FS0501 1 2000

Factors affecting survival and growth

Traditional food preservation

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Stored in cool caves

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Frozen / freeze-dried at high latitudes and altitudes

-

Dried in the sun Smoked / dried over fires Salted and dried Fermented Cheese, fruits, vegetables Meat (Switzerland), potato (Peru) Tomato (Italy), fish, meat (South America), Biltong (South Africa) Kipper (UK, smoked herring) Bacalhau (Portugal - dried cod) Tempeh, cheese, wine, beer, yoghurt, kefir FS0501 2 2000

FS0501 3 2000

Pasteur’s germ theory

Traditional methods work by

‹

Preventing contamination

‹

Destroying microorganisms

‹

Inhibiting growth of undesirable microorganisms

Traditional food preservation

FS0501 4 2000 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 5 2000

Traditional food preservation

Certain “traditional” foods can be manufactured using industrial processes

Example: Soy sauce

FS0501 6 2000

Factors affecting growth of bacteria in food

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Temperature

-

Time

-

pH

-

Water activity (a w )

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Oxygen tension

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Preservatives

-

Microbial interactions

Boiling point Pasteurising temperature FS0501 7 2000 Body temperature Fridge Freezer

Temperature

100 ° 72 ° 60 ° 36.5

° 10 ° 0 ° SAFETY DANGER SAFETY

How temperature affects growth rate of a bacterial population B (Optimum ) FS0501 8 2000 Cold C (Minimum) Temperature A (Maximum) Hot

Growth of S. typhimurium at different temperatures FS0501 9 2000 9 8 7 6 5 4 3 2 1 0 0 25 ° 20 ° 15 ° 10 ° 1 2 3 Time (Days) 4 5

Temperature range for growth of pathogens

Salmonella Campylobacter E. coli S. aureus

C. botulinum (proteolytic) C. botulinum (non-proteolytic)

B. cereus Min.

Temperature °C Opt.

Max.

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 - 50 1 43 2

FS0501 10 2000

FS0501 11 2000 Temperature range for growth of toxigenic moulds

Penicillium verrucosum Aspergillus ochraceus Aspergillus flavus Fusarium moniliforme Temperature °C Min.

Opt.

Max. 0 8 10 3 20 28 32 25 31 37 42 37

Growth of different bacteria at 25 °C Log CFU 9 8 7 6 5 4 3 2 1 0 0 FS0501 12 2000 1 2 Time (Days) 3

S. typhimurium L. monocytogenes Ps. fluorescens B. cereus C. bot-Proteolytic

G+ve Spoilers 4

Effect of temperature on time to botulinum toxin production FS0501 13 2000 10 9 8 7 6 5 4 3 2 1 0 0 Vacuum-packed hot smoked trout Salt concentration = 0.5% 5 10 15

Temperature ( °C)

20 25

FS0501 14 2000

Temperature affects bacteria

‹

Lag phase

‹

Growth rate

‹

Final cell numbers

through the change in

‹

Enzymatic and chemical composition of cells

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Nutritional requirements

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Limits for other factors influencing growth

Effect of salt concentration on time to botulinum toxin production FS0501 15 2000 16 14 12 10 8 6 4 2 0 0 10 °C 14 °C 18 °C 24 °C 0.5 1

Salt Concentration (%)

1.5 2

Limits of pH for growth of pathogens

Min pH

Max.

FS0501 16 2000

Escherichia coli Salmonella typhi Bacillus cereus Clostridium botulinum Staphylococcus aureus Saccharomyces cerevisiae Aspergillus flavus Fusarium moniliforme Penicillium verrucosum 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

FS0501 17 2000

Definition of water activity (a

w

)

a W = p / p o

a w is the ratio of the water vapour pressure of the food (p) to that of pure water (p o ) at the same temperature.

FS0501 18 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

Water activity (a w )

0.97

0.98

FS0501 19 2000 NaCl and glucose concentrations and corresponding a w values at 25 °C

a W

1.00

0.99

0.98

0.96

0.94

0.92

0.90

0.88

0.86

% w/w NaCl

0.00

1.74

3.43

6.57

9.38

11.90

14.18

16.28

18.18

% w/w Glucose

0.00

8.90

15.74

28.51

37.83

43.72

48.54

53.05

58.45

Minimum levels of a W permitting growth at near optimum temperatures a W Moulds Yeasts Bacteria

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

FS0501 20 2000

a w range

> 0.98 0.93 - 0.98 Range of a W in foods and their microbial flora

foods microbial flora

Fresh meats Fresh fish Fresh fruits Fresh vegetables Canned vegetables in brine Canned fruit in light syrup (<3.5% salt, 26% sugar)

(C. perfringens, Salmonella) (Pseudomonas)

Fermented sausages Processed cheese Bread Evaporated milk Tomato paste (10% salt, 50% sugar)

(B. cereus, C. botulinum, Salmonella)

lactobacilli, bacilli and micrococci FS0501 21 2000

a w range

0.85 - 0.93 0.6 - 0.85 < 0.6 Range of a W in foods and their microbial flora

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 FS0501 22 2000

FS0501 23 2000 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