56 ◾ Fundamental Food Microbiology
μ(h –1 ) varies with microbial types, species, and the growth environment. Normally, it is approximately 0.2 for molds and yeasts. A fast-growing bacterial strain under optimum conditions can
have a μ(h –1 ) of 2.5 or higher. Under nonoptimal growth conditions, μ(h –1 ) can range between 0.2–
0.02. The value of doubling time (t d ) can also be determined from the μ value by the relationship
t d /h = 0.69/μ (0.69 is the value of In2). These equations are important for determining predictable
growth rates and population levels (or of other components) in fermentation and shelf life of foods.
Optimum Growth
Many environmental parameters of food, such as storage temperature, acidity (pH), water activity
(A W ), oxidation-reduction (O-R) potential, and nutrients, influence the microbial growth rate.
This aspect is discussed in Chapter 6. If one of the factors (e.g., temperature) is varied, keeping all
other parameters constant during the growth of a microbial strain, and its growth rate is measured,
it is evident that the growth rate is fastest (or generation time is shortest) at a certain temperature.
This temperature is referred to as the optimum growth temperature for the strain under a given
condition. The growth rate slows down on either side of the optimum growth temperature until
the growth stops. The area under the two points on both sides of an optimum growth condition
when minimum growth occurs is the growth temperature range. When the cells of a microbial
species are exposed to a factor (e.g., temperature) beyond the growth range, the cells not only stop
growing but, depending on the situation, may be injured or may lose viability. The growth range
and optimum growth of a microorganism under a specific parameter provide valuable information
for its inhibition, reduction, or the stimulation of growth in a food.
Growth Curve
The growth rate and growth characteristics of a microbial population under a given condition can
be graphically represented by counting cell numbers, enumerating CFUs, or measuring optical
density in a spectrophotometer at a given wavelength (above 300 nm, usually at 600 nm) of a cell
suspension. Cell mass or specific cell components, such as proteins, RNA, or DNA, can also be
measured to determine growth rate. Each method has several advantages and disadvantages. If
the CFU values are enumerated at different times of growth and a growth curve is plotted using
log 10 CFU versus time (log 10 CFU is used because of high cell numbers), a plot similar to the one
presented in Figure 5.2 is obtained. The plot has several features that represent the conditions of
the cells at different times. Initially, the population does not change (lag phase). During this time,
the cells assimilate nutrients and increase in size. Although the population remains unchanged
because of a change in size, both cell mass and optical density show some increase. Following this,
the cell number starts increasing, first slowly and then very rapidly. The cells in the population
differ initially in metabolic rate and only some multiply, and then almost all cells multiply. This is
the exponential phase (also called the logarithmic phase). The growth rate at the exponential phase
follows first-order reaction kinetics and can be used to determine generation time. Following this,
the growth rate slows down, and finally, the population enters the stationary phase. At this stage,
because of nutrient shortage and accumulation of waste products, a few cells die, and a few cells
multiply, keeping the living population stable. However, if one counts the cells under a microscope or measures cell mass, both may show an increase as dead cells may remain intact. After the
stationary phase, the population enters the death phase in which the rate of cell death is higher
than the rate of cell multiplication. Depending on the strain and conditions of the environment,
after a long period of time (which may even be a few years) some cells may still remain viable. This
μ(h –1 ) varies with microbial types, species, and the growth environment. Normally, it is approximately 0.2 for molds and yeasts. A fast-growing bacterial strain under optimum conditions can
have a μ(h –1 ) of 2.5 or higher. Under nonoptimal growth conditions, μ(h –1 ) can range between 0.2–
0.02. The value of doubling time (t d ) can also be determined from the μ value by the relationship
t d /h = 0.69/μ (0.69 is the value of In2). These equations are important for determining predictable
growth rates and population levels (or of other components) in fermentation and shelf life of foods.
Optimum Growth
Many environmental parameters of food, such as storage temperature, acidity (pH), water activity
(A W ), oxidation-reduction (O-R) potential, and nutrients, influence the microbial growth rate.
This aspect is discussed in Chapter 6. If one of the factors (e.g., temperature) is varied, keeping all
other parameters constant during the growth of a microbial strain, and its growth rate is measured,
it is evident that the growth rate is fastest (or generation time is shortest) at a certain temperature.
This temperature is referred to as the optimum growth temperature for the strain under a given
condition. The growth rate slows down on either side of the optimum growth temperature until
the growth stops. The area under the two points on both sides of an optimum growth condition
when minimum growth occurs is the growth temperature range. When the cells of a microbial
species are exposed to a factor (e.g., temperature) beyond the growth range, the cells not only stop
growing but, depending on the situation, may be injured or may lose viability. The growth range
and optimum growth of a microorganism under a specific parameter provide valuable information
for its inhibition, reduction, or the stimulation of growth in a food.
Growth Curve
The growth rate and growth characteristics of a microbial population under a given condition can
be graphically represented by counting cell numbers, enumerating CFUs, or measuring optical
density in a spectrophotometer at a given wavelength (above 300 nm, usually at 600 nm) of a cell
suspension. Cell mass or specific cell components, such as proteins, RNA, or DNA, can also be
measured to determine growth rate. Each method has several advantages and disadvantages. If
the CFU values are enumerated at different times of growth and a growth curve is plotted using
log 10 CFU versus time (log 10 CFU is used because of high cell numbers), a plot similar to the one
presented in Figure 5.2 is obtained. The plot has several features that represent the conditions of
the cells at different times. Initially, the population does not change (lag phase). During this time,
the cells assimilate nutrients and increase in size. Although the population remains unchanged
because of a change in size, both cell mass and optical density show some increase. Following this,
the cell number starts increasing, first slowly and then very rapidly. The cells in the population
differ initially in metabolic rate and only some multiply, and then almost all cells multiply. This is
the exponential phase (also called the logarithmic phase). The growth rate at the exponential phase
follows first-order reaction kinetics and can be used to determine generation time. Following this,
the growth rate slows down, and finally, the population enters the stationary phase. At this stage,
because of nutrient shortage and accumulation of waste products, a few cells die, and a few cells
multiply, keeping the living population stable. However, if one counts the cells under a microscope or measures cell mass, both may show an increase as dead cells may remain intact. After the
stationary phase, the population enters the death phase in which the rate of cell death is higher
than the rate of cell multiplication. Depending on the strain and conditions of the environment,
after a long period of time (which may even be a few years) some cells may still remain viable. This