82 DAIRY BACTERIOLOGY 



The first visible Jfpr.t of iveating^is the gradual destruction^ of 

 the~cTearrr line, and as the public usually judge the richness of the 

 milk according to the thickness of the cream layer which separates 

 out, this matter is not without significance in practice. The cream 

 line is already affected by heating to 63 C. for some time 1 , and as 

 it happens that heating for half an hour at this low temperature 

 is critical for most milk bacteria 2 , especially the colon bacteria, 

 this method of treatment, which we will refer to as low temperature 

 pasteurisation (the American holder process), would seem to be the 

 best. Moreover, the bacteria which cause foot and mouth disease, 

 diphtheria, cholera, dysentery, typhoid and other intestinal 

 diseases, are killed just as surely by low temperature pasteurisation 

 as by short heating to 80 to 85 C. Different opinions have been 

 held with regard to tubercle bacteria, but according to Barthel and 

 Stemstrom's researches 3 there can no longer be any doubt that 

 these organisms will under normal conditions be killed even by 

 only heating to 60 C. for ten to twenty minutes. 



Whichever method of pasteurisation may be adopted, it is of 

 the greatest inapo^ajice to prevent the fora^tionjof s^in and 

 frptli> as these act as insula/tors, prevenETng the transference of 

 heat. This point was first demonstrated by Theobald Smith 4 . 

 Thus, wheji pasteurising at low temperajtur^Jr^ large^vessels, the 

 milk must be in constant though not violent motion. Another 

 condition is that the milk must not contain so much acid or rennet 

 that the heating will precipitate the casein which may thus come 

 to enclose and protect the bacteria. Pasteurisation can by no 

 means be trusted in implicitly to cover all defects, for the result 

 depends largely on the state of the raw milk. Fresh and cleanly 

 handled milk may be practically sterilised on pasteurisation, 

 though the milk flowing from the pasteurisers in some dairies 

 frequently contains several thousand bacteria per cubic centimetre, 

 and the subsequent passage over the coolers will, of course, not 

 improve matters in this respect. The expression of the efficiency 

 of pasteurisation as the percentage of bacteria killed on the 

 number originally present is somewhat misleading ; using the 

 processes described above, the percentage is usually over ninety- 

 nine with bad milk, but it often falls as low as ninety with good 

 milk, although the actual number of organisms is always many 

 times greater in pasteurised bad milk than in pasteurised good 

 milk. 



1 Weigmann, " Mitt, des Deutschen Milchwirtschaftliches Verieins," 1914, 

 Bd. 31 ; Burri, " Schweitzerische Milchzeitung," 1915, Nos. 42 and 43. 



2 Ayers and Johnson, " Journ. of Agric Res.," 1915, Vol. III., No. 5. 



3 " Meddelande," Nos. 117 and 118, fran Centralanstalten for forsok- 

 svasandet pa jordbruks omradet, 1915. 



4 " Journ. of Experimental Medicine," New York, 1899, vol. 4. 



