(3) Psychrophiles. Psychrophilic bacteria have a temperature range from
32 to 50F (0 to 10C) with some species multiplying at temperatures as low as 23F (-
5C) in solutions that do not freeze solid (brine and sugar solutions). Organisms that
grow well at 32 to 41F (0 to 5C) are often encountered as causes of spoilage in
refrigerated foods, but their rate of growth at such temperatures is usually slow and
spoilage is not apparent unless storage is greatly prolonged.
c. Narrow Growth Range. The term microphile is used to designate bacteria
that have a narrow range of temperature for growth. By that, we mean the maximum
and minimum temperatures are relatively close together. Most microphiles are
mesophilic, with a temperature range between 86 to 104F (30 to 40C).
d. Characteristics. Generally speaking, the optimum temperature is much
closer to the maximum temperature than to the minimum temperature. When bacteria
are submitted to temperatures a little above the maximum or a little below the minimum,
they are not necessarily killed but may enter a relatively dormant state.
1-22. EFFECT OF HEAT
a. Mesophilic Bacteria. Most mesophilic bacteria are killed if heated in a liquid
medium to 122 to 149F (50 to 65C) for a few minutes, say 10 minutes. Some
bacteria (those classified in the genera Bacillus and Clostridium) may form spores,
which are highly resistant to boiling. Boiling for minutes to hours does not destroy these
bacteria. The spores of some species can withstand temperatures above the boiling
point of water for a half hour or longer.
b. Thermophilic Bacteria. Thermophilic bacteria, which do not form spores
and which are capable of withstanding temperatures of 140 to 158F (60 to 70C) for
the time required to pasteurize milk, are called thermoduric. These cause considerable
trouble in the milk-processing industry.
c. Resistance Factors. In determining the heat resistance of bacteria, the
following factors are considered jointly:
The length of time during which the bacteria are exposed to the heat.
Whether the bacteria are heated in a moist or a dry condition.
The pH concentration of the medium in which the bacteria are heated.
(5) The other characteristics of the medium. For example, bacteria are
killed at a lower temperature in water than in cream.