Foodborne Listeriosis
611
ripening, and some strains attained levels of 10
6 –10
7 after 65 days of ripening.
110 With an inoculum
of 5 × 10
2 /g and storage at 4
◦ C, L. monocytogenes survived in cold-pack cheese for a mean of 130
days in the presence of 0.30% sorbic acid.
109 On the other hand, in manufactured and stored nonfat
dry milk, a 1–1.5 log reduction in numbers of L. monocytogenes occurred during spray drying, and
more than a 4-log decrease in colony-forming units occurred within 16 weeks when held at 25
◦ C.
30
In ground beef, an inoculum of 10
5 –10
6 remained unchanged through 14 days at 4
◦ C,
64 and inocula
of L. monocytogenes of 10
3 or 10
5 remained unchanged in ground beef and liver for over 30 days,
although the standard plate counts (SPC) increased threefold to sixfold during this time.
114 When
added to a Finnish sausage mix that included 120 ppm NaNO 2 and 3% NaCl, the initial numbers of
L. monocytogenes decreased only by about 1 log during a 21-day fermentation period.
66 When five
strains of L. monocytogenes were added to eight processed meats that were stored at 4.4
◦ C for up to
12 weeks, the organisms survived on all products and increased in numbers by 3 to 4 logs in most.
49
Best growth occurred in chicken and turkey products, due in part to the higher initial pH of these
products. In vacuum-packaged beef in a film with barrier properties of 25–30 ml/m
2 /24 hours/101
kPa, an inoculated strain of L. monocytogenes increased about 4 log cycles on the fatty tissue of strip
loins in 16 days and by about 3 log cycles in 20 days on lean meat when held between 5
◦ and 5.5
◦ C.
49,51
In meat, cheese, and egg ravioli stored at 5
◦ C, a 3 × 10
5 -cfu/g inoculum of strain Scott A survived 14
days.
6 Lettuce and lettuce juice supported the growth of L. monocytogenes held at 5
◦ C for 14 days in
one study, and the organism was recovered from two uninoculated lettuce samples.
122
The studies noted are typical of others and show that the overall resistance of L. monocytogenes in
foods is consistent with its persistence in many nonfood environmental specimens.
REGULATORY STATUS OF L. MONOCYTOGENES IN FOODS
Some countries have established legal limits on the numbers of organisms that are permissible in
foods, especially ready-to-eat products, whereas others have suggested guidelines or criteria that do
not have legal standing.
The United States government has the most rigid policy whereby L. monocytogenes has been
designated as an “adulterant.” This means that any ready-to-eat food that contains this organism can
be considered adulterated and, thus, be subject to recall and/or seizure. The U.S. requirement is the
absence of the organism in 50-g samples. Zero tolerance generally means the absence of the organism
in 25-g samples, which is equivalent to n = 5, c = 0 in a sampling plan (see Chapter 20 for an
explanation). The following summaries were believed to be in effect late in 1995, but it should be
understood that some may have changed.
The European Community (EC) directive on milk and milk-based products specifies zero tolerance
for soft cheeses, and absence of the organism in 1 g of other products.
Great Britain’s provisional guidelines for some ready-to-eat foods establishes four quality groups
based on numbers of L. monocytogenes. Not detected in 25 g is satisfactory; >10
2 /25 g is fairly
satisfactory; 10
2 –10
3 is unsatisfactory; and numbers >10
3 make the product unacceptable.
46
The proposed Canadian compliance criteria of 1993 placed ready-to-eat foods into three categories
relative to L. monocytogenes actions. Category 1 includes products linked to outbreaks, category 2
includes those that have a self-life >10 days, and category 3 includes those that either support growth
with a self-life ≤10 days or those that do not support growth. Among the latter are those that fit one
or more of the following: pH 5.0–5.5 and a w <0.95; pH <5.0 regardless of a w ; a w ≥ 0.92 regardless
of pH; and frozen foods.
77 Recalls of category 3 foods require numbers >10
2 /g.
611
ripening, and some strains attained levels of 10
6 –10
7 after 65 days of ripening.
110 With an inoculum
of 5 × 10
2 /g and storage at 4
◦ C, L. monocytogenes survived in cold-pack cheese for a mean of 130
days in the presence of 0.30% sorbic acid.
109 On the other hand, in manufactured and stored nonfat
dry milk, a 1–1.5 log reduction in numbers of L. monocytogenes occurred during spray drying, and
more than a 4-log decrease in colony-forming units occurred within 16 weeks when held at 25
◦ C.
30
In ground beef, an inoculum of 10
5 –10
6 remained unchanged through 14 days at 4
◦ C,
64 and inocula
of L. monocytogenes of 10
3 or 10
5 remained unchanged in ground beef and liver for over 30 days,
although the standard plate counts (SPC) increased threefold to sixfold during this time.
114 When
added to a Finnish sausage mix that included 120 ppm NaNO 2 and 3% NaCl, the initial numbers of
L. monocytogenes decreased only by about 1 log during a 21-day fermentation period.
66 When five
strains of L. monocytogenes were added to eight processed meats that were stored at 4.4
◦ C for up to
12 weeks, the organisms survived on all products and increased in numbers by 3 to 4 logs in most.
49
Best growth occurred in chicken and turkey products, due in part to the higher initial pH of these
products. In vacuum-packaged beef in a film with barrier properties of 25–30 ml/m
2 /24 hours/101
kPa, an inoculated strain of L. monocytogenes increased about 4 log cycles on the fatty tissue of strip
loins in 16 days and by about 3 log cycles in 20 days on lean meat when held between 5
◦ and 5.5
◦ C.
49,51
In meat, cheese, and egg ravioli stored at 5
◦ C, a 3 × 10
5 -cfu/g inoculum of strain Scott A survived 14
days.
6 Lettuce and lettuce juice supported the growth of L. monocytogenes held at 5
◦ C for 14 days in
one study, and the organism was recovered from two uninoculated lettuce samples.
122
The studies noted are typical of others and show that the overall resistance of L. monocytogenes in
foods is consistent with its persistence in many nonfood environmental specimens.
REGULATORY STATUS OF L. MONOCYTOGENES IN FOODS
Some countries have established legal limits on the numbers of organisms that are permissible in
foods, especially ready-to-eat products, whereas others have suggested guidelines or criteria that do
not have legal standing.
The United States government has the most rigid policy whereby L. monocytogenes has been
designated as an “adulterant.” This means that any ready-to-eat food that contains this organism can
be considered adulterated and, thus, be subject to recall and/or seizure. The U.S. requirement is the
absence of the organism in 50-g samples. Zero tolerance generally means the absence of the organism
in 25-g samples, which is equivalent to n = 5, c = 0 in a sampling plan (see Chapter 20 for an
explanation). The following summaries were believed to be in effect late in 1995, but it should be
understood that some may have changed.
The European Community (EC) directive on milk and milk-based products specifies zero tolerance
for soft cheeses, and absence of the organism in 1 g of other products.
Great Britain’s provisional guidelines for some ready-to-eat foods establishes four quality groups
based on numbers of L. monocytogenes. Not detected in 25 g is satisfactory; >10
2 /25 g is fairly
satisfactory; 10
2 –10
3 is unsatisfactory; and numbers >10
3 make the product unacceptable.
46
The proposed Canadian compliance criteria of 1993 placed ready-to-eat foods into three categories
relative to L. monocytogenes actions. Category 1 includes products linked to outbreaks, category 2
includes those that have a self-life >10 days, and category 3 includes those that either support growth
with a self-life ≤10 days or those that do not support growth. Among the latter are those that fit one
or more of the following: pH 5.0–5.5 and a w <0.95; pH <5.0 regardless of a w ; a w ≥ 0.92 regardless
of pH; and frozen foods.
77 Recalls of category 3 foods require numbers >10
2 /g.