PREPARATION OF LITMUS MILK 25 



The pepsin- and trypsin-like enzymes produced by micro- 

 organisms cannot be thus separated by their activity in a 

 medium of certain reaction; this varies with the species of 

 microorganism and with environmental conditions. Pep- 

 tonization of milk usually takes place in a neutral, slightly 

 alkaline, or more infrequently slightly acid reaction. 



Some organisms peptonize milk without forming a 

 rennet curd. 



(g) Gas Production. This is characterized by the for- 

 mation of gas bubbles in the milk, and is generally accom- 

 panied by the formation of acid curd. Very commonly the 

 curd shrinks, causing extrusion of whey. 



EXERCISE 4. PREPARATION OF LITMUS MILK 



Apparatus. Fresh separated or skimmed milk; titra- 

 tion apparatus; N/20 NaOH; phenolphthalein (indicator); 

 5 c.c. pipette; azolitmin, 2% solution; filling funnel; 

 pinch cock; sterile test tubes; apparatus for steam sterili- 

 zation. 



Method. 1. Fresh separated or skimmed milk should 

 be used. Whole milk is undesirable on account of its fat 

 content. 



2. Titrate and record the reaction of the milk. If the 

 milk titrates above 17 acid, the reaction must be adjusted 

 to +15. Sour, curdled or uncurdled milk, after neutrali- 

 zation, does not make a desirable nutrient medium for 

 microorganisms, therefore, milk whose titre is above 20-25 

 acid should be discarded. 



Fresh milk varies in acidity from 12 to 18. Milk 

 with an acidity above 18 to phenolphthalein will not give 

 a satisfactory blue color with azolitmin, as at 18 it begins 

 to show the acid coloration. 



3. Add 2% of a standard solution of Kahlbaum's azo- 

 litmin. Litmus or azolitmin is added merely as an indi- 

 cator and should be of sufficient strength so as not to dilute 

 the milk to any extent. 



