140 NUTRITION AND METABOLISM. 



if there is not enough acid, there will be no curd, not even after a long 

 time. An acid curd can be brought back to the original state by an ad- 

 dition of alkali, while a rennet curd by no means can be changed back to 

 casein. Rennet-forming bacteria are found in milk and dairy products, 

 in soil and other habitats. They will coagulate milk without causing 

 any appreciable increase of acidity. They all seem to digest the curd 

 after it is formed (see the above table). The relation between proteolytic 

 and rennet enzymes will be discussed in a later chapter. 



Rennet is sometimes called chymosin; the Society of American Bac- 

 teriologists uses the German word "lab." 



ZYMASES. 



The zymases are the agents which furnish the energy for cell life 

 by causing fermentative decompositions. As has been stated before, 

 the processes which provide for energy must take place inside of the 

 cell. Consequently, all intracellular enzymes are endo-enzymes. The 

 difference between the soluble enzymes and the endo-enzymes is very 

 plainly shown in the following table, giving the energy liberated by the 

 various enzymes by acting upon i g. of substance. 



Energy liberated from I g. of substance. 



Soluble Enzymes. Endo-Enzymes. 



Pepsin, trypsin o calories Lactacidase 82 calories 



Lipase 4 calories Alcoholase 160 calories 



Maltase, invertase 10 calories Urease 239 calories 



Lactase 23 calories Vinegar-oxidase . . . . 2,520 calories 



The microbial cell does not lose much energy by the activity of the 

 soluble enzymes outside of the cell, because their energy yield is insignifi- 

 cant. 



The first zymase recognized as such was urease, the enzyme which 

 changes urea to ammonium carbonate. The urease was not considered 

 an exceptional case, and no particular attention was paid to the fact 

 that it was the only zymase known at that time. The actual investiga- 

 gations of the zymases did not start until Buchner had demonstrated 

 that yeast can be ground with infusorial earth until all cells are lacer- 

 ated, and then can be pressed and the juice filtered without losing the 

 power of alcoholic fermentation. Such fermentation cannot be due to 



