

MICROORGANISMS AND FERMENTATIONS 13 



between 65 and 85 C. In the dry state, some enzymes will 

 stand temperatures of about 150 C. 



Enzymes resemble living organisms in being destroyed by heat ; 

 on the other hand they are unaffected by many substances which 

 act as poisons towards living protoplasm, such as toluene, ether, 

 chloroform, ethereal oils, phenol, salicylic acid and benzoic acid. 

 Some such substance, usually toluene, may therefore be added to an 

 enzyme solution to keep it sterile ; boric acid is added to preparations 

 of rennet. Most enzymes act best in feebly acid, all but neutral, 

 solutions ; pepsin is exceptional in acting in presence of the 

 appreciable proportions of acid contained in the gastric juice. 



As a rule, enzymes are named after the substances on which they 

 act, thus maltase acting on maltose, lactase acting on lactose, urease 

 on urea, etc . In some cases, however, previous usage has established 

 other names. Diastase or, better, Amylase (amylum = starch) con- 

 verts starch into dextrin and maltose ; it occurs plentifully in malt, 

 and plays an important pa ; rt in the brewing and distilling industries ; 

 during the mashing process it converts the starch into fermentable 

 products. It also occurs in the saliva and the pancreatic juice of 

 mammals ; as it only makes its appearance some time after birth, 

 newly-born animals are unable to assimilate starchy foods. 

 Invertase hydrolyses saccharose, i.e., cane or beet sugar, into dex- 

 trose (d. glucose) and Isevulose (d. fructose), a mixture which is 

 found in honey and many fruits. Invertase, maltase, lactase and 

 amylase are the chief carbohydrate splitting enzymes. Enzymes 

 which hydrolyse fats into glycerol and fatty acids are known as 

 lipases, and those which split proteins as proteolytic enzymes. 

 Proteins may be hydrolysed by stages, each stage yielding a 

 simpler product, thus : proteins to metaproteins to proteoses to 

 peptones to polypep tides to amino acids. The first decomposition 

 product of casein is paracasein, which, according to Hammarsten, 

 differs from natural casein in being precipitated by the small 

 amounts of calcium salts found in solution in normal cow's milk 1 . 

 A microorganism which secretes proteolytic enzymes will always 

 coagulate milk, and then gradually redissolve, i.e., peptonise, the 

 precipitated paracasein ; unless the organism in question also 

 belongs to the acid-producing group, the process of peptonisation 

 will tend to produce an alkaline reaction in the milk. 



Peptonising organisms generally liquefy gelatine more or 



1 The coagulation of casein by rennet appears to be analogous to the 

 coagulation of other proteins by heat. See Orla Jensen, " Kemiske Under- 

 sogelser over Maelkens Koagulering og Koaglets Oploselighed i Saltvand." 

 Det kgl. danske Videnskabernes Selskabs Oversigter, 1914, No. 4. 

 Chemische Untersuchimgen iiber die Gerinnung der Milch und iiber die 

 Loslichkeit des Ger. in Salzwasser. Zeitschr. f. Physiol. Ohem.. 1914, 

 Bd. XCIII., p. 283. 



