FE-206 Food Microbiology1
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
g can be calculated by
g=t/n=(0.3 t)/”log10N-log10N0 “
Example: Initial population is 103 CFU/ml and incerased to 106 cells in 300 min. What is generation time?
g=”0.3∗300″ /(6-3)=30 min or you can first calculate µ and then calculate g.
2.3log(N/N0)= µ t µ =0.023min-1 and g=0.693/ µ
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)
Killing 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
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  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)/volume=(4πr^2)/(4/3 πr^3 )=3/r high ratio
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
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
Two or more microorganisms help one another during growth in food.
In yogurt;there are two types of lactic acid bacteria.
thermophilus produces small quantities of formic acid and stimulates Lb. Bulgaricus.
Lb. bulgaricus produce aminoacid inturn these products stimulate the growth Str. thermophilus
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.
thermophilus produce 8 ppm Acetaldehyde
Lb. bulgaricus produce 10 ppm Acetaldehyde in milk independently,when they grow together, they produce 30ppm Acetaldehyde .
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.
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.
III. Chemical Changes Caused my microorganisms
1.Changes in nitrogenous organic compounds
2.Changes in organic carbon compounds