164 NINTH REPORT. 



aroma. These begin to develop with the ripening in the milk and continue 

 through the curing and aging process, after the manufacture of the milk i)ro- 

 duct. If the proper bacteria are present we get desirable results not found 

 in unripened or poorly ripened milk. Prominent among the substances 

 formed is lactic acid; but there are also many others, each having its char- 

 acteristic odor or flavor. As an outcome we get many results during the 

 ripening which materially influence the flavor and odor. 



The starter is used to overcome obnoxious micro-organisms and add to 

 the finished product the desired flavor, aroma, keeping quality and perhaps 

 other essential properties. 



There are two general classes of starters — The' Natural and the Artificial 

 or Commercial. 



Under the head of natural are included all those originated by the oj^er- 

 ator himself. These are generally selected by obtaining some milk that 

 has been loppered by the proper kind of lactic bacteria. It must have the 

 desired flavor and odor, also a good firm curd, free from gas bubbles and whey. 

 Skim milk, sour milk, buttermilk ahd whey are sometimes used in this capa- 

 city, though none of these are desirable, for the great amount of acid de- 

 veloped has killed off the better types of lactic bacteria, leaving only the 

 resistant gas and spore producing forms present. 



The commercial starter is a culture of bacteria that is sent out by various 

 firms in solid or liquid form. These are usually cultures of carefully selected 

 and tested bacteria, though not always, "however. While different kinds 

 differ as to activity at a given temperature it is generally a pure culture and 

 when properly handled and developed may be depended upon to give un- 

 varying results for an indefinite length of time. Under ordinary factory 

 conditions this is almost impossible simply because the ordinary factory 

 operator knows little or nothing about the nature of a starter, nor does he 

 have the proper apparatus for obtaining sterile conditions, or of keeping 

 them after he does obtain them. Soon his starter acquires a bitter flavor, 

 a gassy -curd, or a foul odor. These conditions are due to foreign bacteria 

 that have found their way into the starter and developed sufficiently to 

 overcome the lactic germs. Here we soon have similar conditions to those 

 found in the natural starter. 



The natural starter was the first starter used. Some soured milk was 

 saved from day to day in cold weather and added to the milk and cream to 

 hasten the ripening. This was not necessary in summer, because at that 

 temperature the ripening took place rapidly enough. However, a demand 

 on the part of the public for an acid butter led to its continued use. By an 

 acid butter, is meant, one having the peculiar flavor of lactic acid present in 

 an appreciable amount. If sweet cream is churned into butter we get al- 

 most the same flavors that are found in sweet cream; but if ripened cream 

 is churned into butter we get the flavor and odor of lactic acid and other 

 products of decomposition which are present in minute ciuj^itities. 



Workers experienced trouble from the start by undesirable micro-organ- 

 isms l)ecoming so numerous in the starter as to overcome its good results. 

 The culture became gassy, or developed too much acid in the desired length 

 of time, thereby killing the desirable lactic types. Such cultures as this 

 gave no end of trouble and other methods were devised. The use of heat 

 to control bacterial growth in other industries led to its use in the handling 

 of starters. Pasteurization was first resorted to with much better results. 

 This, however, was not entirely satisfactory because of the resistant spore 

 forms remaining, which in most cases exert a i^roteolytic action on the casein, 



