Apr. 24. i9i6 Effect of Pasteurization on Mold Spores 165 



Mold spores in milk seem, therefore, to be destroyed completely or 

 reduced to negligible numbers by both of the standard pasteurization 

 processes. 



Careful study of the cultures showed that the first efifect of heating was 

 to delay germination. This is indicated in the tables by the reports of 

 successive examinations of the same culture. In Table I three reports are 

 given; later only two reports. The third and fourth observations, how- 

 ever, were usually made. At times heating to a degree just under the 

 death point delayed germination almost the full length of the usual 

 growing period of the species. The number of possible sources of error 

 was so great that the results of observations have been tabulated and 

 compared. When essential harmony of results was not obtained, the 

 work was repeated. In a few cases the continued lack of consistent 

 results for particular organisms is indicated by the interrogation point in 

 the tables. Even with these precautions the data obtained can be said 

 to apply only to the strains used. This is indicated by comparing the 

 results given for the Aspergillus flavus group or for the four members of 

 the A. niger group. These results do not prove that other strains of 

 these groups would respond exactly as here tabulated. In fact, more 

 extended studies (as yet unpublished) of these two groups indicate that 

 organisms otherwise undistinguishable may differ greatly if we measure a 

 single physiological reaction. Such quantitative differences may persist in 

 continued cultures,, but are hardly comparable to differences in the kind 

 of reaction as a basis for separating species. Inside the race or strain, 

 conidia transferred from the same culture respond very differently. 

 There is frequently a survival of a few spores where a majority of the 

 spores die. There may be, therefore, a difference of as much as 20° F. 

 (11. 1 C.) between the temperature at which an occasional culture is 

 completely killed and that at which cultures of that species are uniformly 

 killed. These results resemble those obtained in determining the thermal 

 death point of bacteria. 



The applicability of these results to the occurrence of mold spores in 

 substances other than milk has not been tested. The variation in com- 

 position of the substratum together with the heating may at times 

 introduce a considerable variation. In general, however, it is clear that 

 mold spores are easily killed by heat when suspended in fluid. The 

 tables have been studied in an attempt to correlate resistance with size 

 of spore or thickness of spore wall. No such correlation has been found. 

 There is, therefore, no suggestion as to the nature of the difference in 

 these organisms which affects their resistance to heat. 



SUMMARY 



(i) The holder process of pasteurization, in which milk was heated to 

 145° F. (62.8° C.) and maintained at that temperature for 30 minutes, 

 killed the conidia of every species investigated, except those of Asper- 



