273
Chapter 21
new Food Spoilage Bacteria
in Refrigerated Foods
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,
which can range between –1°C and 7°C. A present-day domestic refrigerator is expected to maintain a temperature of approximately 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 terms “psychrophile” and “psychrophilic” are quite specific
and includes those microorganisms that grow optimally at approximately 12°C–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°C–5°C, irrespective of their optimum or the range of growth temperatures. Rather, they seem to grow best at
25°C–30°C and might not grow above 35°C. Thus, they appear to be a subgroup of mesophiles
(growth temperature of mesophiles: optimum 30°C–40°C and range 5°C–45°C) but not a subgroup of psychrophiles. This group includes Gram-positive and Gram-negative aerobic, anaerobic,
and facultative anaerobic, motile and nonmotile, spore formers and non–spore formers, and 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 optimum 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 temperatures). 1,2
Chapter 21
new Food Spoilage Bacteria
in Refrigerated Foods
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,
which can range between –1°C and 7°C. A present-day domestic refrigerator is expected to maintain a temperature of approximately 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 terms “psychrophile” and “psychrophilic” are quite specific
and includes those microorganisms that grow optimally at approximately 12°C–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°C–5°C, irrespective of their optimum or the range of growth temperatures. Rather, they seem to grow best at
25°C–30°C and might not grow above 35°C. Thus, they appear to be a subgroup of mesophiles
(growth temperature of mesophiles: optimum 30°C–40°C and range 5°C–45°C) but not a subgroup of psychrophiles. This group includes Gram-positive and Gram-negative aerobic, anaerobic,
and facultative anaerobic, motile and nonmotile, spore formers and non–spore formers, and 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 optimum 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 temperatures). 1,2