, Apr) Designer catalysts. ChemMatters, pp. 13-15.
- Tocci, S. (1987) Biology projects for young scientists. New York: Grolier.
- VanCleave, J. (1990) Biology for every kid. New York: John Wiley & Sons.
- VanCleave, J. (1989) Chemistry for every kid. New York: John Wiley & Sons.
Additional sources of information
Chemist or biochemist
County extension office
Enzymes are special proteins that help speed up the chemical reactions occurring in
living cells. Enzymes are found in plants and animals. The enzyme catalase splits
ordinary hydrogen peroxide molecules into water and oxygen gas. Can catalase break
down other molecules? Do all plant or animal products contain catalase? Let's find out.
- several plastic cups
- 3% hydrogen peroxide
- potato, bread, apple, turnip, cheese, vinegar, and milk
- Cut off the end of a small potato to expose the inner surface. Do not peel
off the potato skin. Put the end piece of potato into a cup and cover it completely
with hydrogen peroxide. What happens? When you cut the potato, cells are disrupted,
releasing catalase. The enzyme immediately begins reacting with hydrogen peroxide,
producing water and bubbles of oxygen gas. Does catalytic activity occur at both the
peeled and unpeeled areas of the potato? Explain.
- Cut another slice of potato and remove the skin. Cut this slice of potato into
four pieces of equal size. Place each piece in a separate cup. Now pour hydrogen
peroxide into the first cup. Pour vinegar into the second cup. Pour water into the
third cup. Pour milk into the fourth cup. What happens? You'll find that catalase is
highly specific for the hydrogen peroxide molecule. Other molecules do not have the
correct shape to react with this particular enzyme. You can try this with other
- Now pour hydrogen peroxide into four cups. Add a small piece of bread to the
first cup. Add a small piece of apple to the second. Add a small piece of cheese to
the third. Add a small piece of turnip to the fourth cup. What did you observe? Try
this activity with other foods.
- Do all foods contain catalase? What other foods do you think contain catalase?
- What evidence did you see that a reaction was taking place?
- Does catalase decompose all of the liquids used in step 2?
Draw a Venn diagram to represent how many of your classmates enjoy cooked garlic and
how many like it raw. Where do you place the students who do not like garlic at all?
Next, create and conduct a survey to see which garlic-flavored foods your classmates
enjoy. Organize this information on a histogram (bar graph) and a circle graph. Which
graph do you believe will best represent the information? Why?
Usually an increase in temperature will increase the rate of a chemical reaction.
You can demonstrate this effect with two Alka Seltzer tablets. Alka Seltzer tablets
contain dry chemicals that react in water to release bubbles of carbon dioxide gas.
Pour cold water into one jar and hot water into a second jar. Now drop a seltzer
tablet into each jar. What happens?
Researchers are looking at garlic and other foods for help in treating a range of
illnesses. Find out about the healthful benefits of garlic. Make a chart to display
a variety of foods and the illnesses they may prevent.Researchers are looking at
garlic and other foods for help in treating a range of illnesses. Find out about the
healthful benefits of garlic. Make a chart to display a variety of foods and the
illnesses they may prevent.
Garlic odor can be controlled by cutting. In a group, separate a garlic bulb into
individual cloves. Now peel three of the cloves. Place one of the cloves on a paper
plate. On a second plate, cut a clove into four sections. On a third plate, crush or
mush a complete clove. Invite the members of your group to smell each sample and
compare observations of odor intensity.
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