EXPERIMENT STATION BULLETINS. 249 



organisms. Thus it is not difficult to understand why a starter pro 

 duced by natural souring may develop taint or become gassy. 



The Commercial Starter. — The commercial starter is generally de- 

 veloped from a single micro-organism and is built up as a pure cul- 

 ture or a known mixed culture. This class includes those starters orig- 

 inated and offered for sale in solid or liquid form by various com- 

 mercial firms. Though the different brands differ more or less as 

 to activity at a given temperature as well as in the flavor imparted 

 to butter or cheese, yet from the very fact that these are pure cul- 

 tures, uniform growth and acid production may be expected. This 

 being the case, a commercial starter is kept free from contaminations 

 and, developed under the same conditions, may be used for an in-, 

 definite time and produce an unvarying product. 



Comparison of the Natural with the Commercial. — It is essentially 

 true that a pure culture of micro-organisms will hold its quality much 

 longer than one having several species present; it follows that a com- 

 mercial starter will give more constant results than the natural. 



Sterilization. — This term means the destruction of micro-organisms 

 present. Under this head comes sterilization by the use of heat, chem- 

 icals, filters, and other means. 



Heat is the agent most commonly used for this purpose. It may 

 be employed to destroy all micro-organisms present (sterilization) or 

 only a part (pasteurization). There are several purposes which we 

 may desire to accomplish by the use of heat. 1. To destroy all micro- 

 organisms present, thus leaving a clear field for the action of any 

 particular species added as in the case of pure culture starters. 2. To 

 destroy those micro-organisms which cause souring and like fermentative 

 changes. 



The former usually renders the milk unpalatable for use as food, 

 because of the cooked flavor; consequently, in the case of cream for 

 butter and milk for cheese, pasteurization is employed. 



This treatment consists in heating the milk to a temperature of 145^ 

 to 185° F, and then rapidly cooling to below 100° F. Sush pasteuriza- 

 tion generally kills from 95% to 99% of all the bacteria present. 

 Though the remainder constitute a small percentage of those originally 

 present, they are vigorous growers, and with every transfer of the 

 starter their numbers are increased. If a sufficiently high temperature 

 is employed all the bacteria are destroyed at once. But since some 

 of the common milk bacteria have a spore or seed stage, which stage 

 is very resistant to heat, a discontinuous method is used whereby heat 

 is applied for thirty minutes each day to pint bottles and forty-five 

 minutes each day to quart bottles for four consecutive days. The 

 milk in, the meantime being kept in a warm room to hasten the de- 

 velopment into a form more easily killed by heat. If the latter method 

 is employed we get a perfectly sterile condition. A pure culture added 

 to this and allowed to sour gives a pure culture of desirable bacteria 

 which will give uniform results for an indefinite period if properly 

 handled. 



This being the present condition, it seems that a method of handling 

 a starter which insures a permanent quality, and at the same time 

 which lessens the actual expense for starters will be of interest to 

 many. It is due to this fact that an attempt has been made in the 



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