Fundamental Food Microbiology, Third Edition - Fuad Fathir

MICROBIAL GROWTH CHARACTERISTICS 59
Figure 5.1 Photograph of thin sections of Lactobacillus cells by transmission electron microscopy
showing cell wall formation during cell division at earlier stage (a), later stage
(b), and final stage with partial separation of cells (c).
B. Generation Time (or Doubling Time)
The time that a single cell takes to divide into two is called generation time. 1
However, in practice, generation time is referred to as the doubling time for the
entire population. In a population of a microbial species, not all cells divide at the
same time or at the same rate. The generation time of a 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 to 12 min. 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 formula:
where 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 102 and
indicates doubling), t is the duration of study (min), log 10x is initial and log 10z is
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 min, its generation time will be:
\
0.3t
G=
log log
G = 0.3x120
6-4
z- x
10 10
= 18min