ABSTRACT
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. us, psychrotrophs can multiply in these foods, provided other conditions for growth are not restricting.1,2
ere is some confusion about the definitions of the two terms, particularly in relation to their importance in food microbiology. e 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. ese microorganisms can grow in refrigerated and chilled foods. e 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. us, 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. is 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
Several studies have revealed that there are some mesophilic pathogens (e.g., Yersinia enteroco litica 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°C1°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). is 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. e 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 e demand for refrigerated foods has increased dramatically in countries where refrigeration systems are economically available for food processors, retailers, and consumers.3,4 ree major factors can be attributed to this increase. e first is the need and desire of consumers for convenient foods. e 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. ese 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% CO2 or a mixture of CO2 and nitrogen without or with some oxygen). ey 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. ey 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.
