Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Fermentation

217

•

A. Feldman Award in his honor, and the IDSA also created its

own Harry A. Feldman Award.

See alsoAntibody and antigen; Bacteria and bacterial infec-

tion; Chemotherapy; Epidemiology; Infection and resistance;

Meningitis, bacterial and viral; Microbiology, clinical;

Parasites; Poliomyelitis and polio; Protozoa; Serology

FFermentationERMENTATION

In its broadest sense, fermentation refers to any process by

which large organic molecules are broken down to simpler

molecules as the result of the action of microorganisms. The

most familiar type of fermentation is the conversion of sugars

and starches to alcohol by enzymesin yeast. To distinguish

this reaction from other kinds of fermentation, the process is

sometimes known as alcoholic or ethanolic fermentation.

Ethanolic fermentation was one of the first chemical

reactions observed by humans. In nature, various types of

spoil decompose because of bacterial action. Early in history,

humans discovered that this kind of change could result in the

formation of products that were enjoyable to consume. The

spoilage (fermentation) of fruit juices, for example, resulted in

the formation of primitive forms of wine.

The mechanism by which fermentation occurs was the

subject of extensive debate in the early 1800s. It was a key

issue among those arguing over the concept of vitalism, the

notion that living organisms are in some way inherently dif-

ferent from non-living objects. One aspect in this debate cen-

tered on the role of so-called “ferments” in the conversion of

sugars and starches to alcohol. Vitalists argued that ferments

(now known as enzymes) are inextricably linked to a living

cell; destroy a cell and ferments can no longer cause fermen-

tation, they argued.

A crucial experiment on this issue was carried out in

1896 by the German chemist Eduard Buchner. Buchner

ground up a group of cells with sand until they were totally

destroyed. He then extracted the liquid that remained and

added it to a sugar solution. His assumption was that fermen-

tation could no longer occur because the cells that had held the

ferments were dead, so they no longer carried the “life-force”

needed to bring about fermentation. He was amazed to dis-

cover that the cell-free liquid did indeed cause fermentation. It

was obvious that the ferments themselves, distinct from any

living organism, could cause fermentation.

The chemical reaction that occurs in fermentation can

be described easily. Starch is converted to simple sugars such

as sucrose and glucose. Those sugars are then converted to

alcohol (ethyl alcohol) and carbon dioxide. This description

does not adequately convey the complexity of the fermenta-

tion process itself. During the 1930s, two German bio-

chemists, G. Embden and O. Meyerhof, worked out the

sequence of reactions by which glucose ferments. In a

sequence of twelve reactions, glucose is converted to ethyl

alcohol and carbon dioxide. A number of enzymes are needed

to carry out this sequence of reactions, the most important of

which is zymase, found in yeast cells. These enzymes are sen-

sitive to environmental conditions in which they live. When

the concentration of alcohol reaches about 14%, they are inac-

tivated. For this reason, no fermentation product (such as

wine) can have an alcoholic concentration of more than about

fourteen percent.

The alcoholic beverages that can be produced by fer-

mentation vary widely, depending primarily on two factors—

the plant that is fermented and the enzymes used for

fermentation. Human societies use, of course, the materials

that are available to them. Thus, various peoples have used

grapes, berries, corn, rice, wheat, honey, potatoes, barley,

hops, cactus juice, cassava roots, and other plant materials for

fermentation. The products of such reactions are various forms

of beer, wine or distilled liquors, which may be given specific

names depending on the source from which they come. In

Japan, for example, rice wine is known as sake. Wine prepared

from honey is known as mead. Beer is the fermentation prod-

uct of barley, hops, and/or malt sugar.

Early in human history, people used naturally occurring

yeast for fermentation. The products of such reactions

Large vats in which the fermentation process in the brewing of beer

occurs.

womi_F 5/6/03 2:15 PM Page 217