274 ◾ Fundamental Food Microbiology
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°C–
1°C. Spoilage of vacuum-packaged meats by the psychrophilic Clostridium spp. with a growth
range between –2°C 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 designation “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 detect and control them do not differentiate the two groups, a single terminology can be
used for both. The same system has been used in food microbiology for the term “thermoduric,”
the microorganisms that survive pasteurization (or low-heat treatment), and can include thermophiles, mesophiles, and psychrophiles (Clostridium laramie). In this book, the term psychrotroph
is used to include microorganisms that can grow in food stored at refrigeration temperatures irrespective of their being psychrophilic or mesophilic. 1,2
Popularity of Refrigerated Foods
The demand for refrigerated foods has increased dramatically in countries where refrigeration
systems are economically available for food processors, retailers, and consumers. 3,4 Three major
factors can be attributed to this increase. The first is the need and desire of consumers for convenient foods. The changes in socioeconomic pattern, such as an increase in two-income families,
single-parent households, singles, the elderly, and college students living away from home, have
created a demand for convenient foods. Second, there is an increase in awareness and belief that
harshly processed and harshly preserved foods, as well as foods with high fat, cholesterol, and
sodium, are not beneficial for a long and healthy life. Health-conscious consumers are interested
in foods that are natural and fresh. These two factors have created a market for healthy foods
that can be of high quality (restaurant quality), convenient (e.g., take the least time to prepare at
home), and economically affordable. Finally, the technologies necessary for economical production of such foods and subsequent handling until they are consumed are available, especially in
developed countries. As a result, different types of refrigerated foods, including new-generation
refrigerated and chilled foods, sous vide foods, many of which are “ready-to-eat,” “heat-‘n’-eat,”
and “microwave-‘n’-eat” types, are being commercially produced. Many of the new-generation
foods are given low-heat treatment to no heat treatment, contain little or no preservatives, and
many are vacuum-packaged or packaged with modified atmosphere (MA) (with 100% CO 2 or
a mixture of CO 2 and nitrogen without or with some oxygen). They are expected to have a shelf
life of 20–60 days or more. Vacuum- or MA-packaged refrigerated unprocessed or raw foods are
also expected to have a much longer shelf life than the same foods stored in the presence of air.
Sous vide foods are fresh foods (e.g., vegetables, meat, or fish) that are vacuum packaged, cooked
at low heat, and stored in a refrigerator or over ice. They are warmed in the package before serving
and eating, and are expected to have a three-week shelf life. Fresh and ready-to-eat cut fruits and
vegetables are packaged with air and have a shelf life of approximately two weeks at refrigerated
or higher temperatures.
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°C–
1°C. Spoilage of vacuum-packaged meats by the psychrophilic Clostridium spp. with a growth
range between –2°C 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 designation “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 detect and control them do not differentiate the two groups, a single terminology can be
used for both. The same system has been used in food microbiology for the term “thermoduric,”
the microorganisms that survive pasteurization (or low-heat treatment), and can include thermophiles, mesophiles, and psychrophiles (Clostridium laramie). In this book, the term psychrotroph
is used to include microorganisms that can grow in food stored at refrigeration temperatures irrespective of their being psychrophilic or mesophilic. 1,2
Popularity of Refrigerated Foods
The demand for refrigerated foods has increased dramatically in countries where refrigeration
systems are economically available for food processors, retailers, and consumers. 3,4 Three major
factors can be attributed to this increase. The first is the need and desire of consumers for convenient foods. The changes in socioeconomic pattern, such as an increase in two-income families,
single-parent households, singles, the elderly, and college students living away from home, have
created a demand for convenient foods. Second, there is an increase in awareness and belief that
harshly processed and harshly preserved foods, as well as foods with high fat, cholesterol, and
sodium, are not beneficial for a long and healthy life. Health-conscious consumers are interested
in foods that are natural and fresh. These two factors have created a market for healthy foods
that can be of high quality (restaurant quality), convenient (e.g., take the least time to prepare at
home), and economically affordable. Finally, the technologies necessary for economical production of such foods and subsequent handling until they are consumed are available, especially in
developed countries. As a result, different types of refrigerated foods, including new-generation
refrigerated and chilled foods, sous vide foods, many of which are “ready-to-eat,” “heat-‘n’-eat,”
and “microwave-‘n’-eat” types, are being commercially produced. Many of the new-generation
foods are given low-heat treatment to no heat treatment, contain little or no preservatives, and
many are vacuum-packaged or packaged with modified atmosphere (MA) (with 100% CO 2 or
a mixture of CO 2 and nitrogen without or with some oxygen). They are expected to have a shelf
life of 20–60 days or more. Vacuum- or MA-packaged refrigerated unprocessed or raw foods are
also expected to have a much longer shelf life than the same foods stored in the presence of air.
Sous vide foods are fresh foods (e.g., vegetables, meat, or fish) that are vacuum packaged, cooked
at low heat, and stored in a refrigerator or over ice. They are warmed in the package before serving
and eating, and are expected to have a three-week shelf life. Fresh and ready-to-eat cut fruits and
vegetables are packaged with air and have a shelf life of approximately two weeks at refrigerated
or higher temperatures.