1»161 DAIRY FARMING DAIRYING. 777 



Studies on the numbers of bacteria present in milk which has undergone 

 various changes, B. W. Hammeb and R. H. Hix (Iowa Sta. Research Bui. 

 29 (1916), pp. S5-62). — In the work reported an effort was made to secure 

 information regarding the numbers of bacteria required to produce various 

 changes in milli. While changes in the flavor and odor first attracted attention, 

 other changes were considered because of the difficulties presented by all work 

 dealing with changes in flavor and odor. Sterile milk was used for most of the 

 work, although some experiments were carried out with aseptic milk. 



The authors conclude that " from the data presented it appears that changes 

 in milk due to the growth of bacteria therein occur only after large numbers 

 of bacteria are present. The samples of milk which showed changes of one kind 

 or another always contained over 1,000,000 bacteria per cubic centimeter and 

 ordinarily much larger numbers. The sweet curdlers produced changes in milk 

 with the smallest numbers of organisms, and here the smallest number observed 

 with slight coagulation was 1,250,000 per cubic centimeter. With some organ- 

 isms pronounced changes required approximately 1,000.000,000 per cubic centi- 

 meter, and between this value and the minimum already mentioned wide 

 variations were encountered. 



" Wide variations apparently exist in the numbers of organisms present in 

 milk showing the same condition. This is evident from the percentage varia- 

 tion between the minimum and maximum and also by the results obtained when 

 freshly inoculated cultures were plated at two-hour intervals for considerable 

 periods. The difliculty of classifying the conditions observed in milk are, in 

 part, responsible for the variations obtained. 



" When Bacterium lactis acidi was inoculated into aseptic milk a distinct rise 

 in acidity was commonly detectable by the sense of taste before the milk could 

 be classed as sour. There seemed to be no definite relationship between the 

 rise in acidity and the classification of the milk as sour or as showing a 

 distinct rise in acidity. Acidity increases of 0.03, 0.04, or 0.05 per cent (in one 

 case 0.02) were detected by the sense of taste, and this means that quite low 

 acidities (acidities that would be regarded as normal) may be encountered 

 along with acid flavors in the milk." 



Some effects of temperature upon the growth and activity of bacteria in 

 milk, H. S. Reed and R. R. Reynolds (Virginia Sta. Tech. Bui. 10 (1916). 

 pp. S-26). — In this investigation upon the vitality of different species of bacteria 

 in milk at different temperatures the factors studied were (1) the numbers 

 of bacteria, (2) the proportion of acid-forming bacteria to others, (3) the 

 changes affecting the consistency of the milk, (4) the quantity of acid formed 

 in the milk, and (5) the reducing action as measured by the conversion of 

 methylene blue. In securing pure cultures fresh milk was obtained from the 

 college dairy, the cream separated, and samples of 100 cc. placed in Erlenmeyer 

 flasks. The samples were sterilized fractionally by heating to 95° C. for 15 

 minutes on each of three consecutive days; they were then incubated for 

 three days at 32°, and those which showed no signs of bacterial growth were 

 given another fractional sterilization extending over three days. The samples 

 were inoculated with a pure culture of the desired organism after the final 

 sterilization and incubated at four different temperatures for a six weeks' 

 period. During this time the multiplication of the organisms and their activity 

 were studied at temperatures ranging from that employed in commercial milk 

 storage to that of a warm summer day. 



Results in detail are tabulated for each of the 13 organisms studied, a sum- 

 mary of which, showing the effect of age and temperature on bacterial growth, 

 is given in the following table: 



