ai2 MILK AND MILK PRODUCTS 
is added, and two drops of a 2 per cent solution of paraphenylendiamin 
(solution should be renewed quite often), then the fluid is shaken. If 
the milk or cream becomes, at once, indigo blue or the whey violet or 
reddish brown, then this has not been heated, or, at all events, it has 
not been heated higher than 78° C.; if the milk becomes a light 
bluish gray immediately or in the course of half a minute, then it 
has been heated to 79° to 80°C. If the color remains white, the 
milk has been heated at least to 80° C. In the examination of sour 
milk or sour buttermilk, lime water must be added as the color 
reaction is not shown in acid solution. (Report of the Commission 
on Milk Standards.) 
Arnold’s Guaiac Method. A little milk is poured into a test tube 
and a little tincture of guaiac is added, drop by drop. If the milk has 
not been heated to 80° C. (176° F.) a blue zone is formed between the 
two fluids; heated milk gives no reaction, but remains white. The 
guaiac tincture should not be used perfectly fresh, but should have 
stood a few days and its potency have been determined. Thereafter it 
can be used indefinitely. These tests for heated milk are only active in 
the case of milks which have been heated to 176° F. or 80°C. (Jensen’s 
Milk Hygiene, Pearson’s translation, p. 192.) 
Microscopic Test for Heated (Pasteurized) Milk. Frost and 
Ravenel (1911). About 15 c.c. of milk are centrifuged for five minutes, 
or long enough to throw down the leucocytes. The cream layer is then 
completely removed with absorbent cotton and the milk drawn off with 
a pipette, or a fine-pointed tube attached to a Chapman air pump. 
Only about 2 mm. of milk are left above the sediment which is in the 
bottom of the sedimentation tube. 
The stain, which is an aqueous solution of safranin 0, soluble in water, 
is then added very slowly from an opsonizing pipette. The important 
thing is to mix stain and milk so slowly that clotting does not take place. 
The stain is added until a deep opaque rose color is obtained. After 
standing three minutes, by means of the opsonizing pipette, which has 
been washed out in hot water, the stained sediment is then transferred 
to slides. A small drop is placed at the end of each of several slides and 
spread by means of a glass spreader, as in Wright’s method for opsonic 
index determinations. 
In an unheated milk the polymorphonuclear leucocytes have their 
protoplasm slightly tinged or are unstained. 
In heated milk the polymorphonuclear leucocytes have their nuclei 
stained. In milk heated to 63° C. or above, practically all of the leu- 
cocytes have their nuclei definitely stained. When milk is heated at a 
