STERILIZATION OF MILK. 97 



Pathogenic that is, disease-producing germs as well as other 

 dairy microbes of most common occurrence in fermenting milk can 

 be destroyed by a steam heat of 68 to 75 C. for one hour's time, or 

 for three-fourths of that time when the temperature is 100 C. 

 This is so where the amount treated does not exceed one litre. 

 For this reason it is comparatively easy to effect the complete steril- 

 ization without any alteration of its chemical composition, its 

 colour, or the state of its fatty globules, provided the milk does not 

 contain spores of a resistant nature. Unfortunately such pure milk 

 rarely occurs in ordinary practice. Sterilization becomes very 

 difficult in the common case of milk which has been contaminated, 

 through dirty and careless handling, with very resistant spores, 

 such as some bacteria belonging to the species of butyric acid, and 

 hay and potato bacilli (for example, bacillus mesentericus, liodermus, 

 butyricus, and subtilis). 



From what has been already said, it will be seen that milk is 

 sometimes easy and sometimes difficult to sterilize. Milk containing 

 lasting forms of the above-described nature may keep at ordinary 

 temperatures for about six months unchanged if previously heated 

 for 45 minutes to the temperature of boiling water; yet at a 

 temperature favourable to the development of bacteria it may 

 coagulate, often with considerable development of gases, after only 

 three or four days. Where coagulation ensues, this is never effected 

 by the formation of acid, but always by enzymes formed by 

 bacteria, which are of the nature of rennet. It is in the highest 

 degree improbable, that lasting spores which have not been entirely 

 killed in milk treated according to Soxhlet's method and then 

 consumed should be able to germinate during the short, digestive 

 period and exercise a deleterious action, yet it is not absolutely 

 impossible. For this reason every effort should be made to effect 

 the perfect sterilization of milk. Temporary sterilization, which is 

 at present almost universally practised, would gradually become 

 improved and brought nearer to perfect sterilization if it were only 

 possible to obtain milk in ever-increasing quantities capable of 

 being easily sterilized. For this purpose nothing further is wanted 

 than cleanly handling of milk ; and thus avoiding its contamination 

 with such resistant spores of bacteria as above mentioned. How 

 simple this demand seems to be when stated, and yet how extraor- 

 dinarily difficult it is in practice to have proper attention paid to it! 



Hueppe recommends that all milk destined for the use of 



( M 175 ) G 



