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

- Smoked / dried over fires

- Salted and dried

- Fermented

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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

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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

Pasteurising temperature

Body temperature

Fridge

Freezer

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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

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Temperature range for growth of toxigenic moulds

Temperature °C Min. Opt. Max.

Penicillium verrucosum

Aspergillus ochraceus

Aspergillus flavus

Fusarium moniliforme

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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

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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

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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

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Effect of salt concentration on time to botulinum toxin production

16

14

12

10

8

6

4

2

0

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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

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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.

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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 0.97 0.98

Water activity (a w)

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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

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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

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Range of aW in foods and their microbial flora

aw range

> 0.98

0.93 - 0.98

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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

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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

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