1861, Louis Pasteur
 

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 large
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 of
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 made. 
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.