Control of microbial hazards heat treatment

The heat treatment applied during processing will affect the survival of the bacteria contaminating the foods. The extent of death of bacteria is a function of time and temperature. The rate of destruction, expressed as the decimal reduction time, D, is the time required to destroy 90% of the bacterial population (or to effect a 10-fold reduction or a one log (base 10) reduction). The effect of temperature on the D values, expressed as a z value, is the increase in temperature needed to produce a ten-fold reduction in D values. Both D and z values are available from many sources for all pathogenic bacteria including those of concern for the safety of cooked chilled foods containing vegetables.

The D70 value (i.e. decimal reduction time at 70°C) for L. monocytogenes is about 0.1-0.2min (ICMSF, 1996). These values are markedly higher for C. botulinum, illustrating the considerably higher heat resistance of bacterial spores when compared to vegetative cells, with a maximum D121.2 of about 0.21 min for Group I proteolytic C. botulinum and a maximum D82.2 of 2.4 for Group II non-proteolytic C. botulinum, with z values close to 10°C (ICMSF, 1996). These values are subject to multiple sources of variations, including intraspecies variability, type of foods, preparation of bacterial suspensions, and so on. B. cereus is a representative example of strong variations between different strains; D values of 2min have been reported at 90°C as well as at 121.2°C, with z values from 7 to 14°C (ICMSF, 1996).

Heating these bacteria in vegetable substrates affects heat resistance only marginally, the major decreases being observed in acid vegetable substrate. However some specific effects caused by vegetables are observed on non-proteolytic C. botulinum. The measured heat resistance of this bacterium may be increased by addition of juices of vegetable, such as turnip, swede, potato, flat bean or cabbage (Stringer et al., 1999). This activity is attributed to an endogenous lysozyme activity, similar to that of hen egg white lysozyme, which is assumed to aid growth from heat damaged spores of non-proteolytic C. botulinum (Lund and Peck, 2000).

Table 11.2 Fate of Listeria monocytogenes, Bacillus

cereus and Clostridium botulinum in some preparations of cooked vegetables

Bacterium

Food or medium

Temperature (°C)

Parameter

Value

Reference

L. monocytogenes

Vacuum-packed potato

4

Increase in number

+4 log cfu g-1 in 14 days

1

Vacuum-packed potato

15

Increase in number

+4.5 log cfu g-1 in 3 days

1

Vacuum-packed potato

28

Increase in number

+4log cfu g-1 in 12h

1

B. cereus

Vacuum-packed carrot and potato

4 and 8

Increase in number

0 after 90 days

2

Vacuum-packed carrot and potato

room

Increase in number

+4.5 log cfu g-1 in 3 days

2

Courgette broth

20

Lag (h), generation time (h)

<5-20, 2.5—3.1a

3

Courgette broth

14

Lag (h), generation time (h)

5-16, 4.4-5.1

3

Courgette broth

10

Lag (h), generation time (h)

184-333, 3-9

3

Courgette broth

6.5

Lag (h), generation time (h)

ngob

3

Carrot broth

25

Lag (h), generation time (h)

7-8, 0.93-1.1

4

Carrot broth

16

Lag (h), generation time (h)

11-12, 3.2-3.5

4

Carrot broth

12

Lag (h), generation time (h)

26-28, 7.5-23

4

Carrot broth

10

Lag (h), generation time (h)

ngo-51, ngo-14

4

Carrot broth

8

Lag (h), generation time (h)

ngo-110, ngo-15

4

Carrot broth

5

Lag (h), generation time (h)

ngo-156, ngo-63

4

C. botulinum Group I

Vacuum-packed potato

25

Toxin detection

Samples +ve after 6 days

5

Vacuum-packed potato

25

Toxin detection

Samples +ve after 7 days

6

Vacuum-packed potato

20

Toxin detection

Samples +ve after 4 days

7

Vacuum-packed potato

15

Toxin detection

Samples +ve after 14 days

7

C. botulinum Group II

Vacuum-packed potato

10

Toxin detection

Samples +ve after 9 days

7

Vacuum-packed potato

4

Toxin detection

No sample +ve after 60 days

7

Canned peas

8

Toxin detection

Samples +ve after 14 days

8

Mushroom purée

8

Lag (h), generation time (h)

146, 9

9

Mushroom purée

5

Lag (h), generation time (h)

304, 12

9

Cauliflower purée

8

Lag (h), generation time (h)

288, 11

9

Cauliflower purée

5

Lag (h), generation time (h)

383, 9

9

Potato purée

8

Lag (h), generation time (h)

628, 10

9

Vacuum-packed potato, pH 4.8

10 and 20

Toxin detection

Samples +ve after 21 days

10

Vacuum-packed potato, pH 5.2

7 and 10

Toxin detection

Samples +ve after 21 days

10

Vacuum-packed potato, pH 5.2

20

Toxin detection

Samples +ve after 14 days

10

a Several strains tested; b ngo, no growth observed.

1, Juneja et al., 1998; 2, Picoche et al., 1993; 3, Choma et al., 2000; 4, Valero et al, 2000; 5, Lund et al., 1988; 6, Dodds, 1989; 7, Notermans et al., 1981; 8, Gola and Mannino, 1985; 9, Carlin and Peck, 1996; 10, Baumgart, 1987.

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