602 Modern Food Microbiology
whole milk, skim milk, and 11% nonfat milk solids, Donnelly and Briggs
27 found that composition
did not affect heat destruction and that at 62.7
◦ C, the D values were 60 seconds or less. The five
strains employed included serotypes 1, 3, and 4. When milk that was naturally contaminated with a
serotype 1 strain at around 10
4 /ml was subjected to an HTST protocol at temperatures ranging from
60
◦ to 78
◦ C, no viable cells could be detected at processing temperatures of 69
◦ C or above.
37 In
their review of the early studies on the thermal resistance of L. monocytogenes in milk, Mackey and
Bratchell
89 concluded that normal pasteurization procedures will inactivate this organism but that the
margin of safety is greater for the vat protocol (LTLT) than the HTST protocol. Their mathematical
model predicted a 39 D reduction for vat and a 5.2 D reduction for HTST.
Nondairy Products
For liquid whole egg and meat products, D values are generally higher than for milk, a fact not
unpredicted, considering the effect of proteins and lipids on the thermal resistance of microorganisms
(discussed in Chapter 17). For one strain of L. monocytogenes isolated from a chicken product, D
values at 70
◦ C were 6.6–8.4 seconds; they were essentially the same in beef and two poultry meats.
88
In one study, viable cells could be recovered by enrichments from eight of nine samples following
heating in ground beef to 70
◦ C.
8 In a study of blue crabmeat, strain Scott A at levels of about 10
7 had
a D value of 2.61 minutes with a z of 8.4
◦ C, indicating that the crabmeat pasteurization protocol of
30 minutes at 85
◦ C was adequate to render the product safe from this organism.
55 Processing frankfurters to an internal temperature of 160
◦ F (71.1
◦ C) has been shown to effect at least a 3-log cycle
reduction of strain Scott A.
133 The cooking of meat products to an internal temperature of 70
◦ C for 2
minutes will destroy L. monocytogenes.
43,83,89
In liquid whole egg (LWE) exposed to 60
◦ C for 3.5 minutes, the calculated D value for strain Scott
A was 2.1 minutes.
5 However, the same strain in LWE + 10% NaCl heated at 63
◦ C for 3.5 minutes
had a D of 13.7 minutes, whereas LWE + 10% sucrose gave a D of 1.9 minutes under the same
conditions. The 10% NaCl lowered the a w from 0.98 to 0.915, which could account in part for the
higher D value. Higher D values were found for seven serovars incubated at 4
◦ C for 5 days followed
by 37
◦ C incubation for 7 days.
119 In saline, D 60 values were 0.72–3.1 and D 62 were 0.30–1.3 minutes.
In the sausage-type meat employed by Farber,
36 the D value at 62
◦ C was 61 seconds, but when
cure ingredients were added, the D value increased to 7.1 minutes, indicating some heat-protective
effects of the cure compounds, which consisted of nitrite, dextrose, lactose, corn syrup, and 3% (w/v)
NaCl. An approximate doubling in D value in ground beef containing 30% fat, 3.5% NaCl, 200 ppm
nitrite, and 300 ppm nitrate was found by Mackey et al.
88 who attributed the increased heat resistance
to the 3.5% NaCl. The destruction of strain Scott A by microwave cooking was investigated by Lund
et al.,
86 where more than 10
7 cells/g were placed in chicken stuffing and 10
6 –10
7 /g on chicken skin. By
use of a home-type microwave unit, the adequacy of heating to an internal temperature of 70
◦ C for 1
minute was shown to give a 6-log reduction in numbers. The thermal destruction of L. monocytogenes
is similar to that of most other bacteria relative to pH of suspending menstrum where resistance is
higher at pH values closer to 7.0 than values in the acid range. This was demonstrated in cabbage
juice, where D values were higher at a pH of 5.6 than at 4.6.
7
In a study of rainbow trout from retail markets in east Tennessee, 51% of the 74 samples were
positive for L. monocytogenes.
31 The log 10 means for aerobic plate counts (APC) and coliforms were
6.2 and 3.2, respectively, and the higher percentage of L. monocytogenes was associated with samples
that had the highest APC and coliform numbers. See Chapters 4, 5 and 9 for numbers of listeriae in a
variety of foods.
whole milk, skim milk, and 11% nonfat milk solids, Donnelly and Briggs
27 found that composition
did not affect heat destruction and that at 62.7
◦ C, the D values were 60 seconds or less. The five
strains employed included serotypes 1, 3, and 4. When milk that was naturally contaminated with a
serotype 1 strain at around 10
4 /ml was subjected to an HTST protocol at temperatures ranging from
60
◦ to 78
◦ C, no viable cells could be detected at processing temperatures of 69
◦ C or above.
37 In
their review of the early studies on the thermal resistance of L. monocytogenes in milk, Mackey and
Bratchell
89 concluded that normal pasteurization procedures will inactivate this organism but that the
margin of safety is greater for the vat protocol (LTLT) than the HTST protocol. Their mathematical
model predicted a 39 D reduction for vat and a 5.2 D reduction for HTST.
Nondairy Products
For liquid whole egg and meat products, D values are generally higher than for milk, a fact not
unpredicted, considering the effect of proteins and lipids on the thermal resistance of microorganisms
(discussed in Chapter 17). For one strain of L. monocytogenes isolated from a chicken product, D
values at 70
◦ C were 6.6–8.4 seconds; they were essentially the same in beef and two poultry meats.
88
In one study, viable cells could be recovered by enrichments from eight of nine samples following
heating in ground beef to 70
◦ C.
8 In a study of blue crabmeat, strain Scott A at levels of about 10
7 had
a D value of 2.61 minutes with a z of 8.4
◦ C, indicating that the crabmeat pasteurization protocol of
30 minutes at 85
◦ C was adequate to render the product safe from this organism.
55 Processing frankfurters to an internal temperature of 160
◦ F (71.1
◦ C) has been shown to effect at least a 3-log cycle
reduction of strain Scott A.
133 The cooking of meat products to an internal temperature of 70
◦ C for 2
minutes will destroy L. monocytogenes.
43,83,89
In liquid whole egg (LWE) exposed to 60
◦ C for 3.5 minutes, the calculated D value for strain Scott
A was 2.1 minutes.
5 However, the same strain in LWE + 10% NaCl heated at 63
◦ C for 3.5 minutes
had a D of 13.7 minutes, whereas LWE + 10% sucrose gave a D of 1.9 minutes under the same
conditions. The 10% NaCl lowered the a w from 0.98 to 0.915, which could account in part for the
higher D value. Higher D values were found for seven serovars incubated at 4
◦ C for 5 days followed
by 37
◦ C incubation for 7 days.
119 In saline, D 60 values were 0.72–3.1 and D 62 were 0.30–1.3 minutes.
In the sausage-type meat employed by Farber,
36 the D value at 62
◦ C was 61 seconds, but when
cure ingredients were added, the D value increased to 7.1 minutes, indicating some heat-protective
effects of the cure compounds, which consisted of nitrite, dextrose, lactose, corn syrup, and 3% (w/v)
NaCl. An approximate doubling in D value in ground beef containing 30% fat, 3.5% NaCl, 200 ppm
nitrite, and 300 ppm nitrate was found by Mackey et al.
88 who attributed the increased heat resistance
to the 3.5% NaCl. The destruction of strain Scott A by microwave cooking was investigated by Lund
et al.,
86 where more than 10
7 cells/g were placed in chicken stuffing and 10
6 –10
7 /g on chicken skin. By
use of a home-type microwave unit, the adequacy of heating to an internal temperature of 70
◦ C for 1
minute was shown to give a 6-log reduction in numbers. The thermal destruction of L. monocytogenes
is similar to that of most other bacteria relative to pH of suspending menstrum where resistance is
higher at pH values closer to 7.0 than values in the acid range. This was demonstrated in cabbage
juice, where D values were higher at a pH of 5.6 than at 4.6.
7
In a study of rainbow trout from retail markets in east Tennessee, 51% of the 74 samples were
positive for L. monocytogenes.
31 The log 10 means for aerobic plate counts (APC) and coliforms were
6.2 and 3.2, respectively, and the higher percentage of L. monocytogenes was associated with samples
that had the highest APC and coliform numbers. See Chapters 4, 5 and 9 for numbers of listeriae in a
variety of foods.