1-23. EFFECT OF COLD
Low temperatures do not necessarily kill bacteria. Multiplication ceases below
the minimum growth temperature but the organisms may remain viable in the dormant
condition for long periods. Growth and multiplication can resume when the temperature
is raised to the normal range. The resistance of bacteria to low temperature provides a
means of preserving many species for long periods without the necessity of frequent
a. Bacteria Survival. The survival of bacteria at low temperatures is important
in the food industry. However, survival of bacteria in frozen foods can lead to trouble.
Many years ago when ice was used for refrigeration on a wide scale, several outbreaks
of typhoid fever were reported that apparently were caused by bacteria in natural ice
used several months after the ice was harvested. Laboratory tests later showed that
typhoid bacteria could live in ice as long as 22 weeks. Other intestinal pathogens
possess similar survival power.
b. Implications for Frozen Foods. The tremendous expansion of the frozen
food industry emphasizes the importance of bacterial resistance to low temperature. As
we have stated previously, freezing is not necessarily a means of destroying pathogens
in food. Freezing often destroys vegetative cells and frozen foods usually spoil more
rapidly after thawing than in the original unfrozen food. Freezing is believed to damage
the food tissues so that nutrient materials are released upon thawing to promote very
active multiplication of surviving spoilage bacteria. Therefore, it is suggested that
thawed frozen foods be used immediately or else discarded.
Most bacteria are naturally aquatic creatures and therefore, are seldom harmed
by an excess of water. Water is the vehicle by which bacteria, yeasts, and molds
secure food and eliminate waste products. Most bacteria and yeasts prefer media of
very high water content. Molds require much less water. Dehydration restricts the
metabolic activities of bacteria and may lead to death, especially at room temperature
(or above) and in the presence of oxygen. As you might expect, spores are much more
resistant than vegetative cells. Generally speaking, bacteria can live only a short time
without water. In conditions of less than 15 percent moisture, most bacteria will die and
the growth of others will be inhibited. Dehydration is a means of preserving foods.
a. Ultraviolet Light. The rate of killing of microorganisms by radiation is
dependent upon the intensity of the source, the distance from the source, the time of
exposure, and the amount of shielding material. Some viruses and the spores of
bacteria and molds are more resistant to ultra-violet light than are vegetative cells.