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FE-206 Food Microbiology1
Spring 2016
LECTURE4
Microbial Growth -Kinetics First
Order
First Order Kinetics
Food microbiology is concerned with all phases
Of microbial growth (lag,log, stationary, death
phase).Growth curves are normally plotted as
the number of cells on a log scale or log10 cell
number versus time.
Table First order kinetics to describe exponential growth and
inactivation
Growth
2
1a. N=N0eµt
1b. N=N0e
2a. 2.3 log(N/N0)=µt
2b. 2.3 log(N/N0)=-kt
3a.∆t=2.3 log(N/N0)/µ
3b. ∆t=-2.3 log(N/N0)/k
4.a. g=0.693/µ
4b. D=2.3/k
5b. Ea=
Irradiation
−𝑘𝑡
2.3𝑅𝑇1𝑇2 9
∗5
𝑍
N=Final cell number(CFU/ml)
N0=Initial cell number
t=time (h)
µ=Specific growth rate (h-1)
g=Doubling time(generation time)(h)
k=rate constant (h-1)
D=Decimal reduction time(h)
Ea=Activation energy(kcal/mol)
T1 and T2,reference and test temperature(K)
D0=Rate constant (h-1)
Dd=Dose(Gy)
N=N0e-Dd/D0
Growth Kinetics
g can be calculated by
𝑡
0.3 𝑡
g=𝑛=log N−log N
10
10 0
Example: Initial population is 103 CFU/ml and incerased to
106 cells in 300 min. What is generation time?
0.3∗300
g=
=30 min or you can first calculate µ and then
6−3
calculate g.
2.3log(N/N0)= µ t
µ =0.023min-1 and g=0.693/ µ
g=30.13 min
µ can be obtained by slope of straight line when the log
numbers of the cell is plotted against time.
Ex:Ground meat manufactured with N0=1.2*104
CFU/g.
How long it be held at 7°C before reaching a
level of 108CFU/g (for µ=0.025 h-1)
N=N0eµt
108 =1.2*104e0.025t
t=361.12 h
Death KineticsKilling can be by heat, radiation,acid,bacteriocin and
other lethal agents is also governed by first order
kinetics.
D value=amount of time required to reduce N0 by 90%
is the most frequently used constant.
The relationship between k and temperature is
explained by arrhenius equation
Ea/RT
Z value
Zvalue= a number of degees required to change
in the D values by a factor 10, or
It is the temperature required for one log10
reduction in the D-value.
z-value is used to determine the time values with different D-values at
different temperatures with its equation shown below:
where T is temperature in °F or °C.
This D-value is affected by pH of the product where low pH has faster
D values on various foods. The D-value at an unknown temperature
can be calculated [1] knowing the D-value at a given temperature
provided the Z-value is known.
For example: If Dvaue at 121 °C is 1.5 min and z
value is 10 °C. The D value at 131 °C will be 0.15
min.
Importance of Being small size
𝑆𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎 4𝜋𝑟 2
=4 3
𝑣𝑜𝑙𝑢𝑚𝑒
𝜋𝑟
=3/r
high ratio
3
Cell mass is close to cell surface, no circulatory
metabolism are required and this limits the size
of bacteria to microscopic dimensions.
As the cell size increases, the s/v ratio decreases,
which adversely affects the transport of
nutrients into and end-products out of the cell.
Microbial Growth Characteristics in
Foods
1.Competition
2.Metabiotic Growth
3.Symbiotic Growth
4.Synergistic Growth
5.Commensalism
6.Antagonistic Growth
7.Predation
1.Competition
Energy and nutrient sources are often present in
limiting concentrations; microorganisms
compete each other for nutrients and results in
exclusion of slower growing species.
Foods contain a mixed population of
microorganisms. Competition among the
different kinds of microorganisms in food
determines which one will outgrow the others
and cause its characteristics types of changes.
2. Metabiotic (Sequential)Growth
Different types of microorganism present normally in foods, but the
predominant types can change with time during storage.
Ex: If the food is packaged in a bag with a little bit of air(e.g. ground
meat),the aerobes will grow first and utilize O2. The environment will
become anaerobic, in which anaerobes grow favorably.
Ex: In most food fermentations metabiotic growth is observed.
In Sauerkraut fermentation,4 different bacterial species grow in
succession, one creating the favorable conditions for the next one.
First ,coliform grow produce acid and activate the growth of lactic acid
bacteria.
second, Leuconoctoc mesenteroides ;
third Lb. plantarum
Last, acid tolerant Lb. brevis
3.Symbiotic Growth
Two or more microorganisms help one another
during growth in food.
In yogurt;there are two types of lactic acid bacteria.
1. S. thermophilus
2.Lb. bulgaricus
S. thermophilus produces small quantities of formic
acid and stimulates Lb. Bulgaricus.
Lb. bulgaricus produce aminoacid inturn these
products stimulate the growth Str. thermophilus
4.Synergistic Growth
When two types of microorganism grow together
and may able to bring changes which could not
produce alone.
Acetaldehyde is desirable flavor component in
yogurt.
S. thermophilus produce 8 ppm Acetaldehyde
Lb. bulgaricus produce 10 ppm Acetaldehyde in
milk independently,when they grow together, they
produce 30ppm Acetaldehyde .
5.Commensalism
Microorganisms may not effect each other but
one organisms uses the substrate whic
isproduced by other.
For ex: cellulose hydrolyzing microorganisms
produce glucose and cellulose non hydrolyzing
micoorganims use this glucose.
One population benefits while latter remain
unaffected.
6.Antagonistic Growth
Microorganisms can adversely affect each
other, one kill the other. Some Gr(+) bacteria
produce antimicrobial components that can
kill many other types.
For ex: L. lactis ssp. lactis produce bacteriocin
called nisin and inhibits Gr(-)bacteria.
6.Predation Growth
The example for predatory growth is the
attachment of bacteria of the genus
Bdellovibrio, Daptobacter and
Stenotrphomonas maltophilia to Gr(-)
bacteria,penetrating the cell wall, and
subsequently multipying witin the
III. Chemical Changes Caused my
microorganisms
1. Changes in nitrogenous organic compounds
2. Changes in organic carbon compounds
a) Carbohydrates
b)Organic acids
c)Other compounds
d)Lipids
e)Pectic substances