(1) Nonsporeforming microorganisms killed at relatively low temperatures.
This includes nonsporeforming bacteria, yeasts, and molds.
(2) Genus Bacillus--forms spores but is not as heat resistant as Clostridium
organisms. Bacillus organisms must be killed by heat, because they have a high salt
(3) Clostridium organisms--have a high resistance to heat and must be
either killed by heat or held in a dormant spore state by salt, acidity, or heat injury.
(4) Thermoduric organisms--are not ordinarily killed by canning
temperatures, but are held dormant by 5 percent brine concentration.
Each canned meat product is a law unto itself, and it must be processed
according to the cumulative effects of heat, salt, and pH on the
microorganisms normally present. The combined effects of heat and brine
concentration on Clostridium spores are the basis of the canned cured meat
industry in the United States.
c. Swells in Canned Meats. Canned meats are subject to spoilage, which is
classified as biological swells and chemical swells. Biological swells are caused by the
production of carbon dioxide by bacteria. Chemical swells are not common in canned
meats. This is because meats are not highly acid, and chemical swells are caused by
the action of acid on the iron in the can, with the resulting production of hydrogen gas.
3-21. FREEZE DEHYDRATION
Freeze-dehydrated foods are expensive items, but they are easy to store and will
keep in storage indefinitely. They are being used more extensively by the military at the
present time. By way of introducing the discussion of freeze dehydration, we say that
the three states of matter are solid, liquid, and gas. When ice is heated, it changes to
water, and then to vapor, or from a solid to a liquid to a gas. When a solid changes to a
gaseous state without passing through the liquid state, we call it sublimation. A good
example of sublimation at atmospheric pressure is dry ice changing to the gaseous
state. Sublimation is the basic physical process underlying freeze dehydration. Ice
formed in the frozen food product is converted under vacuum to vapor and removed
from the food without passing through the liquid state. Atmospheric pressure at sea
level is 760 mm of mercury per inch. Sublimating ice in foods to vapor without the
intervening liquid phase requires reducing atmospheric pressure to below 4.6 mm of
mercury per inch and applying heat. Vacuum is required to reduce the atmospheric
pressure in this case.
a. Removal of Moisture. In the application of the process to freeze dehydration
of foods, food is frozen in thin layers so that a greater surface area is exposed. The
frozen food is placed in the vacuum chamber of a sublimator where a vacuum is drawn
and pressure is lowered to 1 mm of mercury per inch or lower. Heat is applied, ice