46 BACTERIA IN RELATION TO PLANT DISEASES. 
Milk.—Milk from aclean dairy and free or nearly free from cream should be selected 
for use. If some cream remains it may be filtered out or removed by the centrifuge 
(fig. 43). ‘The milk should not be acid to the taste and should not contain formal- 
dehyd or other antiseptic substances which milk-dealers sometimes add to dirty 
milk to improve its keeping qualities. It should be steamed in wire-crates 15 min- 
utes at 100° C. on each of four consecutive days (10 cc. portions in test-tubes), and 
should not be used until at least a week after the last steaming. Such milk should 
titrate + 12 to+ 17 or thereabouts with sodium hydrate and phenolphthalein. Milk- 
cultures should be kept under observation at least six or eight weeks. 
Observe in particular: 
(@) Separation of the casein without the develop- 
ment of any acid, indicating the presence of 
the lab, or rennet, ferment. The milk usually 
becomes more alkaline. 
(4) Saponification of the fat. The fluid becomes 
transparent without any precipitation of casein; 
but the caseinogen may be thrown down sub- 
sequently by acidifying the clear liquid. 
(c) Ropiness, The fluid becomes viscid, and strings 
when touched. This viscidity is sometimes so 
great that an entire pail of milk may be in- 
verted without immediate loss of its contents. 
See striking figures in Ward's papers (’99 and 
‘or, Bibliog., XLVII). 
(ad) Formation of acids. This occurs with or with- 
out evolution of gas, and usually with the final 
separation of the whey from the casein at room 
temperatures or on boiling. Boil if necessary. 
(€) Re-solution of precipitated casein (trypsin fer- 
ment); formation of crystals (tyrosin, leucin, 
etc. ). 
(/) Gelatinization of old cultures. Milk alkaline. 
(g) Changes in smell, color, and taste. 
In using milk it should not be for- 
gotten that anaerobes are sometimes pres- 
ent (Theobald Smith) and also organisms 
of the dunghill which will grow only at 
temperatures above 40°C. Very resistant 
spores of aerobic species, growing at tem- 
Fig. 42.* peratures below 40° C., are present also 
sometimes, especially in dirty milk, and the milk is then difficult to sterilize. 
Several experiments made by the writer with milk from Washington dairies 
have shown that Franz Lafar’s statement in Technische Mykologie, Bd. I, p. 189, 
while probably true for.the milks which he tested, is not true when stated as a 
general proposition. In brief, this statement is that nine out of ten milks are not 
*Fic. 42.—Section of Arnold steam sterilizer. Water enters the double bottom through a few 
small openings indicated by two arrows in the water-pan. The other arrows show movement of 
the steam. In this form the outer jacket (of copper) is lifted off to put in or remove media. 
