Jan. 1 8, 1877] 



NA TURE 



251 



saccharine liquid. M. Pasteur took two flasks partially 

 filled with a liquid containing in each case 150 grammes 

 of sugar ; the one {a) was almost completely deprived of 

 air, the other {b) contained air, A mere trace of yeast 

 (species, Saccharomyces pastorianus) was added to each. 

 The fermentation in a was sluggish, and after nineteen 

 days, when the experiment was closed, there was still a 

 slight evolution of carbonic acid gas ; in b the fermen- 

 tation was active, and ceased entirely on the ninth day. 

 The weight of yeast formed was determined in both 

 cases, as also that of the sugar left undecomposed ; these 

 were the results : — 



a. Weight ©f yeast formed 

 Sugar undecomposed . 



/'. Weight of yeast formed 

 Sugar undecomposed ., 



I '368 grammes. 

 4'6oo ,, 

 1-970 ,, 



nil. 



In b, therefore, though the liquid was far from being 

 saturated with air, the decomposition of the sugar was 

 completed in eight days, whereas in a, even at the end of 

 nineteen days, there was still some sugar left. The weight 

 of yeast formed was as i to 76 of sugar decomposed in ^, 

 and only as i to 8g in a. In another experiment on a 

 sugar solution completely deprived of air, the whole of the 

 sugar was not decomposed even at the end of three 

 months, the yeast produced was deformed in appearance, 

 and the ratio of yeast to alcohol produced was as i to 

 176. Reversing the experiment so as to obtain a thorough 

 and constant saturation of the liquid with air, the ratio of 

 yeast formed to the alcohol was as high as i to 4. In 

 this latter case the yeast acted more as a mycoderm or as 

 a mould than as a true ferment. We may sum up the 

 bearing of the facts elucidated by Pasteur, by the state- 

 ment that, within given limits defined in Pasteur's experi- 

 ments, the production of yeast is directly, and that of 

 alcohol inversely, as the consumption of free oxygen. 



M. Pasteur having demonstrated that the alterations 

 in the yeast, in the wort, and in the beer, are due to the 

 action of microscopic organisms other than the true yeast 

 cells, and that these ferments of disease are killed at a 

 high temperature, it follows that if a boiling wort be 

 cooled under conditions which preclude the entrance of 

 these germs, and if fermented with pure yeast, a beer will 

 be obtained which, so long as not exposed to germ-laden 

 air, must remain unchanged for an indefinite time. M. 

 Pasteur has conferred an additional favour on the prac- 

 tical man by designing an apparatus by which these con- 

 ditions maybe secured, and has carried on his process of 

 fermentation on a large experimental scale in several French 

 breweries with much success so far as fl avour and soundness 

 of product are concerned. The general adoption of new 

 and costly appliances must necessarily be slow. Nor is 

 it essential that his suggestions should be in all cases 

 carried out in their entirety. The intelligent brewer who 

 thoroughly masters the key to sound and unsound fermen- 

 tation will be enabled to secure the one and avoid the 

 other without replacing costly appliances by others vastly 

 more so. To do this, however, the microscope must be in 

 daily use in order to cultivate a purer and purer yeast crop, 

 and his technical processes of skimming and cleansing 

 must be more and more directed by this invaluable instru- 

 ment. The selection and preservation of yeast, the cul- 

 tivation of it and the gradual elimination of diseased 

 ferments will henceforth be the object of well-guided 



efforts, and not of mere chance, as heretofore. The 

 technical reader will do well to read and re-read the work 

 before us ; as he becomes master of its method of inves- 

 tigation and of the results obtained, so will he master 

 many of the difficulties of his art. 



Some suggestions are given (p. 224, et seq.) on the puri- 

 fication of yeast of interest to the brewer as well as to 

 the experimentalist. It would lengthen this notice too 

 much to give the details of the process ; it will suffice 

 here to state that the author recommends the use of a 

 weak solution of sugar rendered slightly acid with tar- 

 taric acid. In some cases he employs in addition a trace 

 of carbolic acid. Such a liquid destroys some true yeast ; 

 it is, however, far more injurious to the ferments of 

 disease. It should not be forgotten that aeration, while 

 injurious to most ferments of disease (mycoderma vini 

 and aceti, of course, excepted), is a great stimulant to the 

 Saccharomyces ; hence this method may be adopted 

 either alone or in conjunction with others suggested by 

 him. Those interested in this matter, and especially the 

 brewer, will do well to study carefully the work itself, they 

 will find on this subject, as well as on many others, not 

 alluded to here, much invaluable and suggestive informa- 

 tion. 



In concluding our brief examination of Pasteur's bio- 

 logical researches on fermentation, a few words on the 

 products formed will not be out of place. 



Numerous attempts have from time to time been made 

 to express by a chemical equation the decomposition of 

 glucose sugar by the yeast cell. Until the discovery of 

 succinic acid by Schmidt, and of glycerine by Pasteur, 

 the attempts made were far from being satisfactory. 

 According to Pasteur 100 parts of cane-sugar correspond- 

 ing to 105*26 parts of grape-sugar give when fermented — 



Alcohol 



Carbonic acid 



Succinic acid 



Glyceria ... 



Matters united with ferments 



49-42 

 0-67 

 3-06 



I '00 



105*26 

 Therefore ninety-five parts of cane-sugar are broken up 

 into carbonic acid and alcohol, one part is assimilated by 

 the yeast, and four parts are converted into succinic acid 

 and glycerin. 



Monoyer has proposed the following equation to repre- 

 sent the decomposition of these four parts of sugar, e.g:, 



4(Cj2H240i2) + 6HaO = 2(C4He04) + izlCsHgOs) 

 Hydrated cane-iugar. Water. Succinic acid. Glycerin. 



+ 4CO2 + O2, 



Carbonic acid. 



the liberated oxygen supporting the life of the plant. It 

 is impossible at present, however, to formulate the reac- 

 tions which occur, because they vary according to the 

 temperature, pressure, species of yeast employed, and to 

 the nature of the saccharine hquid. M. Pasteur himself 

 has pointed out that the proportions of succinic acid and 

 glycerin vary in different experiments. 



Mr. Horace Brown in some admirable and suggestive 

 experiments on the influence of pressure on the products 

 of fermentation {Journ. Chcm. Soc, 1872, p. 570), has 

 shown that in alcohohc fermentation in addition to 

 carbonic acid there is always formed a small quantity 

 of nitrogen, hydrogen, and a hydrocarbon of the marsh 



