1920.] Wright.—Bacteriology in relation to Meat Products. 209 
BACTERIOLOGY IN RELATION TO COMMERCIAL 
MEAT PRODUCTS.* 
By A. M. Wright, A.I.C. 
Sterilization by Heat. 
The preservation of foods by sterilization in hermetically sealed containers 
was suggested over a hundred years ago. For nearly fifty years the process 
was chiefly confined to the home, and it was only after the middle of last 
century that commercial canning passed the experimental stage. 
At the outset let it be understood that there is a distinction between 
pasteurization, in which the temperature is raised sufficiently high to kill 
vegetative bacterial cells, but not spores, and sterilization, in which the 
temperature is considerably higher—high enough to ensure the destruction 
of all bacterial cells, including spores. 
Bacteriologically there is a thermal death-point for the vegetative forms, 
and this is defined as “ that temperature which certainly kills a watery 
suspension of the organisms in question after an exposure of ten minutes.” 
This has been defined further for spores as “that time of exposure to a fixed 
temperature (moist) of 100° C. necessary to effect the death of all spores 
present in a suspension/’ The thermal death-point can be determined 
with a fair degree of accuracy for each species, and is much higher for 
spores than for the vegetative forms. 
Heat may be applied either in the absence or in the presence of moisture, 
known respectively as “dry heat” or “moist heat.” Dry heat is much 
less effective than moist heat, the death of the protoplasm taking place 
more readily in the presence of moisture. In practical sterilization moist 
heat is therefore used where possible, especially where penetration, as in 
the case of meats, is required. Steam may be applied under pressure, 
when a fifteen-minute exposure at 127° C. suffices to destroy all known 
organi ms, the thermal death-point being lower under more prolonged 
exposure It has also been demonstrated that for most vegetative forms 
a temperature of 66° C. will kill with moisture present, while an exposure 
of an hour and a half at from 120° to 130° C. is necessary to attain the 
same result with dry heat. 
Instead of performing sterilization in one operation, “ discontinuous ” or 
“ fractional ” sterilization may be employed. For this the material is 
exposed to heat for a few minutes on two or three successive days. Any 
spores surviving the first heating germinate, and the resulting organisms 
succumb to the second and third sterilizations. As some spores take days 
or even weeks to germinate, the procedure is not always certain in its 
action, and it is possible for spores to germinate in a material believed 
to be sterile. 
Meat-canning. 
The successful canning of food, including meats, depends upon the 
destruction of all the bacteria present in the material, and the sealing of 
* Part of No. 4 of a series of lectures on “Applied Bacteriology,” delivered at 
Canterbury College, 3rd May, 1920. The author desires to express his thanks to the 
New Zealand Refrigerating Company (Limited), and to the Director-General, New Zealand 
Medical Services (N.Z. Expeditionary Force), for permission to publish the results of 
these investigations carried out in New Zealand ancl with the overseas Forces. 
14—Science. 
