|Influence of oxygen on the development
of yeast and on
the alcoholic fermentation (abstract)
1861, Louis Pasteur
|Pasteur, L. 1861. Influence de l'oxygene sur le developpement de la
levure et la fermentation alcoolique. Bulletin de la Societe chimique de
Paris, June 28, 1861, pages 79-80 (Resume).
M. Pasteur revealed the results of his researches on the fermentation
of sugar and the development of the yeast cell, depending on whether that
fermentation had occurred in the absence or in the presence of free oxygen
gas. Moreover, his results differ from those of Gay-Lussac on grape juice
in the absence or presence of oxygen.
Yeast forms small buds and develops repeatedly in a liquid medium containing
sugar and protein in the complete absence of oxygen or air. In this case
only a small amount of yeast is formed, while a large amount of sugar disappears,
amounting to 60-80 parts of sugar disappearing for one part of yeast formed.
The fermentation is very slow under these conditions.
If the experiment is carried out in the presence of air alld with a
surface of the liquid exposed, the fermentation is rapid. For the same
quantity of sugar disappearing, a much larger amount of yeast is formed.
The air gives up some of its oxygen which is absorbed by the yeast. The
yeast reproduces vigorously, but its fermentative character tends to disappear
under these conditions. Indeed, it is found that for one part of yeast
formed, only 4-10 parts of sugar are removed. Nevertheless this yeast retains
its fermentative characteristics and these are again strongly revealed
if it is allowed to act on sugar in the absence of free oxygen gas. It
seems reasonable to assume there fore that when the yeast is a ferment,
acting in the absence of air, it takes oxygen from the sugar, and this
is the origin of its fermentative characteristics.
M. Pasteur described the vigorous activity that occurs at the
beginning of the fermentation under the influence of the oxygen which is
dissolved in the liquid when the process begins. In addition, the author
has observed that beer yeast, when seeded into an albuminous liquid, such
as yeast water [a water extract of yeast cells], continues to multiply
even when there is no trace of sugar in the liquid, but only if oxygen
from the air is present in large amounts. The same experiment can be repeated
with an albuminous liquid mixed with a solution of non-fermentable sugar
such as crystalline lactose. The results are quite similar.
The yeast formed in this way in the absence of sugar has not changed
its nature. It is able to ferment sugar if it is allowed to act on it in
the absence of air. It should be remarked always that the development of
the yeast is quite poor in the absence of a fermentable material.
In summary, beer yeast behave exactly like an ordinary plant [this
presumably means that beer yeast is able to respire under aerobic conditions
in the same way that green plants do], and the analogy may be completed
if ordinary plants would have an affinity for oxygen which would permit
them to respire with the aid of this element by removing it from compounds
that are not too stable, in which case, following M. Pasteur, they would
be considered to be ferments for these materials.
M. Pasteur stated that he hoped to achieve this result, by finding
conditions in which certain lower plants could live in the absence
of air but in the presence of sugar, in this way bringing about the fermentation
this substance in the same way as beer yeast.
Commentary by Thomas Brock
This is apparently the first description of a phenomenon known
in biochemistry today as the Pasteur effect. It describes the different
behaviour of yeast when grown aerobically or anaerobically. When yeast
grows anaerobically, a large amount of sugar is converted into alcohol,
with some of the sugar being converted into yeast protoplasm. The efficiency
of the process of anaerobic growth is poor. Under aerobic conditions, although
less sugar is used, a greater proportion of the sugar is converted into
yeast protoplasm, with little or no alcohol produced. A point, not stressed
here is that under aerobic conditions, the utilisation of sugar actually
seems to be inhibited. Therefore, although a much greater efficiency of
conversion of sugar into yeast protoplasm exists aerobically, so that greater
amounts of yeast cells are formed, the actual utilisation of sugar is depressed.
The reasons for this are quite complex, and even today are not completely
explained. This latter phenomenon is the Pasteur effect and has been found
in other organisms and in animal tissues, and currently has been studied
considerably in relation to cancer tissue metabolism. The implications
of a discovery can never be realised fully at the time the discovery is
Another observation made by Pasteur in this paper is that yeast seems
to be able to grow anaerobically when sugar is present, but not when some
other carbon and energy source replaces sugar. In the presence of
other carbon sources, the yeast can grow, but only aerobically. The explanation
for this observation has only been made in the last 20 years. We know that
when sugar is present, yeast cells can obtain energy from this source anaerobically
by oxidising part of the sugar into carbon dioxide, while another
part of the sugar is reduced to alcohol. Thus some sugar molecules
are electron acceptors, while other molecules are electron donors. During
this process, called glycolysis, energy is released which is stored as
high energy phosphate bonds. No free oxygen is necessary for this process.
However, when other energy sources are used which are not sugars, such
as amino acids or organic acids, these compounds cannot be handled in the
above manner, and must be oxidised using free oxygen as the electron acceptor.
Therefore yeast can only grow when these materials are the energy sources
when there is oxygen present.
The discovery that yeast could grow both aerobically and anaerobically
is quite important for biochemistry and biology. Pasteur could not see
his discovery in the light of modern work, and therefore could only view
it in terms of the fermentative process he was seeking to explain. His
hope to bring about fermentation of sugar with green plants remains unfulfilled.