14 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



With the lactic-acid organisms which bring about the souring of 

 milk the lactic-acid fermentation is produced. This may also occur 

 as the result of the action of putrefactive bacteria in the alimentary 

 canal. The two stages of the lactic-acid fermentation are represented 

 by the following equations : — 



(1) C,2H220n + H2O = 4C3H6O3 



[lactose] [lactic acid] 



(2) 4C3H6O3 = 2C4Hg02 + 4CO2 + 4H., 



[lactic acid] [butyric acid] 



Maltose is the chief end product of the action of malt diastase on 

 starch, and is also formed as an intermediate product in the action 

 of dilute sulphuric acid on the same substance. It is also the chief 

 sugar formed from starch by the diastatic ferments contained in the 

 saliva (ptyalin) and pancreatic juice (amylopsin).^ It can be ob- 

 tained in the form of acicular crystals ; it is strongly dextro-rotatory. 

 It gives Trommer's test ; but its reducing power, as measured by 

 Fehling's solution, is one-third less than that of dextrose. 



By prolonged boiling with water, or, more readily, by boiling with 

 a dilute mineral acid, or by means of an inverting ferment, such as 

 occurs in the intestinal juice, it is converted into dextrose. 



C,2H220i, + H2O = 2C(;H,206 



[maltose] [dextrose] 



It undergoes readily the alcoholic fermentation. 



The three important physiological sugars (dextrose, lactose, and 

 maltose) may be distinguished from one another by their relative 

 reducing action on Fehhng's solution (I'O : OTl : 0'63), or by the 

 phenyl-hydrazine test described in Lesson XIII. 



POLYSACCHAKIDES. 



Starch is widely diffused through the vegetable kingdom. It 

 occurs in nature in the form of microscopic grains, varying in size 

 and appearance, according to their source. Each consists of a 

 central spot {hilum) round which more or less concentric envelopes 

 of starch proper or granulose alternate with layers of cellulose. 

 Cellulose has very little digestive value, but starch is a most 

 important food. 



Starch is insoluble in cold water ; it forms an opalescent solution 

 in boiling water, which if concentrated gelatinises on cooling. Its 

 most characteristic reaction is the blue colour it gives with iodine. 



On heating starch with dilute mineral acids dextrose is formed. 



' An isomeric sugar called iso-maltose (see Lesson XIII.) is also formed under 

 these circumstances. 



