384 Bacteria in Relation to Country Lije 



much longer by the simple precaution of cooling it 

 quickly and keeping it at a temperature below 50°. 

 Milk has been kept for two weeks and over without 

 curdling and without becoming noticeably injured by 

 the action of bacteria. If a temperature lower than 

 this — in the vicinity of 40°, or below — be maintained, the 

 milk can be kept for an even longer period. The value 

 of these facts to dairymen is clear. They show that, in 

 the keeping of milk, the maintenance of a low tempera- 

 ture is a factor of more significance than any other 

 factor connected with dairying." 



Different species of bacteria may become prominent 

 in milk at different temperatures. At 95°, B. lactis 

 aerogenes develops more vigorously than any of the 

 other lactic-acid bacteria. At 70°, the lactic-acid organ- 

 ism that is most prominent is B. lactis acidi, an organism 

 that, unlike the other, causes the gradual souring of 

 milk, without the formation of gas and without the 

 production of unpleasant tastes. At 50°, neither of the 

 two lactic-acid organisms develops. Instead, there appear 

 various kinds of bacteria that produce undesirable 

 changes, some of them partaking of the nature of putre- 

 faction. At this temperature, therefore, the milk may 

 outwardly remain sweet and wholesome, and yet be 

 filled with millions of bacteria and the unwholesome or 

 even dangerous products of their activity. The cooling 

 of the milk at once, and the constant maintenance of 

 temperatures of 50° Fahj-., or less, will safeguard the 

 dairyman against loss, and will permit him to supply to 

 the consumer milk of superior keeping quality. 



