What Do We Have Left After a
Fermentation Process?
Nestor Bohonos
Assistant Professor and Associate Chemist
Agricultural Experiment Station, Purdue University
Lafayette, Indiana
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The types of products formed during a fermentation may be divided
into two groups: (a) those which are formed as part of the cellular
material or as some excretory product (amino acids, lipids, vitamins,
antibiotics, etc.); and (b) those which are formed in relatively large
quantities and the formation of which is directly associated with the
liberation of energy (the solvents, organic acids, carbon dioxide, etc.).
By analogy to the animal kingdom, the constituents of the body and the
urinary and fecal excretory products would be in the first group, and
carbon dioxide would be in the second.
It is commonplace to state that a human requires a certain number
of calories to supply energy, but we often overlook the fact that energy
is required by all forms of life, including micro-organisms. Animals
oxidize the organic components of feeds to carbon dioxide, but microorganisms are unique in the variety of products which they produce and
which accumulate in the process of their metabolism. The fermentation
products such as ethyl alcohol, acetone, butyl alcohol, lactic acid, etc.,
must be regarded as by-products of the organism's biological furnace:
they are formed incidentally to the growth of the cell. The amount of
energy obtained by a cell from the fermentation of a food material will
depend on the product formed. For example, the conversion of sugar
to ethyl alcohol and carbon dioxide involves a release of energy corresponding to about one-twelfth of that released when carbon dioxide and
water are the end products. Anaerobic fermentations usually involve
both oxidation and reduction; and less energy is released in these processes than in aerobic processes, which are largely oxidative. Since the
growth of the fermenting organisms is dependent on the available energy,
it is understandable that aerobic growth results in a higher yield of cells
per unit of substrate. In the production of feed yeast or yeast for
inoculum, aeration is employed so that the yield of cells will be greater
than if the process were anaerobic, as in alcoholic fermentation.
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