67*» 



Common yeast is Saccharomyces cerevisiae, the composition bakers use has 

 very small cells and is called S. Minor. The yeast that grows on malt liquor when 

 left to spontaneous fermentation, as is the practice in Belgium, is S. Apiculatus. 

 Other species appear on musts of wines, and juices of stone fruit. The species 

 that is so important in this district, because it effects the transformation of apple 

 juice into cider, appears under the microscope to be identical with that which is 

 found on malt liquor, viz., S. Apiculatus. Pasteur has proved, by a simple experi- 

 ment, that germs or spores of Saccharomyces exist on the surface of grapes. He 

 introduced boiled grape juice into a series of thirty flasks ; of these ten were 

 immediately sealed up ; into a second ten he dropped a minute quantity of a liquid 

 prepared by washing the surface of some ripe untouched grapes ; into the third ten 

 he passed some of the same liquid boiled ; in 48 hours the first ten were unaltered, the 

 second ten were in full fermentation, and filled with flakes of mycelium ; the third 

 ten were unaffected. There is reason for believing that all saccharine fruits have 

 on their surface spores which remain quiescent till a concurrence of circumstances 

 brings them into contact \vith the enclosed juices, then subaqueous gro\vth com- 

 mences, accompanied by the decomposition of the sugar, so long as the subaqueous 

 growth continues propagation of the fungus takes place by budding, but as soon 

 as the sugar is exhausted the fungus comes to the surface and forms spores. Sac- 

 charomyces cerevisife, or common yeast, is seen under the microscope to consist of 

 a multitude of granular cells, diffused through a turbid liquid called yeast water. 

 The cells are "OOS of an inch in diameter, and like all other vegetable cells in 

 their simplest stage, consist of a speck of jelly called protoplasm, enclosed in a non- 

 nitrogenous envelope. Yeast is composed principally of albuminous and amyla- 

 ceous matter, but it contains a large proportion of phosphates of potash and 

 magnesia. The remarkable feature in its composition is its richness in nitrogen. 

 Funguses contain more nitrogen than any other class of plants. The Chantarelle 

 contains 3 "62 per cent. ; Boletus edulis, 4 "25 ; Lactarius dehciosus, 4*60 ; Mush- 

 room, 7 "26 ; and Yeast, 10 ; so that it closely approaches animal matter. These 

 agarics have been selected for comparison, because they have been often set before 

 us at our Fungus Foray dinners. Knowing the chemical composition of yeast, we 

 should expect the medium in which it flourishes to contain the nitrogenous and 

 mineral matters which it requires. It has been proved by experiment that yeast 

 will not exert its peculiar action on sugar unless these matters are present in 

 solution. 



We all know that if yeast be added to a liquor at a suitable temperature in 

 which malt or some saccharine fruit has been digested, certain occurrences will 

 ensue. The liquor vdU shortly become turbid, effervescence will take place from 

 the escape of free carbonic acid, the sweetness will disappear, alcohol will become 

 evident to the taste and smell, and a large increase will take place in the bulk of 

 the fungus. 



There are several varieties of sugar much alike in their chemical composition 

 and properties. The two principal are saccharose and glucose. Yeast acts dif- 

 ferently on each, so that it will be well to trace back their relation to, and forma- 



