Intrinsic
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Factors affecting microbial activity in foods Intrinsic Factors Extrinsic Factors Bacterial growth curve Incubation time Log bacterial No. Possible fate of a microorganism after gaining access to a food product Storage time Number of cells Growth Death Survival
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Transcript of Intrinsic
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Factors affecting microbial activity in foods
Intrinsic FactorsExtrinsic Factors
Bacterial growth curve
Incubation time
Log
bac
teri
al N
o.
Possible fate of a microorganism after gaining access to a food product
Storage time
Num
ber
of c
ells
Growth
Death
Survival
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Intrinsic Factors pHMoisture contentOxidation-reduction potential (Eh)Nutrient contentAntimicrobial constituentsBiological structures
Extrinsic Factors
Temperature of storageTime of storageGaseous environmentRelative humidity
Intrinsic factors
Intrinsic: Pertaining to the food product
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pH
Effects on microorganisms:Affects enzymatic functionAffects nutrient transport into the cell
pHCytoplasmic pH is neutralWhen microorganisms are in a low pH environment they must prevent H+ intake, or free H+ as the same rate as it is taken. Effect of H+ transport:
Increase in denaturization of membrane proteins and transport enzymes
Affects cell premeability and decreases intracellular pH
Optimal pH values for growth of different types of microorganisms
Type ofmicroorganism Optimum pH
Bacteria 6.7 – 7.5
Yeasts 4.0 – 6.5
Molds 4.5 – 6.8
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pH of selected foods
Food pHEgg yolk 7.6 – 9.5
Seafood 6.8 – 8.0
Meats 5.3 – 6.4Liver 5.3 – 6.3
Vegetables 5.3 – 6.3
Fruits 2.0 – 4.2
Effect of food pH on microbial activity
For canned foods product pH determines the type of treatment to apply
Minimal pH for production of botulintoxin = 4.5≥4.5 = 12D treatment (1012-fold reduction)
Effect of food pH on microbial activity
Beef carcassesDFD (dry, firm, dark)
When cattle are subjected to stress before slaughter a glycogen depletion occursThere are no lactic acid sources during rigor mortispH of normal meat: 5.5-5.7, pH of DFD meat: >6.0
DFD meat is more susceptible to bacterial deterioration and is of lower quality
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Most common causes of food deterioration by product pH
Yeasts, molds2.0 – 4.0Juices, soft drinks
Bacteria5.3 – 6.4Meat
Bacteria5.3 – 6.3Vegetables
Yeasts, molds2.0 – 4.2Fruits
Most common causes of spoilagepHProduct
Growth of E. coli in the presence of acetic acid (Diez-Gonzalez and Russell, Microbiol., 143:1175. 1997)
0
0.05
0.1
0.15
0.2
0.25
0.3
0 40 80 120 160
Acetate concn. (mM)
OD
60
0
O157:H7K-12
6
6.2
6.4
6.6
6.8
7
7.2
0 40 80 120 160
Acetate concn. (mM)
pH
O157:H7K-12
Internal pH
Moisture content
Water activity (aw)
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Water activity (aw)Definitions
Physical chemical:Aw = p/p0
p = vapor pressure of food systemp0 = vapor pressure of solvent (water)
Biochemical:Amount of molecules available to serve as vehicle for biochemical reactions
Microbial growth
Water activity (aw)
AtmosphericpressureVapor
pressure
Water activity (aw)
O2
H
H- +
Na+
Na+
Cl-
Cl-
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Minimum Aw requirements for growth of different types of microorganisms
Type ofmicroorganism
Minimum requiredAw
Most bacteria 0.90
Yeasts 0.80
Molds 0.70
Aw values for selected foods
Food AwMeat 0.95 - 0.999MilkFruitsVegetablesBreadEvaporated milkTomato paste
0.93 – 0.95
CheeseCondensed milk
0.85 – 0.93
JelliesSyrup
0.60 – 0.85
CrackersPasta
<0.60
Water activity (aw)Mostly used for preservation purposesLimited effect on food safety
Good control measure for pathogens that need to grow in order to become a hazard
Staphylococcus aureusClostridium botulinum
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Minimum aw for growth of S. aureus
Minimum for growth0.86
Minimum for enterotoxin B production
0.90
Survival of Salmonella in meringue powder as affected by aw
0
50
100
150
200
250
Day
s of
sto
rage
0.3 0.2 0.1
aw
REDOX Potential (Eh)Definitions
OxidationRelease of electrons from a compound
ReductionIncorporation of electrons into a compound
Always occur in pairsOxidation - Reduction
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REDOX Potential (Eh)When an oxidation-reduction reaction occurs, an electric potential is created
Oxidated = positiveReduced = negative
Depending on the concentration of oxidizing and reducing compounds:
Greater reduction or oxidation capacityEh value
Eh and microbial growth
Anaerobiosis≤ -200 mV
Aerobiosis≥ 200 mV
-200 a 200 mV includes growth of microaerobic, aerobic and anaerobic organisms
Eh values of selected foods
300 to 400Fruits, vegetables, juices
-20 a -200Cheeses200Ground meat-200Meat300Milk
Eh (mV)Food
Adapted from Jay, J. M., 5th. Ed., 1998
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Eh of meat
With microbial growth
Without microbial growth
-250 mV
-130 mV30 h post mortem
250 mVImmediately after slaughter
Barnes and Ingram. J. Sci. Food Agr. 6:448. 1955
Nutrient contentAvailability
Ex. Ground beef – Breaking tissues promotes fluid leakage
More nutrient availability
ContentMost foods can support microbial growth
Antimicrobial constituentsSpices
Essential oils Eugenol in clove and cinnamon Alicin in garlicCinnamic aldehyde in cinnamon Carvacrol and thymol in oreganoAllyl isothiocianyde in mustard
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Antimicrobial constituents
Raw cow’s milkLactoferrinLactoperoxidaseLysozymeImmunoglobulines
Egg whiteLisozymeConalbumen
FruitsOrganic acids
Biological structuresNatural covers. Prevent microbial access and the resulting food spoilage
Hull of fruits and vegetables Nut shellEgg shell
After harvests woundsStem scar in fruits. Promotes bacterial penetration
