398 BACTERIA IN DAIRY PRODUCTS 



physical properties of milk powder are evidently not due to bacterial action but to 

 oxidation and other chemical reactions. 



ICE CREAM 



The cream, sugar, condensed milk, gelatin, and other ingredients used in ice 

 cream contain bacteria, sometimes in large numbers. During the manufacturing proc- 

 ess the total number may increase and, unless measures are taken to reduce the bac- 

 terial content, the frozen product is likely to contain excessive numbers of bacteria. 

 These may be millions per gram. A high bacterial count carries with it a possibility 

 of infection with pathogenic bacteria. Fabian' has compiled records of thirty-five 

 epidemics due to ice cream including typhoid, scarlet fever, and diphtheria. In re- 

 cent years the manufacturing processes and the sanitary conditions of the factories 

 have been much improved. Pasteurization of the mix is now a common practice, 

 and the better factories produce an ice cream consistently containing less than 

 100,000 bacteria per gram.^ In the freezing there is an apparent increase in numbers 

 due to the violent agitation of the freezer, but in the hardening-room where the prod- 

 uct is held at 0° to -10° F. there is a slow decrease in numbers.^ 



BUTTER 



In the customary process of butter-making the cream is soured or "ripened" by 

 inoculation with lactic bacteria to develop acidity and flavor. In the better factories 

 the cream is pasteurized and the cultures are propagated with great care. St. lactis 

 is used for this purpose, but a strictly pure culture forming lactic acid almost exclu- 

 sively, with only a small proportion of volatile acid, does not give the most satis- 

 factory flavor to the butter. It is now known that the best "starters" are not pure 

 strains but contain, in addition to St. lactis, one or more streptococci which have 

 little effect in the souring of the milk but, through the formation of acetic acid and 

 other volatile products, increase the aroma of the starter and especially that of the 

 butter.4 



Butter is susceptible to many influences which not only affect its flavor and aroma 

 when fresh from the churn but which also cause it to take on various off-flavors that 

 seriously affect its market value. The natural flavor of butter is due partly to the 

 natural flavor of the cream and partly to volatile compounds produced by bacteria 

 in the milk and especially in the ripening of the cream. Up to a certain point these 

 volatile substances improve the flavor; beyond this they cause a deterioration. The 

 off-flavor may be slight and undefined or it may be pronounced and definite. Thus 

 butter may acquire an oily flavor of varying intensity or it may be metallic and fishy. 



' Fabian, F. W.: Am. J. Pitb. Health, 16, 873. 1926. 



^ Fay, A. C, and Olson, N. E.: J. Dairy Sc, 7, 330. 1924; Fabian, F. W.: Michigan Agric. 

 Expcr. Sta. Spec. Bull. 158. 1926. 



3 Ellenberger, H. B.: Cornell Agric. Expcr. Sta. Mem. 18. 1919. 



" Boekhout, F. W. J.: Vet. Exploit. Procfziiivclbocrderij Hoorti, Vcrslag, iQiy, p. 48. 1918; Storch 

 v.: Beret. Forsgslah. K. Vet. og Landbohjskoles {Denmark), p. 102. 1919; Hammer, B. W., and Bailey, 

 D. E.: Iowa Agric. Exper. Sta. Research Bull. 55, 1919; Hammer, B. W.: ibid., Bull. dj. 1920; 

 ibid.. Bull. 65. ig2o; ibid., Bull. 67. 1921; Ayers, S. H., and Mudge, C. S.: J. Dairy Sc. ,4, 240. 1921. 



