EXPERIMENT STATION BULLETINS. 481 



going inito the mix and then follows the mix through eac^h snecessive 

 operation as it goes tlirongh the plant and 'as it comeisi ooit a finished pro- 

 duct, S'amples l)'eing taken before and after each openation and plated 

 immediately to determine tlie influence of each particular operation on 

 the bacterial content of the mix. In this way it was lioped to throw some 

 light on the influence of each of the various operations on the number of 

 bacteria as determined by the plate metbod. 



METHODS 



All samples were collected with sterile pipettes and plated immedi- 

 ately. Samples of the mix up to the time it was frozen were easily col- 

 lected and plated. After the mix was frozen a slightly different proced- 

 ure was followed. The method used by Ayers and Jobnson (9) with 

 slight modifications was used for obtaining tbe number of bacteria in the 

 ice cream after it had been frozen and placed in cans. The top of the 

 can was rinsed with water, the cover removed and the top inch or so of 

 ice cream taken off with a stei'ile spoon. A sterile butter trier was then 

 pmished to the bottom of the can and the siolid 'vOire placed in a sterile 

 flask. Three cores from each ican were taken in this way from different 

 parts of the can and placed in the flask. The Suiinples of ice 'cream cqI- 

 lected f ix>m the freezers were caught in sterile wide-moimhed bottles as at 

 flowed from tihe freezer. A gallon or so wais always allowed to ran out 

 before the sample was collected. 



The samples were then placed in a water bath at 40° C (104° F) for 

 15 minutes or until melted. After melting tliey were islhaken 30 times to 

 get rid of the air whidh is very abundant in frozen ice cream. In all cases 

 snitable dilutions were made uising 90 cc. tand 99 cc. respectively, of sterile 

 plhysiolo'gical salt isolution. The dilution flasks were shaken 25 times and 

 1 cc. was plated from each dilution desired. 



The agar used in plating wais Standard nutrient agar mlade according 

 to the method recommended by the American Public Health Association 

 ( 10) . The pipettes used were standard baicteriological pipettes graduated 

 to deliver a calibrated amount between two miarks, thus avoiding any 

 error due to blowing. All calculations are based upon cubic centimeters 

 of ice cream and not on grams except for gelatin. In the case of gelatin, 

 one gram was weig^hed on a sterile filter paper and the contents poured 

 into 99 c. c. of sterile physiological salt solution. Suitable dilutioris 

 were nuade from this and bacteria calculated per gram. All plates were 

 made in duplicate and incubated at 37° C for 48 hours. The cou'nts rep- 

 resent an average of the two plates containing from 30 to 300 colonies or 

 those coming the nearest to 300. Determinations were made only on 

 vanilla ice cream. 



THE RAW PRODUCTS ',' 



The raw products that went into the mix were analyzed for the num- 

 ber of bacteria per c. c. or gram in the case of gelaitin. 



The data are presented in Table 1. * 



16 ' ' 



