CHAPTER XXXIV 

 CONTAMINATION AND DETERIORATION OF FOOD 



CHARLES THOM 



Microbiological Laboratory, Bureau of Chemistry, U.S. Department of Agriculture 



THE MICROBIC CONTENT OF FOODS 



The ubiquity of bacteria has already been emphasized by others. The surfaces 

 of raw fruits and vegetables as ordinarily handled are in constant contact with cur- 

 rents of air-carrying dust particles, visited by insects and brushed by man and many 

 other animals. Many products are moist from transpired water, often more or less 

 sticky or mucilaginous from the osmotic products deposited by the evaporating juices 

 or exudates. They thus present a lodging-place for bacteria, yeasts, and molds, and 

 hence not infrecjuently support a large population of micro-organisms of many species 

 measurably influenced in variety by the nature of the exudates. 



Some conception of the numbers present is furnished by studies in the Bureau of 

 Chemistry in which a colony count was found to average some 250,000 per gram in 

 spinach and about 135,000 per kernel within the husk and in contact with the indi- 

 vidual kernels of corn. 



These numbers can be reduced by thorough washing under strictly controlled con- 

 ditions, but many investigations of routine washing practices show many of them to 

 add to, rather than to reduce, the bacterial content of food. Further, under even good 

 conditions, after washing, contaminated surfaces are wet enough to accelerate the 

 multiplication of the bacteria present. 



The findings from fruits and vegetables are repeated when we turn to flesh foods. 

 The healthy flesh of the living animal is believed to be almost free from contamina- 

 tion, but invasion by bacteria from the alimentary canal follows quickly after the 

 animal is killed, while handling processes are notoriously careless from a bacteriologi- 

 cal standpoint. 



We know comparatively little thus far of the extent of the invasion of tissue areas, 

 for example, the intercellular spaces of plants, by bacteria without the production of 

 evident pathological conditions. Enough has been done to indicate that such invasion 

 through the stomata does occur in many plants, but the extent and nature of the 

 species invading remain largely untested. Similarly with animal foods, entire absence 

 of bacteria is doubtful even in living tissues of apparently healthy food animals, while 

 positive invasion or contamination at time of slaughter is almost inescapable. 



The student of food problems is thus faced with the fact of contamination with 

 micro-organisms in dealing with raw foods. Inquiry as to the relation of this popula- 

 tion to fitness of products for human consumption takes several forms: microbiology 

 of freshness and soundness in the foods themselves, the changes of flora involved in 

 the handling processes encountered in current practices of the gathering and market- 



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