46 



BACTERIA IN RELATION TO PLANT DISEASES. 



Milk. — Milk from a clean dairy and free or nearly free from cream should be selected 

 for use. If some cream remains it ma}' be filtered out or removed by the centrifuge 

 (fig. 43). The milk shoirld 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 observ^ation at least six or eight weeks. 



Observe in particular : 



( a ) 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. 



(b) Saponification of the fat. The fluid becomes 

 transparent without anyprecipitation 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 

 '01, Bibliog., XLVII). 



{d) 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. 



(e) Re-solution of precipitated casein (trypsin fer- 

 ment); formation of crystals (tyrosin, leucin, 

 etc. ). 



(/) Gelatinization of old cultures. Milk alkaline. 



(,<') 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 tern- 

 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 L,afar'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 



*FiG. 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. 



