266 UNSWEETENED CONDENSED MILK DEFECTS 



Blown Evaporated Milk (Gaseous Fermentation). 



General Description. The ends of the cans bulge out very 

 noticeably, frequently so much so that the seams of the cans 

 burst open. This is due to gaseous fermentation causing high 

 pressure in the cans. The pressure is of 'ten so great that upon 

 opening the cans, most of the contents are blown out with tre- 

 mendous force. In some cases of blown evaporated milk, the 

 contents have an acid odor, pleasant and aromatic. In most 

 instances, however, they give off very foul odors and suggesting 

 hydrogen sulfide, not unlike aggravated cases of Limburger 

 cheese. These -odors are exceedingly penetrating and difficult to 

 remove from anything they come in contact with. 



Causes and Prevention. The bacteria causing gaseous fer- 

 mentations in evaporated milk usually belong to the anaerobic 

 group of butyric acid species and in most cases, though not al- 

 ways, the putrefactive types prevail, such as Bacillus putrificus, 

 Plectridium novum and Plectridium foetidum, especially the lat- 

 ter, because of its extraordinary power of resistance to heat. 

 Plectridium foetidum is an obligatory anaerobe and it absolutely 

 refuses to grow under aerobic conditions. It is an actively motile, 

 medium-sized organism with flagella and spores. At one end it 

 has an Indian club-like enlargement, in which appears the spore. 

 The bacillus resembles a kettle-drum stick similar to B. tetani. 

 Under strictly anaerobic conditions, and incubated at 90 degrees 

 P., it ferments milk in four days. The milk first curdles, then 

 gradually the curd dissolves (digests) completely, leaving a clear 

 yellow liquid, similar in appearance to butter oil. The fermenta- 

 tion is accompanied by the evolution of a penetrating foul odor. 

 This organism survives exposure for 15 minutes to 245 degrees 

 F. Its thermal death point lies between 245 and 250 degrees P. 1 



Plectridium foetidum, as well as most of the other species of 

 anaerobic, spore-bearing butyric acid bacilli and bacteria, is 

 present abundantly in cultivated soil, in field crops and even on 

 the kernels of the grain. Since this type of evaporated milk 

 defect is characteristic, especially, of the product manufactured 

 during the late summer and early fall months, it is very probable 

 that the dust incident to the harvesting of the field crops, fur- 



1 Hunziker, A Study of Gaseous Fermentation in Evaporated Milk. 



