1 54 FERMENT A TION IN MILK 



able change. As we point out elsewhere, milk may be drawn 

 from the udder and kept in sealed vessels for weeks and months 

 without any apparent change whatever. The souring of milk is 

 therefore due to some extraneous agent or ferment capable of 

 setting up these two chief changes when introduced into milk : 

 ^/ firstjLthe^feriiientation of the lactose with the production of lactic 

 acid; and secondly and subsequently, the curdling of the casein 



L present in milk, which becomes held in suspension in a semi- 

 colloid al fofmT Tf the acidity exceeds a certain percentage, the 

 casein is precipitated and the milk becomes coagulated.^ 



Such is the simple process of lactic acid fermentation. Yet when 

 we come to examine these changes closely, many points of interest 

 arise concerning the mode of the fermentative action, the ferment- 

 ing agent or agents, the constitution of the acid produced, the 

 external conditions favouring the fermentation, and the purposes 

 to which the fermentation may be subservient in the economy of 

 nature.^ 



The earliest chemical investigation of the souring of milk was 

 probably that instituted in 1833 by Pelouze and Gay-Lussac, but 

 from that date fully twenty-five years elapsed before the know- 

 ledge that this process is a manifestation of vital activity on the 

 part of sundry micro-organisms assumed definite shape. But as 

 early as 1701, Andry had noticed that sour milk contained living 

 organisms, and Blondeau, in 1847, made a microscopic examina- 

 tion of milk, and distinguished therein two types of micro- 



^ As we have already stated, milk is often acid the moment it passes from 

 the udder. This is evidently not due to lactic fermentation and should be 

 differentiated from it. Various text-books are responsible for the idea commonly 

 held that milk is absolutely neutral as it passes from the udder. We examined 

 milk on one occasion by testing with sensitive litmus paper. Twenty-six 

 healthy cows (Shorthorns and their cross) were milked in the evening, and as 

 the milk passed from the udder to the pail the litmus paper was used. Out of 

 the twenty-six milks, twenty-four gave an acid reaction (92-3 per cent.). 



2 The equation of lactic acid fermentation is generally expressed as follows : 

 C12H22O11 (Lactose) + HjO = 4C3HBO3, or more simply CgHjaOg = 2C3H80;j. Actu - 

 ally the process is not simple but complex, and CO.^ is liberated, and alcohol ana 

 volatile acids formed. Warington has shown that the amount of acid varies 

 greatly in different species. The organisms producing lactic acid are inimically 

 affected if the acid produced passes above a certain slightly variable point. 

 Variations depend on time, temperature, and the particular species of lacti 

 bacteria. Ordinary lactic acid exists in three forms characterised by then 

 optical properties. One turns the plane of polarised ray to the left, a second 

 turns it to the right, and a third is inactive. The most common is dextro-lactit 

 acid. 



