592 



NATURE 



[April 21, 1898 



determines thereby a purification of it, separating its limpid from 

 its turbid constituents. Valentin realised that alcohol makes 

 its appearance in the fermenting liquid, but supposed that it was 

 in some way pre-existent in the extract of the germinated barley 

 grains, and became active and capable of distillation only after 

 being liberated from impurities which accompanied it, and which 

 masked its special properties. 



Van Helmont, who wrote in 1648, though confusing fermenta- 

 tion with effervescence, like most writers of the time, yet dis- 

 tinguished that a special gas, which he called the " gas of wine," 

 was produced during the former action, and pointed out that it 

 was different from the spirit of the wine. 



Attention was drawn to a distinction between effervescence 

 and fermentation by de la Boe in 1659 and by Lemery in 1675. 

 The first of these writers held the sounder view as to this 

 difference, stating that in effervescence the chief reaction is one 

 of combination, while in fermentation it is a question of de- 

 composition. Lemery held, less accurately, that the chief 

 difference between them was one of relative rapidity, fermen- 

 tation being a slower and more complicated process. This 

 theory of fermentation is stated by himself in the following 

 words : — 



" Pour expliquer cet efifet, il faut savoir que le moust contient 

 beaucoup de sel essential ; ce sel comme volatil faisant effort 

 dans la fermentation pour se detacher des parties huileuses par 

 lesquelles il etait comme lie, il les penetre, il les divise et il les 

 ecarte jusqu'a ce que par ses pointes subtiles et tranchantes, il 

 les ait rarefiees en esprit ; cet effort cause I'ebullition qui arrive 

 au vin, et en meme temps sa purification ; car il en fait separer 

 et ecarter les parties les plus grossieres en forme d'ecume, dont 

 une portion s'attache et se petrifie aux cotes du vase, et I'autre 

 se precipite au fond, c'est ce qu'on appelle le tartre et la lie. 

 L'esprit inflammable du vin n'est done autre chose qu'une huile 

 exaltee par des sels." 



Apparently the first ideas on the subject that may be re- 

 garded as at all clear, were advanced by Becher in 1682, and 

 they mark an epoch in the development of our knowledge of it. 

 This author ascertained the fundamental fact that only sac- 

 charine liquids are capable of undergoing alcoholic fermenta- 

 tion, and he showed that the alcohol does not exist as such in 

 the original must of wine, but is formed during the operation of 

 fermentation. Becher thought its formation to be due to a 

 kind of combustion, as he ascertained that access of air is 

 needed to set up the phenomenon. 



About the same time a theory of the nature of fermentation 

 was advanced, which has much in common with the ideas 

 maintained in later times by Liebig. This was due to Willis 

 and to Stahl, both of whom entertained similar opinions on 

 the subject. It was that the ferment which they recognised 

 as the factor that started the operation is a body possessing 

 a peculiar internal movement or vibration, and that it transmits 

 this vibration to the fermenting material. Of course in the 

 condition of chemical science at the time, there was no satis- 

 factory statement possible as to the nature of the changes caused 

 by such vibration, but Stahl suggested that various decomposi- 

 tions and recombinations resulted therefrom. 



The next marked advance in our knowledge must be 

 associated with the name of Lavoisier a century or more later. 

 Up to his time no quantitative researches into the .subject had 

 been carried out. The bodies capable of fermentation had 

 been ascertained up to a certain point ; besides the alcoholic, 

 the acetic fermentation had been discovered, and a general 

 analogy had been established between fermentation and putre- 

 faction. The products of these fermentations had been ascer- 

 tained to be carbonic dioxide, alcohol, and acetic acid. Very 

 little acquaintance had been made with the ferment, which was 

 shortly to be recognised as a definite vegetable organism. 



While Becher first pointed out the necessity for the presence 

 of sugar in the fermenting liquid, Lavoisier studied quantitatively 

 the relations of the sugar to the derivatives of it formed during 

 the fermentation, and came to the conclusion that the operation 

 consists of a separation of the sugar into two parts, one of which 

 becomes oxygenated to form carbonic dioxide, while the other 

 is converted into alcohol. He says that if it were possible to 

 recombine these two substances, alcohol and carbonic dioxide, 

 sugar would again be formed. 



It is apparent that though Lavoisier's methods of analysis 



were imperfect, and his figures inaccurate in consequence, yet 



his general conclusions were sound. Towards the year 181 5 



. analyses by Gay-Lussac, Thenard, and de Saussure fixed 



NO. i486, VOL. 57] 



definitely the composition of sugar and alcohol. These more 

 accurate analyses confirmed Lavoisier's position, but revealed a 

 discrepancy which for a long time remained unexplained. Com- 

 putation of the composition of sugar t>ased upon the CO., and 

 alcohol formed during its fermentation pointed to its having the 

 formula CgHigOg (taking the modern values of the atomic weights),, 

 the decomposition being capable of expression by the equatiorv 

 CeHjoOg = 20211^0-1-2002. The analysis made by Gay-Lussac 

 and Thenard of cane-sugar itself demanded the formula 

 ^\^^vf^w These authors were unable to account for the dis- 

 crepancy which remained unexplained till Dubrunfaut in 1832: 

 observed that before cane-sugar could ferment, it became trans- 

 formed into another form of sugar which is non-crystallisable. 

 Dumas and Boullay in 1828 tried to reconcile the discrepancy by 

 assuming that the fermentation is accompanied by the absorption^ 

 of water. We have in the work of these three investigators the 

 substance of what we now know to be true, that the fermentation 

 of cane-sugar involves two processes, the hydrolysis of the cane- 

 sugar with the formation of hexoses, and the decomposition of 

 these with the formation of alcohol and carbonic dioxide. 



During the progress of these investigations into the chemistry 

 of fermentation a certain study of the fermenting body was being 

 carried on by various observers. As long ago as 1680 the yeast 

 of beer was examined microscopically by Leuwenhoek, who stated 

 that it was composed of little ovoid or spherical globules, but was- 

 not able to determine their nature. Subsequent writers con- 

 sidered them to be of animal origin, but very little was definitely 

 ascertained about them till the fourth decade of the present 

 century, when Cagniard de Latour, repeating Leuwenhoek's- 

 experiments, saw that yeast is composed of a mass of organised 

 globules capable of reproduction by budding, and appearing to 

 belong to the vegetable kingdom. He concluded that very 

 probably they disengaged the carbonic dioxide and fermented 

 the liquid by some effect of their vegetation. Before de Latour 

 and writers contemporary with him the yeast was generally 

 considered to be of an animal rather than a vegetable nature, 

 this view being promulgated especially by Fabroni, Desmazieres, 

 and Astier, the latter of whom held that it could only live at the 

 expense of the .sugar which it decomposed. Since the time of 

 Meyen the true systematic position of yeast has been recognised. 

 The work of Astier and of C. de Latour laid the foundation 

 for the more complete and satisfactory views of Pasteur, whose 

 researches have thrown so much light upon the whole process- 

 of fermentation. 



One of the most important discoveries that we owe to Pasteur 

 is that alcoholic fermentation is accompanied by the coincident 

 formation of glycerine and succinic acid, and that, therefore, 

 the equation given above by no means represents all that is 

 taking place in a fermenting liquid. Without committing him- 

 self to an equation to represent the whole decomposition, Pasteur 

 determined by quantitative methods that about 4 per cent, of 

 the sugar which disappears in the process of fermentation does 

 not give rise to alcohol, but to glycerine and succinic acid. 

 In addition to ordinary ethylic alcohol, also small quantities of 

 higher alcohols, in varying quantities, are always or generally 

 formed. 



Pasteur's theory of fermentation is the natural outcome and 

 the completion of the ideas of Astier and of C. de Latour. 

 In his own words it may be stated : " Mon opinion la plus 

 arretee sur la nature de la fermentation alcoolique est celle-ci : 

 L'acte chimique de la fermentation est essentiellement un 

 phenomene correlatif d'un acte vital, commen9ant et s'arretant 

 avec ce denier. Je pense qu'il n'y a jamais fermentation al- 

 coolique sans qu'il y ait simultanement organisation, develop- 

 pement, multiplication de globules, ou vie continuee, pour 

 suivie, des globules deja formes." 



This hypothesis, originally advanced by C. de Latour, did 

 not, however, obtain acceptance at once. It was opposed 

 strongly by Liebig, who put forward a view which is a modi- 

 fication of that advocated so long before by Willis and by 

 Stahl. According to Liebig, the cause of fermentation is an 

 internal molecular movement or vibration which a body in the 

 course of its decomposition communicates to other matters whose 

 elements are held together' with very feeble affinity. Liebig 

 says : " La levure de biere, et en general toutes les matieres 

 animales et vegetales en putrefaction, reportent sur d'autres 

 corps I'etat de decomposition dans lequel elles se trouvent elles- 

 memes ; le mouvement qui, par la perturbation d'equilibre, 

 s'imprime a leurs propres elements, se communique egalement 

 aux elements des corps qui se trouvent en contact avec elles." 



