396 Modern Food Microbiology
between 0
◦ C and 7
◦ C and produce visible colonies (or turbidity) within 7–10 days in this temperature
range. Because some psychrotrophs can grow at temperatures at least as high as 43
◦ C, they are, in
fact, mesophiles. By these definitions, psychrophiles would be expected to occur only on products
from oceanic waters or from extremely cold climes. The organisms that cause the spoilage of meats,
poultry, and vegetables in the 0–5
◦ C range are psychrotrophs.
Because all psychrotrophs do not grow at the same rate over the 0–7
◦ C range, the terms eurypsychrotroph (eurys, wide or broad) and stenopsychrotroph (stenos, narrow, little, or close) have been
suggested. Eurypsychrotrophs typically do not form visible colonies until sometime between 6 and 10
days; stenopsychrotrophs typically form visible colonies in about 5 days.
34 It has been suggested that
psychrotrophs can be distinguished from nonpsychrotrophs by their inability to grow on a nonselective
medium at 43
◦ C in 24 hours, whereas the latter do grow.
50 It has been shown that some bacteria that
grow well at 7
◦ C within 10 days also grow well at 43
◦ C, and among these are Enterobacter cloacae, Hafnia alvei, and Yersinia enterocolitica (ATCC 27739).
34 These are eurypsychrotrophs, although
there are others that grow well at 43
◦ C but only poorly at 7
◦ C in 10 days. Typical of stenopsychrotrophs
are Pseudomonas fragi (ATCC 4973) and Aeromonas hydrophila (ATCC 7965), which grow well at
7
◦ C in 3–5 days but do not grow at 40
◦ C.
34
There are three distinct temperature ranges for low-temperature stored foods. Chilling temperatures
are those between the usual refrigerator (5–7
◦ C) and ambient temperatures, usually about 10–15
◦ C.
These temperatures are suitable for the storage of certain vegetables and fruits such as cucumbers,
potatoes, and limes. Refrigerator temperatures are those between 0
◦ C and 7
◦ C (ideally no higher than
40
◦ F or 4.4
◦ C). Freezer temperatures are those at or below −18
◦ C. Under normal circumstances,
growth of all microorganisms is prevented at freezer temperatures; nevertheless, some can and do
grow within the freezer range but at an extremely slow rate.
TEMPERATURE GROWTH MINIMA
Bacterial species and strains that can grow at or below 7
◦ C are rather widely distributed among the
Gram-negative and less so among Gram-positive genera (Tables 16–1 and 16–2). The lowest recorded
temperature of growth for a microorganism of concern in foods is −34
◦ C, in this case a pink yeast.
Growth at temperatures below 0
◦ C is more likely to be that of yeasts and molds, than bacteria. This
is consistent with the growth of fungi under lower water activity (a w ) conditions. Bacteria have been
reported to grow at −20
◦ C and around −12
◦ C.
45 Foods that are likely to support microbial growth
at subzero temperatures include fruit juice concentrates, bacon, ice cream, and certain fruits. These
products contain cryoprotectants that depress the freezing point of water.
PREPARATION OF FOODS FOR FREEZING
The preparation of vegetables for freezing includes selecting, sorting, washing, blanching, and
packaging prior to actual freezing. Foods in any state of detectable spoilage should be rejected for
freezing. Meats, poultry, seafoods, eggs, and other foods should be as fresh as possible.
Blanching is achieved either by a brief immersion of foods into hot water or by the use of steam.
Its primary functions are as follows:
1. inactivation of enzymes that might cause undesirable changes during freezing storage;
2. enhancement or fixing of the green color of certain vegetables;
3. reduction in the numbers of microorganisms on the foods;
between 0
◦ C and 7
◦ C and produce visible colonies (or turbidity) within 7–10 days in this temperature
range. Because some psychrotrophs can grow at temperatures at least as high as 43
◦ C, they are, in
fact, mesophiles. By these definitions, psychrophiles would be expected to occur only on products
from oceanic waters or from extremely cold climes. The organisms that cause the spoilage of meats,
poultry, and vegetables in the 0–5
◦ C range are psychrotrophs.
Because all psychrotrophs do not grow at the same rate over the 0–7
◦ C range, the terms eurypsychrotroph (eurys, wide or broad) and stenopsychrotroph (stenos, narrow, little, or close) have been
suggested. Eurypsychrotrophs typically do not form visible colonies until sometime between 6 and 10
days; stenopsychrotrophs typically form visible colonies in about 5 days.
34 It has been suggested that
psychrotrophs can be distinguished from nonpsychrotrophs by their inability to grow on a nonselective
medium at 43
◦ C in 24 hours, whereas the latter do grow.
50 It has been shown that some bacteria that
grow well at 7
◦ C within 10 days also grow well at 43
◦ C, and among these are Enterobacter cloacae, Hafnia alvei, and Yersinia enterocolitica (ATCC 27739).
34 These are eurypsychrotrophs, although
there are others that grow well at 43
◦ C but only poorly at 7
◦ C in 10 days. Typical of stenopsychrotrophs
are Pseudomonas fragi (ATCC 4973) and Aeromonas hydrophila (ATCC 7965), which grow well at
7
◦ C in 3–5 days but do not grow at 40
◦ C.
34
There are three distinct temperature ranges for low-temperature stored foods. Chilling temperatures
are those between the usual refrigerator (5–7
◦ C) and ambient temperatures, usually about 10–15
◦ C.
These temperatures are suitable for the storage of certain vegetables and fruits such as cucumbers,
potatoes, and limes. Refrigerator temperatures are those between 0
◦ C and 7
◦ C (ideally no higher than
40
◦ F or 4.4
◦ C). Freezer temperatures are those at or below −18
◦ C. Under normal circumstances,
growth of all microorganisms is prevented at freezer temperatures; nevertheless, some can and do
grow within the freezer range but at an extremely slow rate.
TEMPERATURE GROWTH MINIMA
Bacterial species and strains that can grow at or below 7
◦ C are rather widely distributed among the
Gram-negative and less so among Gram-positive genera (Tables 16–1 and 16–2). The lowest recorded
temperature of growth for a microorganism of concern in foods is −34
◦ C, in this case a pink yeast.
Growth at temperatures below 0
◦ C is more likely to be that of yeasts and molds, than bacteria. This
is consistent with the growth of fungi under lower water activity (a w ) conditions. Bacteria have been
reported to grow at −20
◦ C and around −12
◦ C.
45 Foods that are likely to support microbial growth
at subzero temperatures include fruit juice concentrates, bacon, ice cream, and certain fruits. These
products contain cryoprotectants that depress the freezing point of water.
PREPARATION OF FOODS FOR FREEZING
The preparation of vegetables for freezing includes selecting, sorting, washing, blanching, and
packaging prior to actual freezing. Foods in any state of detectable spoilage should be rejected for
freezing. Meats, poultry, seafoods, eggs, and other foods should be as fresh as possible.
Blanching is achieved either by a brief immersion of foods into hot water or by the use of steam.
Its primary functions are as follows:
1. inactivation of enzymes that might cause undesirable changes during freezing storage;
2. enhancement or fixing of the green color of certain vegetables;
3. reduction in the numbers of microorganisms on the foods;