Fundamental Food Microbiology, Third Edition - Fuad Fathir

290 FUNDAMENTAL FOOD MICROBIOLOGY
I. MICROORGANISMS THAT GROW IN REFRIGERATED FOODS
(PSYCHROTROPHS)
In food microbiology, the terms psychrophiles and psychrotrophs are used to identify
those microorganisms that can grow in food stored at low temperatures, namely,
chilling and refrigeration, that can range between –1�C and +7�C. A present-day
domestic refrigerator is expected to maintain a temperature of ca. 40�F (4.4�C),
whereas commercial refrigeration can be lower or higher than this, depending on
the particular food stored and shelf life expected. Highly perishable foods are either
refrigerated or chilled on ice. Thus, psychrotrophs can multiply in these foods,
provided other conditions for growth are not restricting. 1,2
There is some confusion about the definitions of the two terms, particularly
in relation to their importance in food microbiology. The term psychrophile or
psychrophilic is quite specific and includes those microorganisms that grow optimally
at ca. 12 to 15�C and have a growth temperature range between £ –5�C and
22�C. These microorganisms can grow in refrigerated and chilled foods. The
definition of psychrotrophs is, however, not clear-cut. Originally, in 1960, this
term was introduced to include those microorganisms that grow at 0 to 5�C,
irrespective of their optimum or the range of growth temperatures. Rather, they
seem to grow best at 25 to 30�C and might not grow above 35�C. Thus, they
appear to be a subgroup of mesophiles (growth temperature of mesophiles: optimum
30 to 40�C and range 5 to 45�C), but not a subgroup of psychrophiles. This
group includes Gram-positive and Gram-negative aerobic, anaerobic, and facultative
anaerobic, motile and nonmotile, sporeformers and nonsporeformers, coccus
and rod-shaped bacteria, as well as many yeasts and molds. Later in 1976, the
International Dairy Foundation defined psychrotrophs as those microorganisms
that grow at 7�C, irrespective of their optimal and range of growth temperatures.
However, this does not indicate whether it includes only the subgroup of mesophiles
with the ability to grow at or below 7�C, or both psychrophiles and the
subgroup of mesophiles that grow at the lower temperature range. More recently,
the term psychrotrophs includes mesophilic subgroups that can grow at 40�F or
below (£ 4.4�C, refrigerated temperature). 1,2
Several studies have revealed that there are some mesophilic pathogens, (e.g.,
Yersinia enterocolitica and Listeria monocytogenes) and spoilage bacteria (e.g.,
Leuconostoc spp., several Lactobacillus spp., and Serratia spp.), that can grow in
vacuum- and modified-air (MA) packaged foods at 0 to 1�C. Spoilage of vacuumpackaged
meats by the psychrophilic Clostridium spp. with a growth range between
–2 and 20�C was also reported. It is important to have understandable communication
among the people associated with food production, regulations, sanitation, academic
and research activities, and others without creating confusion by using the designations
psychrotrophic spoilage for the mesophilic subgroup and psychrophilic spoilage
for the second group. For easier understanding, it is probably not unscientific
to use the term psychrotrophs for the microorganism that can grow (in food) at
refrigerated or chilled storage temperature (£ 40�F or 4.4�C). This includes both the
mesophile subgroup that can grow at lower temperature and psychrophiles. As both
groups are important in spoilage and foodborne diseases, and the methods used to