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Factors affecting survival and growth
FS0501 1 2000
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
- Smoked / dried over fires
- Salted and dried
- Fermented
FS0501 2 2000
Tomato (Italy), fish, meat (South America), Biltong (South Africa)
Kipper (UK, smoked herring)
Bacalhau (Portugal - dried cod)
Tempeh, cheese, wine, beer, yoghurt, kefir
Pasteur’s germ theory
Traditional methods work by
‹ Preventing contamination
‹ Destroying microorganisms
‹ Inhibiting growth of undesirable microorganisms
FS0501 3 2000
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.
FS0501 4 2000
Traditional food preservation
Certain “traditional” foods can be
manufactured using industrial
processes
Example: Soy sauce
FS0501 5 2000
Factors affecting growth of bacteria in food
- Temperature
- Time
- pH
- Water activity (aw)
- Oxygen tension
- Preservatives
- Microbial interactions
FS0501 6 2000
Temperature
Boiling point
Pasteurising temperature
Body temperature
Fridge
Freezer
FS0501 7 2000
100°
72°
60°
36.5°
10°
0°
SAFETY
DANGER
SAFETY
How temperature affects growth rate of a bacterial population
B (Optimum)
C (Minimum) A (Maximum)
Cold Hot
TemperatureFS0501 8 2000
Growth of S. typhimurium at different temperatures
9 8
7 25°
6 20°
5 15°
4 10°
3 2 1 0
0 1 2 3 4 5
Time (Days)FS0501 9 2000
Temperature range for growth of pathogens
Temperature°C
Min. Opt. Max.
Salmonella 5 35 - 37 47Campylobacter 30 42 47E. coli 10 37 48S. aureus 6.5 37 - 40 48C. botulinum (proteolytic) 10 50C. botulinum (non-proteolytic) 3.3 25 - 37B. cereus 4 30 - 35 48 - 501
432
FS0501 10 2000
Temperature range for growth of toxigenic moulds
Temperature °C Min. Opt. Max.
Penicillium verrucosum
Aspergillus ochraceus
Aspergillus flavus
Fusarium moniliforme
FS0501 11 2000
0 20 31
8 28 37
10 32 42
3 25 37
Growth of different bacteria at 25°C
9
8
7
6
5 Log 4 CFU 3
2
1
0
FS0501 12 2000
0 1 2
Time (Days)
S. typhimurium L. monocytogenes Ps. fluorescens B. cereus C. bot-Proteolytic G+ve Spoilers
3 4
Effect of temperature on time to botulinum toxin production
10 9
8 7 6 5
4 3 2 1 0
FS0501 13 2000
0
Vacuum-packed hot smoked trout
Salt concentration = 0.5%
5 10 15
Temperature (°C)
20 25
Temperature affects bacteria
‹ Lag phase ‹ Growth rate ‹ Final cell numbers
through the change in
‹ Enzymatic and chemical composition of cells
‹ Nutritional requirements ‹ Limits for other factors influencing growth
FS0501 14 2000
Effect of salt concentration on time to botulinum toxin production
16
14
12
10
8
6
4
2
0
FS0501 15 2000
0
10°C 14°C 18°C 24°C
0.5 1
Salt Concentration (%)
1.5 2
Limits of pH for growth of pathogens
pH Min Max.
Escherichia coli 4.4 8.5Salmonella typhi 4 - 4.5 8 - 9.6Bacillus cereus 4.9 9.3Clostridium botulinum 4.6 8.5Staphylococcus aureus 4 9.8Saccharomyces cerevisiae 2.3 8.6Aspergillus flavus 2.0 11.2Fusarium moniliforme 2.5 10.7Penicillium verrucosum 2.0 10.0
FS0501 16 2000
Definition of water activity (aw)
aW = p / po
aw is the ratio of the water vapour pressure of the food (p) to that of pure water (po) at the same temperature.
FS0501 17 2000
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 0.97 0.98
Water activity (a w)
FS0501 18 2000
NaCl and glucose concentrations and corresponding aw values at 25°C
aW % w/w % w/wNaCl Glucose
1.00 0.00 0.000.99 1.74 8.900.98 3.43 15.740.96 6.57 28.510.94 9.38 37.830.92 11.90 43.720.90 14.18 48.540.88 16.28 53.050.86 18.18 58.45
FS0501 19 2000
Minimum levels of aW permitting growth at near optimum temperatures
aW
Moulds Aspergillus chevalieri 0.71Aspergillus ochraceus 0.78Aspergillus flavus 0.80Penicillium verrucosum 0.79Fusarium moniliforme 0.87
Yeasts Saccharomyces rouxii 0.62Saccharomyces cerevisiae 0.90
Bacteria Bacillus cereus 0.92Clostridium botulinum (proteolytic) 0.93Clostridium botulinum (non-proteolytic) 0.97Escherichia coli 0.93Salmonella 0.95Staphylococcus aureus 0.83
FS0501 20 2000
Range of aW in foods and their microbial flora
aw range
> 0.98
0.93 - 0.98
FS0501 21 2000
foods
Fresh meats Fresh fish Fresh fruits Fresh vegetables Canned vegetables
in brine Canned fruit
in light syrup (<3.5% salt, 26% sugar)
Fermented sausages Processed cheese Bread Evaporated milk Tomato paste (10% salt, 50% sugar)
microbial flora
(C. perfringens, Salmonella)
(Pseudomonas)
(B. cereus, C. botulinum, Salmonella) lactobacilli, bacilli and micrococci
Range of aW in foods and their microbial flora
aw range
0.85 - 0.93
0.6 - 0.85
< 0.6
FS0501 22 2000
foods Dry fermented sausages
Raw ham (17% salt, saturated sucrose)
Dried fruit Flour Cereals Salted fish Nuts
Confectionery Honey Noodles Dried egg, milk
microbial flora S. aureus
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
FS0501 23 2000