2
Prior to 1800, the association of yeast or leaven with fer-
mentation had been noted, but the nature of these agents was
not understood. Experiments conducted independently by C.
Cagniard-Latour, F. T. Kützing, and T. Schwann in 1837 indicated
that yeast is a living organism and is the cause of fermenta-
tion. This view was opposed by such leading chemists as J.
von Liebig and F. Wöhler, who sought a chemical rather than a
biological explanation of the process. The biological concept
became generally accepted following the work of Louis Pasteur,
who concluded that fermentation is a physiological counter-
part of oxidation and permits organisms to live and grow in the
absence of air (anaerobically). This linked fermentation and pu-
trefaction as comparable processes; both represent decomposi-
tions of organic matter brought about by microorganisms in
the absence of air. The difference is determined by the nature of
the decomposable material; sugary substances generally yield
products with pleasant odor and taste (fermentation), whereas
proteins give rise to evil-smelling products (putrefaction).
Pasteur also discovered the fermentations of lactic acid
(Fig. 2) and butyric acid, and he concluded that each kind of
fermentation was caused by a specific microbe. Later work
supported this idea to a large extent and considerably in-
creased the number of specific fermentations.
During fermentation, organic matter is decomposed in the
absence of air (oxygen); hence, there is always an accumulation
of reduction products, or incomplete oxidation products. Some
of these products (for example, alcohol and lactic acid) are of
importance to humans, and fermentation has been used for
their manufacture on an industrial scale. There are also many
microbiological processes that go on in the presence of air while
yielding incomplete oxidation products. Good examples are the
formation of acetic acid (vinegar) from alcohol by acetic acid bac-
teria, and of citric acid from sugar by certain molds (for example,
Aspergillus niger). These microbial processes, too, have gained
industrial importance and are often referred to as fermenta-
tions, even though they do not conform to Pasteur's concept of
fermentation as a decomposition in the absence of air.
Fermentation (continued)
Fig. 2: Lactic acid fermentation can take place in yeasts, bacteria, and animal cells, such as muscle cells. This process is similar to alcoholic
fermentation, except that lactate is produced instead of ethanol. (Copyright © McGraw-Hill Education)
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