The controversy between Pasteur and Liebig on fermentation

1 The controversy between Pasteur and Liebig on fermentation (Story taken from Acevedo & García-Carmona, 2016) The association of yeast fermentation with food has accompanied mankind throughout history. Around 6000 B.C., the Babylonians learnt to brew beer. And around 4000 B.C., the Egyptians learnt how to use yeast to make bread. Obtaining wine by the fermentation of grapes is mentioned in the Old Testament in the Bible. Other artisan fermentation procedures known from ancient times are those used to prepare vinegar, yoghurt, and cheese. Fermentation was also a favourite problem among alchemists. During the eighteenth and nineteenth centuries, chemists were attempting to formulate alcoholic fermentation as chemical reactions, following the methods that had provided them with so much success with other natural phenomena. Illustrious French chemists such as Lavoisier (1743-1794), Gay-Lussac (1778-1850), Thénard (1777-1857), and Dumas (1800-1884) studied the transformation of sugar cane into alcohol with quantitative methods. Lavoisier obtained a simple formulation of the process which gave the impression that the nature of the phenomenon had been discovered, in which there was no place for yeast. All the chemists of the time took for granted that yeast accompanied, and probably began, the fermentation. However, even if it were the initiator of the reaction, it did not seem to take any part in it. Berzelius (1779-1848) called this strange phenomenon catalysis, and defined the term ferment as an example of catalytic activity. Shortly thereafter, Schwann (1810-1882) found pepsin to be the substance responsible for the digestion of albuminous substances in the stomach. He believed this was what Berzelius had defined as catalysts, or the force of the chemical reactions of minerals, organic substances, and living matter. Liebig (1803-1873) opposed the use of the terms catalyst and pepsin since they were only a vague idea. In 1837, Cagniard de La Tour (1777-1859), Kützing (1807-1893), and Schwann independently identified yeast as a living organism that thrives when sugar ferments. In 1839 in Paris, at the request of the French Academy of Sciences, Turpin (1772-1853) confirmed the microscopic observations of Cagniard de La Tour. Berzelius, Liebig, and Wöhler (1880-1882) rejected this idea of a vitalist nature. That same year, Liebig and Wöhler published a paper on the role of yeasts in alcoholic fermentation, grotesquely ridiculing the organic nature of yeast. In 1858, Traube (1826-1894) stated that all fermentations produced by living organisms are based on chemical reactions rather than on any vital force. There was a deeper reason for scientists of the time to be leery of the vitalist interpretation of alcoholic fermentation. Since this process could be described by a simple chemical reaction, there was no sense in explaining it in terms of a living organism instead of referring to chemical and physical interactions. But the fact is that, in the mid-nineteenth century, it was unclear what fermentation was. There were three main theories: chemical, vitalist, and modified chemical. The first of these, the chemical theory, was that of the German chemist Liebig. Following Lavoisier s ideas, he considered that fermentation was a chemical decomposition caused by the putrefaction of an animal or vegetable substance, but which did not require the intervention of microorganisms. I.e., it was not a biological process. Liebig s interpretation of the process was that vibrations from the decomposition of the organic matter extended to the sugar. In the case of alcoholic fermentation, he believed that the yeast used was a putrefied vegetable matter, of an albuminous type, which decomposed the sugar by triggering a kind of putrefaction. Alcohol, carbon dioxide, and other by-products were produced in this decomposition, and a product was deposited that Liebig defined as an insoluble ferment. This nitrogen-rich ferment could cause fermentation in another sugar solution. The change was facilitated by the ferment or yeast with its characteristics of a nitrogenated compound in a state of putrefaction. The ferment is susceptible to change. It undergoes

2 decomposition by the action of air (providing oxygen), water (providing moisture), and a favourable temperature. Before coming into contact with oxygen, the components are together but without interacting. By means of the oxygen, the resting, or equilibrium, state of the attractive forces holding the elements of the ferment together is disturbed. As a result, there forms a separation or new arrangement of its elements. The fermentation is produced by transfer of the ferment s molecular instability (atoms in movement) to the sugar molecules, and continues for as long as its decomposition lasts. Thus, Liebig s theory of fermentation corresponds to the mechanistic view of the world in the philosophical sense that was given to the term in the seventeenth century on the basis of Newton s contributions. Liebig also held that rotting organic substances (urine, blood, etc.) had analogous fermentative properties. Thus, different phenomena were explained in the same way both classical fermentations (wine, beer, etc.) as well as the putrefaction and transformations undergone by food when digested. The first notes on fermentation made by the French chemist Pasteur (1822-1895) are found in his laboratory notebooks in September 1855. But it was in 1857 that he made the development of his theory about fermentation public in his Mémoire sur la Fermentation Appelée Lactique (henceforth, Mémoire), as well as in other subsequent works. In this publication, he collected together the ideas of Cagniard de La Tour and Schwann, put forth considerations about the molecular dissymmetry of organic substances in reference to the laevo (left-handed) and dextro (right-handed) forms of tartaric acid, and emphasized the speculative nature of his theory. Historians of science consider the Mémoire to be one of Pasteur's most important writings, although it offered no rigorous empirical evidence on the involvement of living organisms in fermentation. He provided these later in Nouvelles Recherches sur la Fermentation Alcoolique (1858) and, above all, in Mémoire sur la Fermentation Alcoolique published in 1860. With regard to the importance of the Mémoire, Geison (1974) writes: With two striking exceptions this memoir contains the central theoretical and methodological features of all of Pasteur's work on fermentation the biological conception of fermentation as the result of the activity of living microorganisms; the view that the substances in the fermenting medium serve as food for the causative microorganism and must therefore be appropriate to its nutritional requirements; the notion of specificity, according to which each fermentation can be traced to a specific microorganism; the recognition that particular chemical features of the medium can promote or impede the development of any one microorganism in it; the notion of competition among different microorganisms for the aliment contained in the media; the assumption that air might be the source of the micoorganisms that appear in fermentation; and the technique of directly and actively sowing the microorganism presumed responsible for a given fermentation in order to isolate and purify it. The two missing features, which soon completed Pasteur's basic conception, were the techniques of cultivating microorganisms (and thereby producing fermentation) in a medium free of organic nitrogen and his notion of fermentation as life without air (Geison, 1974: 362, quoted by Latour, 1991: 131-132). In this same Mémoire, Pasteur wrote about Liebig s interpretation of fermentation: In the eyes of [Liebig] a ferment is an excessively alterable substance that decomposes itself and thereby excites fermentation in consequence of its alteration, by disrupting through communication and by disassembling the molecular group of the fermentable matter. According to Liebig, such is the primary cause of all fermentations and the origin of most contagious diseases. Berzelius believes that the chemical act of fermentation is to be referred to the action of contact (Latour, 1991: 133). The choice of lactic fermentation, responsible for the souring of milk, instead of alcoholic fermentation on which he had worked during 1855 and 1856, was due to strategic reasons. This fermentation was chemically the simplest because, in it, the sugar is split into two halves which are the lactic acid molecules. If he could demonstrate that the process needed a living organism (yeast) then it could serve as a general model for other fermentations.

3 In subsequent experiments on alcoholic fermentation, Pasteur used two flasks, one with a gooseneck. He poured broth into both flasks and heated them at the bottom. After boiling the liquid, he let it cool. Then he noted that the broth in the gooseneck flask was clear, unless shaken. He interpreted this fact by saying that the neck of the flask had been able to stop the passage of most particles, and therefore the liquid remained unchanged. In contrast, the liquid in the other flask degenerated. He concluded that this fermentation needed yeast, which was alive but did not require oxygen. I.e., this was an anaerobic fermentation, which Pasteur described as respiration without air, characteristic of some microorganisms (such as certain bacteria and yeasts). When the yeast is allowed to grow, the substance rots over time. Pasteur s interpretation of these observations was that living organisms are responsible for the process of fermentation. It is therefore a biological process, not a chemical process of oxidation-reduction. One can thus say that his theory of fermentation falls under vitalism. In sum, for Pasteur the cause of fermentation was the biological activity of certain yeasts (microorganisms). Unlike Liebig, he hypothesized that yeast was a living organism (vitalist position), and that its action on the sugar had nothing to do with processes of disorganization or putrefaction. This is made clear at the end of his 1857 Mémoire: [ ] whoever judges impartially the results of this work and that which I shall shortly publish will recognize with me that fermentation appears to be correlative to life and to the organization of globules, and not to their death and putrefaction; no more than fermentation is a phenomenon due to contact, in which the transformation of sugar would take place in the presence of the ferment without giving up anything to it or taking anything from it (Latour, 1991: 133). Was this concept of fermentation valid for all the phenomena that Liebig tried to explain with his theory? Pasteur resolved it in his own way, considering that true fermentations, according to his theory, were due to the action of microorganisms. The third theory was that of the French chemist Berthelot (1827-1907). Like Pasteur, he admitted that there might be a relationship between fermentation and the activity of a yeast, but he did not believe that this justified a vitalist interpretation. According to Berthelot, the yeast secreted a chemical substance that acted on the sugar and transformed it. This substance would be comparable to what are now called enzymes. Berthelot recalled that the existence of these soluble ferments had been shown experimentally, and they could even act without the presence of the organism from which they came. In his work Organic Chemistry Founded on Synthesis (1860), he stated that ferment is not a living organism, but the substance produced by one. It is noteworthy that this interpretation, together with the notion of enzyme as biocatalyst, is what is considered correct today. In alcoholic and lactic fermentations, the transformation of the sugar is caused by enzymes. The main difficulty at the time was due to the vague meaning of the term fermentation, in both everyday language and scientific language. Berthelot himself mentioned sulfuric acid as a ferment. However, some order had been established differentiating between insoluble ferments (microorganisms) and soluble ferments, including many substances that today are classified as enzymes. Pasteur s arguments to legitimize his theory were not always precise. Instead, they would apply a semantic strategy which consisted in converting the theory into a definition. Thus, he established biological fermentations as being the only true ones, excluding chemical fermentations which for him were not authentic. Indeed, Pasteur often used the expression fermentations in the strict sense, as he wrote in 1860: [ ] consequently, the opposition that Mr Berthelot believes to find between my statements and the real facts only responds to the extension that he gives to the word ferment, while I have applied it only to the substances which produce fermentations in the strict sense (Thuillier, 1990: 425).

4 Pasteur made sure that he was always right with this kind of circular reasoning. Indeed, he was very skilful in the use of rhetoric and semantic strategies in order to persuade others, skills that he successfully deployed in this controversy and in that of spontaneous generation against Pouchet (Farley & Geison, 1994; Geison, 1995). While it is true that the presence of living cells is necessary for alcoholic or lactic fermentation to take place, Pasteur's theory was far from the explanation that is accepted today. Nonetheless, his interpretation of the phenomenon paved the way for other very fruitful research, and, above all, was successful in many practical aspects of improving the techniques of fermentation, hence the success he had in his time. On the contrary, Liebig was right in his insistence on the chemical nature of fermentations, but his conception of ferments as substances in a state of decomposition gave an idea of the phenomenon which was not only very inaccurate, but also not very fruitful. Strictly speaking, Pasteur did not define fermentation. To do so, he would have had to have a structural theory available to him that could account for the fermentation mechanism with precision. This was something that no one knew then. He limited himself to indicating, in a very general form, the causes of fermentation, and to giving a list of reactions to illustrate this process. In the same way as Liebig had done, Pasteur selected certain cases. Although his list of examples was incorrect, debatable, and even arbitrary, it served a dual purpose. It attempted on the one hand to put order into a heterogeneous and poorly understood set of phenomena, and on the other to set out some general schemes that would allow that classification to be both experimentally verified and explained. This approach was a good starting point from which to generate other useful research, so that in this sense Pasteur s programme of investigation was reasonable, although it might seem somewhat irrational. His list of cases was closer to classical fermentations than to that of the supporters of Liebig s chemical theory: I call fermentations in the strict sense [ ] the fermentations that I have studied and that consist of all the best characterized fermentations, those as old as the world, involved in the formation of bread, wine, beer, sour milk, transforming urine into ammonia, etc., those whose ferments are, according to my investigations, living beings that are born and multiply during the act of fermentation (Thuillier, 1990: 431). Speaking in his Mémoire about his research on fermentation, Pasteur recognized that he started from preconceived ideas, that he went beyond the facts, and that, strictly speaking, his ideas could not be demonstrated irrefutably, but that, despite all this, he was proceeding in a logical fashion. No matter how rash Pasteur s generalizations were, they enabled him to advance. This is what is known, in the philosopher Lakatos s terms, as a progressive research programme one which proved productive for a time. And all this even though in his most scientific texts he frequently had recourse to disputable facts, dogmatic statements, rhetorical tricks, allusions to the needs of industry, and even nationalistic patriotism as in the following: In this moment when I undertake here, against Mr Liebig, the defence of an opinion that, after all, belongs to French science, why does Mr Fremy proclaim himself, in a form that is at least inopportune, a champion of German science, with which I am eager to return to a battle from which I had unwillingly separated myself? (Thuillier, 1990: 432). In the last quarter of the twentieth century, these aspects gave rise to the sociological perspective being emphasized in many publications about Pasteur. Epistemological analyses passed into the background to be replaced by social, economic, political, ideological, and religious aspects of his work. For example, that Pasteur was a faithful follower of Emperor Louis Napoleon (Napoleon III) is highlighted, noting that the emperor supported Pasteur s more practical research by helping him to find financial support for it. Another economic facet that is noted is that Pasteur used a new method to eliminate the microorganisms which can degrade wine. This was to put the liquid in well-sealed vats and raise the temperature to 45 C-50 C for a short time, protecting it

5 from oxygen. Despite the wine industry s initial rejection of heating wine, controlled experiments with batches of heated and unheated wine demonstrated the effectiveness of the procedure. Thus was born pasteurization. Pasteur and his colleague Claude Bernard (1813-1878) performed the first pasteurization on 20 April 1864. Fermentation can produce nutrients or eliminate antinutrients. Food may be preserved by fermentation since this uses energy from the food and can create conditions that are unsuitable for undesirable organisms; e.g., in vinegar, the acid produced by the dominant bacteria inhibits the growth of other microorganisms. Although the foregoing aspects allow one to have clearer knowledge of some very interesting contextual historical realities, it is necessary to try to maintain a balance. In studies of Pasteur s work, the sociological perspective must not be rejected, but neither must the epistemological perspective since they are two sides of the same coin. The controversy between Pasteur and Liebig on the nature of alcoholic fermentation was clarified years later by the German chemist Eduard Buchner (1860-1917). Influenced by his brother, the bacteriologist Hans Buchner (1850-1902), he became interested in the process of alcoholic fermentation in which yeast breaks sugar down into alcohol and carbon dioxide. He published his first article in 1885, showing that fermentation can occur in the presence of oxygen (aerobic fermentation), a conclusion contrary to Pasteur s theory. By 1893, Eduard Buchner devoted himself entirely to the search for the active agent of fermentation. He obtained pure samples of the fluid inside yeast cells by pulverizing them in a mixture of sand and diatomaceous earth, then passing the mixture through a cloth filter. This procedure avoided the use of solvents and high temperatures, a drastic and destructive method that had thwarted previous research. He supposed that the liquid collected would be incapable of producing fermentation because the yeast cells were dead. However, when he kept the liquid in the sugar concentrate he noted that carbon dioxide was released, a sign that fermentation was taking place. Buchner then established the hypothesis that fermentation was caused by an enzyme which he called zymase. In 1897, he published that fermentation was the result of chemical processes, both inside and outside cells. The dispute between Liebig and Pasteur in some way slowed the advance of science in the area of fermentation and enzymes, since neither of them had been correct. However, their conflicting ideas had also accelerated research in that same field from contributions by other scientists. Buchner s work on fermentation paved the way for the study of enzymes and fermentation a key moment in the history of modern chemistry.

6 Questions about NOS aspects related to the story of the controversy Pasteur vs Liebig. Q1. Why do you think that there might be important differences in scientific interpretations of a natural phenomenon, as in the case of Pasteur and Liebig regarding fermentation? Q2. According to what you have read in the text, how would you explain what a scientific theory is? Q3. According to what you have read in the text, to what extent do you agree that scientific research develops mainly through successive processes of experimentation and testing? Q4. From what you have read about the scientific controversy on fermentation, how important for the development of science do you think the mistakes that scientists make are? Q5. According to what you read in the text, what role do you believe scientists creativity and imagination have in their research? Q6. For what reason do you think Pasteur s ideas on fermentation had more success than Liebig s at their time? Q7. How do you think the sociocultural, political, economic, etc. contexts of each age can influence the development of science? Explain it for this case of fermentation. Q8. What interest do you think there can be for the advancement of science in the existence of disputes or disagreements among scientists about a research problem? References Acevedo, J. A., & García-Carmona, A. (2016). Uso de la historia de la ciencia para comprender aspectos de la naturaleza de la ciencia. Fundamentación de una propuesta basada en la controversia Pasteur versus Liebig sobre la fermentación. Revista Iberoamericana de Ciencia, Tecnología y Sociedad, 11(33). Farley, J. y Geison, G. L. (1994). Ciencia, política y generación espontánea en la Francia del siglo diecinueve: el debate Pasteur-Pouchet. In C. Solís (Ed.), Razones e intereses. La historia de la ciencia después de Kuhn (pp. 219-263). Barcelona: Paidós. Geison, G. (1974). Louis Pasteur. In C. C. Gillispie (Ed.), Dictionary of Scientific Biography, vol. 10. New York: Charles Scribner's Sons. Geison, G. (1995). The Private Science of Louis Pasteur. Princeton, NJ: Princeton University Press. Latour, B. (1991). Pasteur on Lactic Acid Yeast: A Partial Semiotic Analysis. Configurations, 1(1), 129-146. Thuillier, P. (1990). De Arquímedes a Einstein. Las caras ocultas de la invención científica. Madrid: Alianza.