Chapter 3
Intrinsic and Extrinsic
Parameters of Foods That
Affect Microbial Growth
As our foods are of plant and/or animal origin, it is worthwhile to consider those characteristics
of plant and animal tissues that affect the growth of microorganisms. The plants and animals that
serve as food sources have all evolved mechanisms of defense against the invasion and proliferation of
microorganisms, and some of these remain in effect in fresh foods. By taking these natural phenomena
into account, one can make effective use of each or all in preventing or retarding the growth of
pathogenic and spoilage organisms in the products that are derived from them.
INTRINSIC PARAMETERS
The parameters of plant and animal tissues that are an inherent part of the tissues are referred to as
intrinsic parameter.
33 These parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures
Each of these substrate-limiting factors is discussed below, with emphasis placed on their effects
on microorganisms in foods.
pH
It has been well established that most microorganisms grow best at pH values around 7.0 (6.6–7.5),
whereas few grow below 4.0 (Figure 3–1). Bacteria tend to be more fastidious in their relationships
39
Intrinsic and Extrinsic
Parameters of Foods That
Affect Microbial Growth
As our foods are of plant and/or animal origin, it is worthwhile to consider those characteristics
of plant and animal tissues that affect the growth of microorganisms. The plants and animals that
serve as food sources have all evolved mechanisms of defense against the invasion and proliferation of
microorganisms, and some of these remain in effect in fresh foods. By taking these natural phenomena
into account, one can make effective use of each or all in preventing or retarding the growth of
pathogenic and spoilage organisms in the products that are derived from them.
INTRINSIC PARAMETERS
The parameters of plant and animal tissues that are an inherent part of the tissues are referred to as
intrinsic parameter.
33 These parameters are as follows:
1. pH
2. Moisture content
3. Oxidation–reduction potential (Eh)
4. Nutrient content
5. Antimicrobial constituents
6. Biological structures
Each of these substrate-limiting factors is discussed below, with emphasis placed on their effects
on microorganisms in foods.
pH
It has been well established that most microorganisms grow best at pH values around 7.0 (6.6–7.5),
whereas few grow below 4.0 (Figure 3–1). Bacteria tend to be more fastidious in their relationships
39