1 5 2 FERMENT A TION IN MILK 



part of the subject of fermentation, namely, that occurring in milk, 

 falls to be considered in these pages, but in this brief introductory 

 note we desire to make it clear that we look upon the fermenta- 

 tions which occur in milk as illustrative of the great laws of 

 fermentation generally, wherever and however it occurs in nature. 



Kinds of Fermentation 



The process of fermentation is for convenience divided into 

 two chief kinds,^ namely, (i) When the action is direct and the 

 chemical changes involved in the process occur only in the presence 

 of the cell, the ferment or cell is described as an organised ferment ; 

 (2) when the action is indirect and the changes are the result of 

 the presence of a soluble material secreted by the cell, acting apart 

 from the cell, this soluble substance is termed an unorganised 

 ferment or enzyme. The organised ferments are yeasts, bacteria, or 

 vegetable cells allied to them ; the enzymes are ferments found in 

 the secretions of highly specialised cells in plants or animals, and 

 have their best illustrations in the digestive agents which bring 

 about assimilation of pabulum in the human body. This function is 

 performed in some cases by the enzyme combining with the sub- 

 stance on which it is acting, and then by decomposition yielding 

 the " new " digested substance and regenerating the enzyme ; in 

 other cases the enzyme, by its own molecular movement, sets up 

 molecular movement in the substance it is digesting, and thus 

 changes its condition. 



The unorganised ferments may be classified as follows : — 

 I. Amylolytic, converting starch and glycogen into sugar — e.g. 

 ptyalin (in the saliva), diastase, and amylopsin. Diastase is de- 

 veloped in the process of malting. Fermi states that (/n albuminous 

 media the bacilli of the subtilis group [B. anthracis)-S. megatherium, 

 etc.) and other species, including Micrococcus mastitidis, produce 

 amylolytic ferments. Most of these varieties afterwards utilise the 

 sugar further to form acid, while others, e.g. B. subtilis, do not.^ 



i 



2. Proteolytic ferments, those which change proteids into pro- 

 teoses and peptones — e.g. pepsin, trypsin. This fermentation is a notj 

 uncommon accompaniment of bacterial growth. The liquefaction 

 of the glue in gelatine is a sure indication of the presence of a 



1 Text-book of Physiology, vol. i., p. 312, E. A. Schafer, F.R.S. 



2 Atlas and Principles of Bacteriology, Lehmann and Neumann, 1901, 

 part ii., p. 63. In this book will be found further matter on bacterial fer- 

 mentation in general, pp. 58-92. 



