Microbial Growth Characteristics ◾ 55
microbial species under different conditions provides valuable information for developing methods
to preserve foods. In general, under optimum conditions of growth, bacteria have the shortest generation time, followed by yeasts and molds. Also, among bacterial species and strains, generation
time under optimum conditions varies greatly; some species, such as Vibrio parahaemolyticus, under
optimum conditions, can have a generation time as low as 10–12 minutes. Generally, in food systems, microorganisms have longer generation times than in a nutritionally rich bacteriological broth.
The generation time of a microbial population can be calculated mathematically from the differences in population during a given time period. Because of large numbers, the calculation is
done in logarithmatics (log 10 ) by using the following formula:
G
t
z
x
=
.
−
0 3
10
10
log
log
in which G is generation time (with time unit in minutes; also expressed as doubling time, t d ,
with time unit usually in hours), 0.3 is a constant (value of log 10 2 and indicates doubling), t is the
duration of study (minutes), log 10 x is initial, and log 10 z is the final cell numbers per milliliter or
colony-forming units (CFUs), per milliliter. For example, if, under a given growth condition, the
initial population of 10 4 cells/mL of a bacterial species increases to 10 6 cells/mL in 120 minutes,
its generation time will be
G =
. ×
−
=
0 3 120
6 4
18min
For the same bacterial strain, this value changes by changing the growth conditions.
Specific Growth Rate
The rate of growth of microorganisms during the exponential growth phase can also be determined mathematically by measuring cell numbers, OD 600 nm, cell mass (wet or dry weight), or
cell constituents (proteins, RNA, or DNA). If N 0 at time t 0 and N t at time t are the quantity and
time of any of these components, respectively; the growth rate of microorganisms is determined
by the equation:
N N
t
t t
=
−
0
0
e
µ(
)
in which μ is the specified growth rate in unit time (e.g., h –1 ). The equation can be logarithmically
expressed as follows:
ln
(
)
log
log
(
)
(log
N
N
t t
N
N
t t
N
t
t
t
0
0
0
0
2 3
2 3
=
−
−
= .
−
=
.
µ
µ
µ
− −
−
log )
N
t t
0
0
microbial species under different conditions provides valuable information for developing methods
to preserve foods. In general, under optimum conditions of growth, bacteria have the shortest generation time, followed by yeasts and molds. Also, among bacterial species and strains, generation
time under optimum conditions varies greatly; some species, such as Vibrio parahaemolyticus, under
optimum conditions, can have a generation time as low as 10–12 minutes. Generally, in food systems, microorganisms have longer generation times than in a nutritionally rich bacteriological broth.
The generation time of a microbial population can be calculated mathematically from the differences in population during a given time period. Because of large numbers, the calculation is
done in logarithmatics (log 10 ) by using the following formula:
G
t
z
x
=
.
−
0 3
10
10
log
log
in which G is generation time (with time unit in minutes; also expressed as doubling time, t d ,
with time unit usually in hours), 0.3 is a constant (value of log 10 2 and indicates doubling), t is the
duration of study (minutes), log 10 x is initial, and log 10 z is the final cell numbers per milliliter or
colony-forming units (CFUs), per milliliter. For example, if, under a given growth condition, the
initial population of 10 4 cells/mL of a bacterial species increases to 10 6 cells/mL in 120 minutes,
its generation time will be
G =
. ×
−
=
0 3 120
6 4
18min
For the same bacterial strain, this value changes by changing the growth conditions.
Specific Growth Rate
The rate of growth of microorganisms during the exponential growth phase can also be determined mathematically by measuring cell numbers, OD 600 nm, cell mass (wet or dry weight), or
cell constituents (proteins, RNA, or DNA). If N 0 at time t 0 and N t at time t are the quantity and
time of any of these components, respectively; the growth rate of microorganisms is determined
by the equation:
N N
t
t t
=
−
0
0
e
µ(
)
in which μ is the specified growth rate in unit time (e.g., h –1 ). The equation can be logarithmically
expressed as follows:
ln
(
)
log
log
(
)
(log
N
N
t t
N
N
t t
N
t
t
t
0
0
0
0
2 3
2 3
=
−
−
= .
−
=
.
µ
µ
µ
− −
−
log )
N
t t
0
0