Factors affecting survival
and growth
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Traditional food preservation
- Stored in cool caves
Cheese, fruits, vegetables
- Frozen / freeze-dried at
Meat (Switzerland),
high latitudes and altitudes potato (Peru)
- Dried in the sun
Tomato (Italy),
fish, meat (South America),
Biltong (South Africa)
- Smoked / dried over fires
Kipper (UK, smoked herring)
- Salted and dried
Bacalhau (Portugal - dried cod)
- Fermented
Tempeh, cheese, wine,
beer, yoghurt, kefir
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Pasteur’s germ theory
Traditional methods work by
‹
Preventing contamination
‹
Destroying microorganisms
‹
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Inhibiting growth of undesirable
microorganisms
Traditional food preservation
Traditional preservation sometimes fails to
preserve the food, or to kill the pathogens.
In these cases, spoilage or food poisoning may
occur.
Examples: salmonellosis from dried beef
(Salmonella St. Paul ) and poisoning from
tempeh bonkrek in Indonesia.
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Traditional food preservation
Certain “traditional” foods can be
manufactured using industrial
processes
Example: Soy sauce
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Factors affecting growth of
bacteria in food
- Temperature
- Time
- pH
- Water activity (aw)
- Oxygen tension
- Preservatives
- Microbial interactions
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Temperature
Boiling
point
100°
SAFETY
Pasteurising
temperature
72°
60°
Body
temperature
Fridge
36.5°
10°
0°
Freezer
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DANGER
SAFETY
How temperature affects
growth rate of a bacterial population
B (Optimum)
C (Minimum)
Cold
Hot
Temperature
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A (Maximum)
Growth of S. typhimurium
at different temperatures
9
8
7
6
5
4
3
2
1
0
25°
20°
15°
10°
0
1
2
3
Time (Days)
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4
5
Temperature range
for growth of pathogens
Temperature°C
Min.
Opt.
Max.
Salmonella
Campylobacter
E. coli
S. aureus
C. botulinum (proteolytic)
C. botulinum (non-proteolytic)
B. cereus
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5
30
10
6.5
10
3.3
4
35 - 37
42
37
37 - 40
30 - 35
47
47
48
48
50
25 - 37
48 - 501
432
Temperature range for growth
of toxigenic moulds
Temperature °C
Min. Opt. Max.
Penicillium verrucosum
0
20
31
Aspergillus ochraceus
8
28
37
10
32
42
3
25
37
Aspergillus flavus
Fusarium moniliforme
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Growth of different bacteria at 25°C
9
8
7
6
Log
CFU
S. typhimurium
L. monocytogenes
Ps. fluorescens
B. cereus
C. bot-Proteolytic
G+ve Spoilers
5
4
3
2
1
0
0
1
2
Time (Days)
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3
4
Effect of temperature on time
to botulinum toxin production
10
9
8
7
6
5
4
3
2
1
0
Vacuum-packed hot
smoked trout
Salt concentration =
0.5%
0
5
10
15
Temperature (°C)
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20
25
Temperature affects bacteria
‹
‹
‹
Lag phase
Growth rate
Final cell numbers
through the change in
‹
‹
‹
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Enzymatic and chemical composition
of cells
Nutritional requirements
Limits for other factors influencing growth
Effect of salt concentration on
time to botulinum toxin production
16
14
12
10
10°C
14°C
18°C
24°C
8
6
4
2
0
0
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0.5
1
Salt Concentration (%)
1.5
2
Limits of pH for growth of pathogens
pH
Escherichia coli
Salmonella typhi
Bacillus cereus
Clostridium botulinum
Staphylococcus aureus
Saccharomyces cerevisiae
Aspergillus flavus
Fusarium moniliforme
Penicillium verrucosum
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Min
Max.
4.4
4 - 4.5
4.9
4.6
4
2.3
2.0
2.5
2.0
8.5
8 - 9.6
9.3
8.5
9.8
8.6
11.2
10.7
10.0
Definition of water activity (aw)
aW = p / p o
aw is the ratio of the water vapour
pressure of the food (p) to that of pure
water (po) at the same temperature.
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Effect of water activity on lag time
of S. aureus in UHT milk at 12°C
120
100
80
60
40
20
0.93
0.94
0.95
0.96
Water activity (a w)
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0.97
0.98
NaCl and glucose concentrations and
corresponding aw values at 25°C
aW
1.00
0.99
0.98
0.96
0.94
0.92
0.90
0.88
0.86
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% w/w
NaCl
% w/w
Glucose
0.00
1.74
3.43
6.57
9.38
11.90
14.18
16.28
18.18
0.00
8.90
15.74
28.51
37.83
43.72
48.54
53.05
58.45
Minimum levels of aW permitting growth
at near optimum temperatures
aW
Moulds
Yeasts
Bacteria
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Aspergillus chevalieri
Aspergillus ochraceus
Aspergillus flavus
Penicillium verrucosum
Fusarium moniliforme
Saccharomyces rouxii
Saccharomyces cerevisiae
Bacillus cereus
Clostridium botulinum (proteolytic)
Clostridium botulinum (non-proteolytic)
Escherichia coli
Salmonella
Staphylococcus aureus
0.71
0.78
0.80
0.79
0.87
0.62
0.90
0.92
0.93
0.97
0.93
0.95
0.83
Range of aW in foods
and their microbial flora
aw range
foods
> 0.98
Fresh meats
Fresh fish
Fresh fruits
(C. perfringens,
Fresh vegetables
Salmonella)
Canned vegetables
in brine
Canned fruit
(Pseudomonas)
in light syrup
(<3.5% salt, 26% sugar)
0.93 - 0.98
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Fermented sausages
Processed cheese
Bread
Evaporated milk
Tomato paste
(10% salt, 50% sugar)
microbial flora
(B. cereus,
C. botulinum,
Salmonella)
lactobacilli,
bacilli and
micrococci
Range of aW in foods
and their microbial flora
aw range
foods
microbial flora
0.85 - 0.93
Dry fermented
sausages
Raw ham
(17% salt,
saturated sucrose)
S. aureus
0.6 - 0.85
< 0.6
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Dried fruit
Flour
Cereals
Salted fish
Nuts
Confectionery
Honey
Noodles
Dried egg, milk
Mycotoxin
producing moulds
Spoilage yeasts
and moulds
Xerophilic fungi
Halophiles
Osmophilic yeasts
No growth but
may remain viable
Factors affecting microbial growth
Key messages
Temperature, pH, water activity and oxygen
tension are the principal factors affecting
microbial growth
There are optimum ranges for these parameters
These optima are interdependent
They can be selected to inhibit the growth of
certain organisms within limits related to the
palatability of food
Certain foods are suited for the growth of certain
flora
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