54
Modern Food Microbiology
it was shown to reduce thermal D values at 57.8
◦ C by around 80% for L. monocytogenes and by
around 86% for S. aureus at 55.2
◦ C.
27 Although the mechanism of this enhanced thermal destruction
is unclear, some interesting implications can be envisioned.
Biological Structures
The natural covering of some foods provides excellent protection against the entry and subsequent
damage by spoilage organisms. In this category are such structures as the testa of seeds, the outer
covering of fruits, the shell of nuts, the hide of animals, and the shells of eggs. In the case of nuts such
as pecans and walnuts, the shell or covering is sufficient to prevent the entry of all organisms. Once
cracked, of course, nutmeats are subject to spoilage by molds. The outer shell and membranes of eggs,
if intact, prevent the entry of nearly all microorganisms when stored under the proper conditions of
humidity and temperature. Fruits and vegetables with damaged covering undergo spoilage much faster
than those not damaged. The skin covering of fish and meats such as beef and pork prevents the contamination and spoilage of these foods, partly because it tends to dry out faster than freshly cut surfaces.
Taken together, these six intrinsic parameters represent nature’s way of preserving plant and animal
tissues from microorganisms. By determining the extent to which each exists in a given food, one
can predict the general types of microorganisms that are likely to grow and, consequently, the overall
stability of this particular food. Their determination may also aid one in determining age, and possibly
the handling history of a given food.
EXTRINSIC PARAMETERS
The extrinsic parameters of foods are not substrate dependent. They are those properties of the
storage environment that affect both the foods and their microorganisms. Those of greatest importance
to the welfare of foodborne organisms are as follows:
1. temperature of storage
2. relative humidity of environment
3. presence and concentration of gases
4. presence and activities of other microorganisms
Temperature of Storage
Microorganisms, individually and as a group, grow over a very wide range of temperatures. Therefore, it is well to consider at this point the temperature growth ranges for organisms of importance in
foods as an aid in selecting the proper temperature for the storage of different types of foods.
The lowest temperature at which a microorganism has been reported to grow is −34
◦ C; the highest
is somewhere in excess of 100
◦ C. It is customary to place microorganisms into three groups based on
their temperature requirements for growth. Those organisms that grow well at or below 7
◦ C and have
their optimum between 20
◦ C and 30
◦ C are referred to as psychrotrophs (see Chapter 16). Those that
grow well between 20
◦ C and 45
◦ C with optima between 30
◦ C and 40
◦ C are referred to as mesophiles,
whereas those that grow well at and above 45
◦ C with optima between 55
◦ C and 65
◦ C are referred to
as thermophiles. (Physiological properties of these groups are treated in Chapters 16 and 17.)
Modern Food Microbiology
it was shown to reduce thermal D values at 57.8
◦ C by around 80% for L. monocytogenes and by
around 86% for S. aureus at 55.2
◦ C.
27 Although the mechanism of this enhanced thermal destruction
is unclear, some interesting implications can be envisioned.
Biological Structures
The natural covering of some foods provides excellent protection against the entry and subsequent
damage by spoilage organisms. In this category are such structures as the testa of seeds, the outer
covering of fruits, the shell of nuts, the hide of animals, and the shells of eggs. In the case of nuts such
as pecans and walnuts, the shell or covering is sufficient to prevent the entry of all organisms. Once
cracked, of course, nutmeats are subject to spoilage by molds. The outer shell and membranes of eggs,
if intact, prevent the entry of nearly all microorganisms when stored under the proper conditions of
humidity and temperature. Fruits and vegetables with damaged covering undergo spoilage much faster
than those not damaged. The skin covering of fish and meats such as beef and pork prevents the contamination and spoilage of these foods, partly because it tends to dry out faster than freshly cut surfaces.
Taken together, these six intrinsic parameters represent nature’s way of preserving plant and animal
tissues from microorganisms. By determining the extent to which each exists in a given food, one
can predict the general types of microorganisms that are likely to grow and, consequently, the overall
stability of this particular food. Their determination may also aid one in determining age, and possibly
the handling history of a given food.
EXTRINSIC PARAMETERS
The extrinsic parameters of foods are not substrate dependent. They are those properties of the
storage environment that affect both the foods and their microorganisms. Those of greatest importance
to the welfare of foodborne organisms are as follows:
1. temperature of storage
2. relative humidity of environment
3. presence and concentration of gases
4. presence and activities of other microorganisms
Temperature of Storage
Microorganisms, individually and as a group, grow over a very wide range of temperatures. Therefore, it is well to consider at this point the temperature growth ranges for organisms of importance in
foods as an aid in selecting the proper temperature for the storage of different types of foods.
The lowest temperature at which a microorganism has been reported to grow is −34
◦ C; the highest
is somewhere in excess of 100
◦ C. It is customary to place microorganisms into three groups based on
their temperature requirements for growth. Those organisms that grow well at or below 7
◦ C and have
their optimum between 20
◦ C and 30
◦ C are referred to as psychrotrophs (see Chapter 16). Those that
grow well between 20
◦ C and 45
◦ C with optima between 30
◦ C and 40
◦ C are referred to as mesophiles,
whereas those that grow well at and above 45
◦ C with optima between 55
◦ C and 65
◦ C are referred to
as thermophiles. (Physiological properties of these groups are treated in Chapters 16 and 17.)