442 BACTERIOLOGICAL ANALYSES 



(a) fore-milk, (b) mid-milk, (c) strappings, and, if possi- 

 ble, from each quarter of the udder. " Mixed " milk, on 

 the other hand, by the time it leaves the retailer's 

 hands, usually contains as many micro-organisms as 

 an equal volume of sewage and indeed during the ex- 

 amination it is treated as such. 



It is possible however to collect and store mixed 

 milk in so cleanly a manner that its germ content does 

 not exceed 5000 microorganisms per cubic centimetre. 

 Such comparative freedom from extraneous bacteria is 

 usually secured by the purveyor only when he resorts 

 to the process of pasteurisation (heating the milk to 

 65 C. for twenty minutes or to 77 C. for one minute) 

 or the simpler plan of adding preservatives to the 

 milk. Information regarding the employment of these 

 methods for the destruction of bacteria should always 

 be sought in the case of mixed milk samples, and in 

 this connection the following tests will be found useful : 



1. Raw Milk (Saul). 



To 10 c.c. milk in a test tube, add i c.c. of a i per cent, aqueous 

 solution of ortol (ortho-methyl-amino-phenol sulphate), recently 

 prepaied and mix. Next add 0.2 c.c. of a 3 per cent, peroxide of 

 hydrogen solution. The appearance of a brick red color within 

 30 seconds indicates raw milk. Milk heated to 74 C. for thirty 

 minutes undergoes no alteration in color; if heated to 75 C. for ten 

 minutes only, the brick red color appears after standing for about 

 two minutes. 



2. Boric Acid. 



Evaporate to dryness, 50 c.c. of the milk which has been 

 rendered slightly alkaline to litmus, then incinerate. 



Dissolve in distilled water, add slight excess of dilute hydrochloric 

 acid and again evaporate to dryness. 



Dissolve the residue in a small quantity of hot water and moisten 

 a piece of turmeric paper with the solution. Dry the turmeric 

 paper. Rose or cherry-red color = borax or boric acid. 



3. Formaldehyde (Hehner). 



To 10 c.c. milk in a test tube add 5 c.c. concentrated commercial 

 sulphuric acid slowly, so that the two fluids do not mix. Hold the 

 tube vertically and agitate very gently. Violet zone at the junc- 

 tion of the two liquids = formaldehyde. 



4. Hydrogen Peroxide. 



To 10 c.c. milk (diluted with equal quantities of water) in a test 



