Heat Resistance in Liquids of Enterococcus spp., Listeria spp., Escherichia coli, Yersinia enterocolitica, Salmonella spp. and Campylobacter spp.

Acta vet. scand. 2003, 44, 1-19.
Review article
Heat Resistance in Liquids of Enterococcus spp.,
Listeria spp., Escherichia coli, Yersinia enterocolitica,
Salmonella spp. and Campylobacter spp.
By S. Sörqvist
Department of Food Hygiene, Faculty of Veterinary Medicine, Swedish University of Agricultural Sciences,
Uppsala, Sweden.
Sörqvist S: Heat resistance in liquids of Enterococcus spp., Listeria spp., Es-
cherichia coli, Yersinia enterocolitica, Salmonella spp. and Campylobacter spp. Acta
vet. scand. 2003, 44, 1-19. – The aim of the work was to collect, evaluate, summarize
and compare heat resistance data reported for Campylobacter, Enterococcus, Es-
cherichia, Listeria, Salmonella and Yersinia spp. The work was limited to resistance in
liquids with pH values 6-8. Results obtained under similar experimental conditions were
sought. Thermal destruction lines for the various bacterial groups studied were con-
structed using log
D values and treatment temperatures. There was a good linear rela-
10
tionship between log
D and temperature with Escherichia coli, listerias and salmonel-
10
las. For campylobacters, enterococci and yersinias the relationships were weaker but,
nevertheless, present. Using the slopes of the lines and their 95% con?dence limits, z
values and their 95% con?dence limits were calculated. z values were compared with z
values obtained from reports. The equations for the lines were also used for calculation
of predicted means of D values at various treatment temperatures. 95% con?dence lim-
its on predicted means of D values and on predicted individual D values were also cal-
culated. Lines and values are shown in ?gures and tables. Differences in heat resistance
noted between and within the bacterial groups studied are discussed.
Campylobacter jejuni/coli; Enterococcus faecalis; Enterococcus faecium; Es-
cherichia coli; Listeria innocua; Listeria ivanovii; Listeria monocytogenes; Listeria
seeligeri; Listeria welshimeri; Salmonella; Yersinia enterocolitica; thermal resis-
tance; in?uencing factors; methods of determination; differences between species;
differences between strains.
Introduction
Microbiologists now and then need heat resis-
sults reported must be taken into consideration
tance data for various microorganisms. In the
(for general information on in?uencing factors,
literature, data of this kind are frequently based
see e.g. Hansen & Riemann 1963, Stumbo
on reports from few investigations. To collect
1973, P?ug & Holcomb 1983). Furthermore,
the data required, however, may be a laborious
the presentations of results often differ essen-
and time-consuming task for the individual
tially.
user. The literature is generally extensive and
The aim of the present work was to collect,
many factors that may have in?uenced the re-
summarize, evaluate and compare heat resis-
Acta vet. scand. vol. 44 no. 1-2, 2003

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2
S. Sörqvist
tance data reported for Campylobacter, Entero-
Murano & Pierson 1992, 1993, Clavero &
coccus, Escherichia, Listeria, Salmonella and
Beuchat 1995, Clementi et al. 1995, Jackson et
Yersinia spp. As it was well known that consid-
al. 1996, Blackburn et al. 1997, Williams &
erably more heat resistance results were pub-
Ingham 1997, George & Peck 1998, Kaur et al.
lished from investigations with liquids than
1998) and enterococci have recently emerged
from those with other heating menstrua, it was
as one of the leading causes of nosocomial,
considered appropriate to base the work on re-
non-food-associated, infections (Kearns et al.
sults obtained in liquids. Moreover, results of
1995).
this kind could be expected to re?ect the inher-
ent heat resistance of the bacteria investigated
Experimental conditions
better than those obtained in more complex
Growth of test bacteria
heating menstrua.
In most cases the bacteria were grown in con-
Reports published until 2000 were studied.
ventional media. In some investigations the
Data produced under experimental conditions
growth media were milk, liquid egg products or
as similar as possible were sought. This meant
clari?ed cabbage juice. The pH values of the
that results from some kinds of experiments
media were given in some cases. The values
were excluded. The various types of excluded
varied from 5.6 to 7.4. Enterococci, E. coli, lis-
data are given below under the different sub-
terias and salmonellas were incubated aerobi-
headings in Experimental conditions. It should
cally at 30-37 °C and Y. enterocolitica aerobi-
be mentioned here that extensive reviews of
cally at 25-37 °C. Campylobacters were grown
heat resistance data reported for Escherichia
microaerobically at 35-43 °C. In the great ma-
coli O157:H7, Listeria monocytogenes and Sal-
jority of cases the bacteria were incubated for
monella spp. have been published recently by
12-48 h, i.e. they could be considered to have
Stringer et al. (2000), Doyle et al. (2001) and
reached the late logarithmic or stationary
Doyle & Mazzotta (2000), respectively. How-
growth phase. At stationary growth phase, bac-
ever, the aims and the selections and analyses of
terial heat resistance is at a maximum (Elliker
data in these reviews differ from those in the
& Frazier 1938, White 1953, Krishna Iyengar
present work.
et al. 1957, Lemcke & White 1959, Beuchat &
Lechowich 1968, Ng et al. 1969, Humphrey et
Bacteria
al. 1990, Jackson et al. 1996, Lou & Yousef
The work deals with the following bacteria:
1996, Kaur et al. 1998, Pagán et al. 1998,
Campylobacter jejuni/coli, Enterococcus fae-
1999).
calis, Enterococcus faecium, Escherichia coli,
Heat resistance results obtained for bacteria
Listeria innocua, Listeria ivanovii, Listeria
grown under carbon, glucose or nitrogen star-
monocytogenes, Listeria seeligeri, Listeria
vation or other stress conditions (see e.g. Ng et
welshimeri, Salmonella spp. and Yersinia ente-
al. 1969, Jenkins et al. 1988, Lou & Yousef
rocolitica. Some of these bacteria are well-
1996) were not used in the present work.
known food-associated human pathogens, oth-
ers are utilized - enterococci and E. coli - or
Conditions between growth and heat treatment
proposed - L. innocua (Foegeding & Stanley
Results recorded for bacteria subjected to stress
1991, Fairchild & Foegeding 1993) - as indica-
conditions prior to heat treatment were not
tors. Some types of E. coli also appear as food-
used: sublethal heat shock (see e.g. Mackey &
linked human pathogens (Morgan et al. 1988,
Derrick 1986, 1987b, 1990, Bunning et al.
Acta vet. scand. vol. 44 no. 1-2, 2003

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Heat resistance of bacteria
3
1990, 1992, Murano & Pierson 1992, 1993,
?asks or cups placed with the menstruum levels
Boutibonnes et al. 1993, Humphrey et al.
under the water level and in some cases shaken,
1993a, Flahaut et al. 1996, 1997, Shenoy &
and heating using pasteurizers, two-phase slug
Murano 1996, alkaline stress (Humphrey et al.
?ow heat exchangers (Bradshaw et al. 1985,
1991, 1993b), acid stress (Farber & Pagotto
Bunning et al. 1986, 1988, 1992, Konvincic et
1992, Leyer & Johnson 1993, Williams & Ing-
al. 1991, Clementi et al. 1995), submerged-coil
ham 1998), osmotic stress (Jørgensen et al.
heating apparatuses (Anderson et al. 1991, Jør-
1995) or other types of stress (see e.g. Bouti-
gensen et al. 1995, 1996, Blackburn et al. 1997,
bonnes et al. 1993, Flahaut et al. 1996, 1997).
Juneja et al. 1998), thermoresistometers (Read
et al. 1968, Pagán et al. 1998, 1999) and an "at-
Heating menstrua
temperated dilution blank method" (Magnus et
Heating menstrua used were milk and liquid
al. 1986, 1988).
milk products, broths, physiological saline and
Results from experiments using rising heating
other salt solutions, liquid egg products, diluted
temperatures (Tsuchido et al. 1974, Mackey &
soups, scalding waters used at chicken or pig
Derrick 1987a, Quintavalla et al. 1988, Steph-
slaughter, and some other liquids. Heat resis-
ens et al. 1994) were excluded.
tance results obtained in menstrua with pH val-
ues of approx. 6-8 were used in the present
Recovery of heat-treated bacteria
work, as the bacterial species investigated are
In the great majority of cases the recovery of
known to have their maximum heat resistances
heat-treated bacteria was performed on agar
in this pH range ( see e.g. Anellis et al. 1954,
plates. Enterococci and E. coli were incubated
Krishna Iyengar et al. 1957, White 1963,
aerobically at 30-37 °C for 24 h-7 days, liste-
Garibaldi et al. 1969a, Humphrey et al. 1981,
rias, salmonellas and Y. enterocolitica aerobi-
Sanz Pérez et al. 1982, Okrend et al. 1986,
cally at 25-37 °C for 24 h-7 days and campy-
Blackburn et al. 1997, Pagán et al. 1998, 1999).
lobacters microaerobically at 37-43 °C for
Results from experiments where salts, fats, car-
24-72 h. In some studies anaerobic recovery
bohydrates, proteins or other substances were
was used: L. monocytogenes (Knabel et al.
added to the heating menstrua with the aim of
1990, George et al. 1998), E. coli (Murano &
in?uencing the heat resistance of the test bacte-
Pierson 1992, 1993, Gadzella & Ingham 1994,
ria were excluded (see e.g. Lategan & Vaughn
Blackburn et al. 1997, George et al. 1998,
1964, Calhoun & Frazier 1966, Baird-Parker et
George & Peck 1998) and salmonellas (Xavier
al. 1970, Goepfert et al. 1970, Vrchlabski &
& Ingham 1993, Blackburn et al. 1997, George
Leistner 1970, Corry 1974, Anderson et al.
et al. 1998). Most Probable Number (MPN)
1991, Palumbo et al. 1995, Blackburn et al.
techniques were applied in some investigations.
1997, Knight et al. 1999).
Procedures for repair of heat-injured bacteria
were studied by Ahmad et al. (1978), Northolt
Heat treatment
et al. (1988), Meyer & Donnelly (1992),
Various methods of heat treatment were ap-
Sörqvist (1993, 1994) and George et al.
plied, e.g. heating in water baths using glass
(1998).
capillary tubes, sealed glass tubes, glass am-
Results from experiments where heat-treated
poules or polythene pouches completely im-
bacteria were recovered on selective or other
mersed in the water, test tubes placed with the
media known to inhibit growth of heat-injured
water level to the bases of the test tube plugs,
cells were excluded.
Acta vet. scand. vol. 44 no. 1-2, 2003

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4
S. Sörqvist
Ta bl e 1 . z values reported from investigations where the experimental conditions laid down in this study
were ful?lled.
Bacterium/
z* values (°C)
Bacterial group
Range
Mean ± SD
No. of
References
values
Enterococcus
3.63-12.82
8.4 ± 2.5
14
Sanz Pérez et al. (1982), Magnus et al. (1986),
faecium
Quintavalla et al. (1988), Gordon & Ahmad (1991),
Simpson et al. (1994), Mulak et al. (1995)
Enterococcus
2.24-9.06
6.0 ± 2.5
10
Gardner & Patton (1975), Sanz Pérez et al. (1982),
faecalis
Magnus et al. (1986), Quintavalla et al. (1988)
Listeria
4.65-6.9 5.8
± 0.8
8
Quintavalla & Barbuti (1989), Foegeding &
innocua
Stanley (1991), Fairchild & Foegeding (1993),
Palumbo et al. (1995)
Listeria
4.30-11.45
6.1 ± 1.2
85
Bradshaw et al. (1985, 1987b, 1991), Beuchat et al.
monocytogenes
(1986), Bunning et al. (1986, 1988), Donnelly &
Briggs (1986), El-Shenawy et al. (1989), Lemaire et
al. (1989), Quintavalla & Barbuti (1989), Foegeding
& Leasor (1990), Linton et al. (1990), Foegeding
& Stanley (1991), Quintavalla & Campanini
(1991), Fairchild & Foegeding (1993), Sörqvist
(1993, 1994), Bartlett & Hawke (1995), Palumbo
et al. (1995), Muriana et al. (1996), Schuman &
Sheldon (1997), Casadei et al. (1998),
Pagán et al. (1998), Rowan & Anderson (1998),
Knight et al. (1999)
Listeria ivanovii
6.3-6.6
6.5 ± 0.2
2
Bradshaw et al. (1991)
Listeria seeligeri
6.4-6.9
6.7 ± 0.3
2
Bradshaw et al. (1991)
Listeria welshimeri
6.1-6.9
6.5 ± 0.5
2
Bradshaw et al. (1991)
Escherichia
3.4-6.0
5.1 ± 0.8
11
Read et al. (1961), Dega et al. (1972), Morgan et al.
coli
(1988), Clementi et al. (1995), Blackburn et al.
(1997), Williams & Ingham (1998)
Yersinia
4.0-5.78
4.8 ± 0.6
10
Lovett et al. (1982), Sörqvist (1989), Sörqvist &
enterocolitica
Danielsson-Tham (1990), Pagán et al. (1999)
Salmonella spp.
3.24-9.5
5.5 ± 1.7
36
Anellis et al. (1954), Garibaldi et al. (1969b),
(except Salm.
Dega et al. (1972), Gibson (1973), Bradshaw et al.
senftenberg 775W)
(1987a), Sörqvist & Danielsson-Tham (1990),
Shah et al. (1991), Xavier & Ingham (1993), Wolfson
& Sumner (1994) Palumbo et al. (1995), Blackburn
et al. (1997), Schuman & Sheldon (1997),
Humpheson et al. (1998), Michalski et al. (1999)
Salm.
5.3-6.8
6.0 ± 0.4
13
Anellis et al. (1954), Thomas et al. (1966),
senftenberg 775W
Baird-Parker et al. (1970), Gibson (1973)
Campylobacter
2.8-5.81
4.8 ± 0.7
14
Blankenship & Craven (1982), Waterman (1982),
jejuni/coli
Sörqvist (1989), Sörqvist & Danielsson-Tham (1990)
* The z value is the number of degrees of temperature change needed to change the D value by a factor of 10 (The term D
value, see Table 3).
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Heat resistance of bacteria
5
Ta bl e 2 . z values obtained using the slopes of thermal destruction lines constructed in the study and their 95%
con?dence limits and, for comparison, summaries of reported and calculated z values.
Bacterium/
z values (°C)*
Bacterial group
Obtained value and its
Reported and calculated** values
95% con?dence interval
Range
Mean ± SD
No. of values
Enterococcus faecium
9.6 (8.8 - 10.5)
3.63 - 14.3
10.2 ± 3.3
24
Enterococcus faecalis
9.5 (8.5 - 10.8)
2.24 - 14.2
8.1 ± 3.2
36
Listeria innocua
5.0 (4.5 - 5.6)
4.65 - 7.3
6.0 ± 0.9
9
Listeria monocytogenes
5.7 (5.6 - 5.9)
4.30 - 11.45
6.3 ± 1.3
103
Listeria ivanovii,
L.seeligeri, L. welshimeri †
6.4 (6.1 - 6.7)
6.1 - 6.9
6.5 ± 0.3
6
Escherichia coli
6.0 (5.9 - 6.1)
3.2 - 9.1
5.4 ± 1.5
33
Yersinia enterocolitica
6.7 (6.0 - 7.7)
4.0 - 13.7
6.6 ± 2.7
20
Salmonella spp. (except
5.2 (5.1 - 5.3)
3.24 - 9.5
5.1 ± 1.6
63
Salm. senftenberg 775W)
Salmonella senftenberg 775W
5.8 (5.4 - 6.4)
4.5 - 9.1
6.2 ± 1.1
16
Campylobacter jejuni/coli
6.4 (5.8 - 7.0)
2.8 - 8.0
5.5 ± 1.1
24
** Calculated z values were ?gured out from reported or calculated D values (see Types of collected data and statistical
analysis) and reported treatment temperatures.
† Listeria ivanovii, L. seeligeri and L. welshimeri are taken together.
Types of collected data and statistical
the line is log
D = a - bt, where D is the deci-
10
analysis
mal reduction time in s, a the intercept, -b the
D and z values were collected from the studied
slope and t the treatment temperature in °C. The
literature. The D value is the time of heat treat-
degree of linear relationship between the tem-
ment required at a certain temperature to de-
peratures used and the logarithms of D values
stroy 90% of the bacterial cells, and the z value
recorded was expressed by the coef?cient of
is the number of degrees of temperature change
correlation, r (Colton 1974). Using the absolute
needed to change the D value by a factor of 10
and inverse values of the slope and its 95% con-
(Stumbo 1973). When not reported, D values
?dence limits, the z value and its 95% con?-
were, where possible, calculated from bacterial
dence limits were calculated (Stumbo 1973,
counts and periods of time of heat treatment
Colton 1974).
given in texts, tables or ?gures. Some z values
95% con?dence limits on predicted means
were worked out from reported or calculated D
(Colton 1974) of D values were calculated (the
values and reported treatment temperatures.
predicted mean is the same as D in the equa-
For each of the bacterial species/groups studied,
tion). 95% con?dence limits on predicted indi-
the log
of D values recorded were plotted vs
vidual D values (Colton 1974) were also ?g-
10
temperature and a thermal destruction line
ured out (From a practical point of view it may
(Stumbo 1973) was ?tted using the method of
be more interesting to know these limits than
least squares (Colton 1974). The equation for
those on predicted means).
Acta vet. scand. vol. 44 no. 1-2, 2003

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6
S. Sörqvist
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Acta vet. scand. vol. 44 no. 1-2, 2003

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Heat resistance of bacteria
7
Fi g u r e 1 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Enterococcus faecium. The equation for the line is log
D = 9.3080 - 0.10412t (r = -0.84748; total number of log
D values =
10
10
195). The 95% con?dence limits on predicted individual log
D values are shown by (- -). The ?gure is based on data from: Greenberg
10
& Silliker (1961), Zivanovic et al. (1965), Ienistea et al. (1970), Vrchlabsky & Leistner (1970), Sanz Pérez et al. (1982), Magnus et al.
(1986, 1988), Quintavalla et al. (1988), Gordon & Ahmad (1991), Kornacki & Marth (1992), Patel & Wilbey (1994), Simpson et al.
(1994), Kearns et al. (1995), Mulak et al. (1995), Renner & Peters (1999).
Fi g u r e 2 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Enterococcus faecalis. The equation for the line is log
D = 8.9359 - 0.10531t (r = -0.72968; total number of log
D values =
10
10
244). The 95% con?dence limits on predicted individual log
D values are shown by (- -). The ?gure is based on data from: Richards
10
& White (1949), White (1953), Krishna Iyengar et al. (1957), White (1963), Zivanovic et al. (1965), Beuchat & Lechowich (1968), Clark
et al. (1968), Ienistea et al. (1970), Shannon et al. (1970), Vrchlabsky & Leistner 1970), Dabbah et al. (1971a, c), Gardner & Patton
(1975), Sanz Pérez et al. (1982), Magnus et al. (1986, 1988), Quintavalla et al. (1988), Boutibonnes et al. (1993), Kearns et al. (1995),
Flahaut et al. (1996, 1997).
Fi g u r e 3 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Listeria monocytogenes. The equation for the line is log
D = 12.3787 - 0.17401t (r = -0.95631; total number of log
D values
10
10
= 474). The 95% con?dence limits on predicted individual log
D values are shown by (- -). The ?gure is based on data from: Bradshaw
10
et al. (1985), Beuchat et al. (1986), Bunning et al. (1986), Donnelly & Briggs (1986), Bradshaw et al. (1987b), Donnelly et al. (1987),
Fernández Garayzabal et al. (1987), Bunning et al. (1988), Farber et al. (1988), Golden et al. (1988), Northolt et al. (1988), Steinmeyer
(1988), El-Shenawy et al. (1989), Fedio & Jackson (1989), Lemaire et al. (1989), Quintavalla & Barbuti (1989), Suárez Fernández et
al. (1989), Boyle et al. (1990), Bunning et al. (1990), Foegeding & Leasor (1990), Knabel et al. (1990), Linton et al. (1990), Mackey et
al. (1990), Anderson et al. (1991), Bradshaw et al. (1991), Foegeding & Stanley (1991), Konvincic et al. (1991), McKenna et al. (1991),
Quintavalla & Campanini (1991), Bunning et al. (1992), Farber & Pagotto (1992), Holsinger et al. (1992), Meyer & Donnelly (1992),
Fairchild & Foegeding (1993), Sörqvist (1993, 1994), Stephens et al. (1994), Bartlett & Hawke (1995), Jørgensen et al. (1995, 1996),
Palumbo et al. (1995, 1996), Lou & Yousef (1996), Muriana et al. (1996), Patchett et al. (1996), Schuman & Sheldon (1997), Casadei
et al. (1998), George et al. (1998), Juneja et al. (1998), Pagán et al. (1998), Rowan & Anderson (1998), Knight et al. (1999).
Fi g u r e 4 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Escherichia coli. The equation for the line is log
D = 11.6471 - 0.16768t (r = -0.97349; total number of log
D values = 332).
10
10
The 95% con?dence limits on predicted individual log D values are shown by (- -). Data used are from: Elliker & Frazier (1938), Katzin
10
et al. (1943), Solowey et al. (1948), Chambers et al. (1957). Read et al. (1957), Lemcke & White (1959), Read et al. (1961), Calhoun &
Frazier (1966), Evans et al. (1970), Goepfert et al. (1970), Dabbah et al. (1971c), Dega et al. (1972), Tsuchido et al. (1974), Ahmad et
al. (1978), Katsui et al. (1981), Yamamori & Yura (1982), D´Aoust et al. (1988), Jenkins et al. (1988), Morgan et al. (1988), Murano &
Pierson (1992, 1993), Gadzella & Ingham (1994), Ahmed & Conner (1995), Clavero & Beuchat (1995, 1996), Clementi et al. (1995),
Jackson et al. (1996), Teo et al. (1996), Blackburn et al. (1997), Williams & Ingham (1997, 1998), George et al. (1998), George & Peck
(1998), Kaur et al. (1998), Semanchek & Golden (1998).
Thermal destruction line for an unusually heat-resistant strain of E. coli reported by Holland & Dahlberg (1940) is also shown (- · -).
Fi g u r e 5 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Yersinia enterocolitica. The equation for the line is log
D = 10.4176 - 0.14896t (r = -0.86082; total number of log
D values =
10
10
88). The 95% con?dence limits on predicted individual log
D values are shown by (- -). The ?gure is based on data from: Hanna et
10
al. (1977), Francis et al. (1980), Norberg (1981), Lovett et al. (1982), D´Aoust et al. (1988), Sörqvist (1989), Sörqvist & Danielsson-
Tham (1990), Toora et al. (1992), Shenoy & Murano (1996), Pagán et al. (1999).
Fi g u r e 6 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Salmonella spp. The equation for the line is log
D = 12.9511 - 0.19282t (r = -0.92147; total number of log
D values = 647).
10
10
The 95% con?dence limits on predicted individual log
D values are shown by (- -). Data used are from: Solowey et al. (1948), Anellis
10
et al. (1954), Osborne et al. (1954), Lategan & Vaughn (1964), Davidson et al. (1966), Ng (1966), Thomas et al. (1966), Corry & Barnes
(1968), Read et al. (1968), Garibaldi et al. (1969a, b), Ng et al. (1969), Baird-Parker et al. (1970), Evans et al. (1970), Goepfert et al.
(1970), Rossebø (1970), Dabbah et al. (1971a, b), Moats et al. (1971), Dega et al. (1972), Gibson (1973), Corry (1974), Thompson et
al. (1979), Humphrey (1981), Humphrey et al. (1981), Mackey & Derrick (1986), Okrend et al. (1986), Bradshaw et al. (1987a), D´Aoust
et al. (1987), Mackey & Derrick (1987a, b), Baker (1990), Bunning et al. (1990), Humphrey et al. (1990), Humphrey (1990), Mackey &
Derrick (1990), Sörqvist & Danielsson-Tham (1990), Humphrey et al. (1991), Shah et al. (1991), Humphrey et al. (1993a, b), Leyer &
Johnson (1993), Xavier & Ingham (1993), Wolfson & Sumner (1994), Humphrey et al. (1995), Palumbo et al. (1995, 1996), Muriana et
al. (1996), Teo et al. (1996), Blackburn et al. (1997), Schuman & Sheldon (1997), George et al. (1998), Humpheson et al. (1998), Michal-
ski et al. (1999).
Thermal destruction line for the extremely heat-resistant Salm. senftenberg 775W is also shown (- · -) ; for references, see text.
Fi g u r e 7 . Heat resistance data (Mean ± SD) recorded at the different treatment temperatures used and ?tted thermal destruction line
(-) for Campylobacter jejuni/coli. The equation for the line is log
D = 10.3432 - 0.15717t (r = -0.89853; total number of log
D val-
10
10
ues = 112). The 95% con?dence limits on predicted individual log
D values are shown by (- -). The ?gure is based on data from: Doyle
10
& Roman (1981), Gill et al. (1981), Blankenship & Craven (1982), Christopher et al. (1982), Waterman (1982), Oosterom et al. (1983),
Humphrey & Cruickshank (1985), Okrend et al. (1986), Humphrey & Lanning (1987), D´Aoust et al. (1988), Sörqvist (1989), Sörqvist
& Danielsson-Tham (1990).
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8
S. Sörqvist
Ta bl e 3 . Heat resistance values at 4 temperatures for bacteria studied in the work. The values are based on re-
sults reported from investigations where the experimental conditions laid down in the work were ful?lled.
D* values (s)
95% con?dence interval
Bacterium/
Temperature
Mean
For the mean
For a predicted
Bacterial group**
(°C)
individual value
Enterococcus faecium
55
3813
3095-4697
1041-13969
60
1150
1017-1300
317-4166
65
347
315-381
96-1254
72
65
53-79
18-237
Enteroccus faecalis
55
1393
1089-1783
220-8816
60
415
361-476
66-2593
65
123
108-141
20-771
72
23
17-30
3.5-144
Listeria innocua
55
1635
1050-2549
474-5644
60
162
127-207
50-529
65
16
13-20
5.0-52
72
0.6 †
0.4-1.0
0.2-2.2
Listeria monocytogenes
55
643
577-715
150-2754
60
87
81-93
20-371
65
12
11-13
2.7-50
72
0.7
0.6-0.8
0.2-3.0
Escherichia coli
55
266
239-297
53-1338
60
39
35-42
8-194
65
5.6
5.1-6.2
1.1-28
72
0.4
0.3-0.5
0.1-1.9
Yersinia enterocolitica
55
168
124-227
23-1244
60
30
24-37
4.1-221
65
5.4
4.0-7.4
0.7-40
72
0.5
0.3-0.9
0.1-3.9
Salmonella spp.
55
222
208-237
64-771
(except Salm. senftenberg 775W)
60
24
23-26
7.0-84
65
2.6
2.3-2.9
0.8-9.1
72
0.1
0.1-0.2
0.1-0.4
Campylobacter jejuni/coli
55
50
44-57
13-190
60
8.2
6.5-10
2.1-32
65
1.3 †
0.9-2.0
0.3-5.4
72
0.1 †
0.1-0.2
0.1-0.5
* The D value is the time of heat treatment required at a certain temperature to destroy 90% of the bacterial cells.
** The bacteria are arranged according to their mean heat resistances at 60 and 65°C.
† Extrapolated value.
Acta vet. scand. vol. 44 no. 1-2, 2003

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Heat resistance of bacteria
9
Summaries of data
-0.72968 to -0.89853, recorded for Ent. fae-
Reported z values are summarized in Table 1.
calis, Ent. faecium, Y. enterocolitica and Camp.
Reported and calculated z values taken together
jejuni/coli indicate weaker but, nevertheless,
are given in Table 2, where z values ?gured out
good linear relationships (Colton 1974). The
in the work by means of the equation men-
following should be noted here: The number of
tioned, etc. are also shown. Thermal destruction
Y. enterocolitica strains investigated is low. The
lines for the bacteria studied, except those for L.
results reported, however, indicate that great
innocua, L. ivanovii, L. seeligeri and L. wel-
variation in heat resistance exists between
shimeri, are depicted in Figures 1-7, where 95%
strains of this species. As to enterococci, non-
con?dence limits on predicted individual log
logarithmic survivor curves were reported in
10
D values are also illustrated graphically. In
several works (Zivanovic et al. 1965, Dabbah et
Table 3, some D values at these limits are
al. 1971a,c, Sanz Pérez et al. 1982, Magnus et
shown for the seven bacterial groups and also
al. 1986, Gordon & Ahmad 1991, Boutibonnes
for L. innocua. Equations for the thermal de-
et al. 1993).
struction line of L. innocua and that of L.
ivanovii, L. seeligeri and L. welshimeri taken
Listeria monocytogenes
together, are given below under the headings
Mackey & Bratchell (1989) published a similar
Listeria innocua and Listeria ivanovii, L. see-
review of the heat resistance of L. monocyto-
ligeri and L. welshimeri, respectively.
genes. Equations were given for heat treatments
in: (a) various menstrua and (b) milk. The treat-
Comments and further information
ments in (b) had been performed by a sealed
D and r values
tube method (b1) or a slug ?ow heat exchanger
The order of death of bacteria subjected to heat
(b2). The equations for (a), (b1) and (b2) were
at a constant lethal temperature is often loga-
log
D = 10.888 - 0.14519t, log
D = 11.931 -
10
10
rithmic (Hansen & Riemann 1963, Stumbo
0.1635t and log
D = 10.126 - 0.1348t (D is in
10
1973, P?ug & Holcomb 1983), i.e. when the
s in the equations). The means of D values ob-
logarithm of survivors is plotted against the
tained by the 3 equations for 55, 60, 65 and
time of heating, the curve obtained, the survivor
72 °C are shown in Table 4. The means in (a),
curve, is a straight line. The D value can then
(b1) and (b2) except that in (b2) for 55 °C are
easily be calculated using the slope of the line.
higher than the corresponding ones (c) re-
Deviations from the logarithmic order of death,
corded for L. monocytogenes in the present
however, are rather frequent and non-logarith-
work (Table 3). The differences between (a) and
mic survivor curves of some different types are
(c) may, at least to some extent, be explained by
obtained (Hansen & Riemann 1963, Stumbo
the fact that some of the heating menstrua in (a)
1973, P?ug & Holcomb 1983). Deviations of
were solids. The differences between (b1) or
this kind often make determinations of D values
(b2) and (c) are therefore of greater interest, as
dif?cult.
all data for these 3 groups were obtained in li-
The r values, varying from -0.92147 to
quids. A probable explanation of these differ-
-0.99405, obtained for Salmonella spp., E. coli
ences is that heat resistance data for several
and the 3 Listeria groups indicate very good
"new" strains have been published later than the
linear relationships (Colton 1974) between the
review by Mackey & Bratchell (1989) and have
log
D values recorded and the treatment tem-
thus been included in the present work. Fur-
10
peratures used. The r values, varying from
thermore, the methods of determining the heat
Acta vet. scand. vol. 44 no. 1-2, 2003

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10
S. Sörqvist
Ta bl e 4 . D values for Listeria monocytogenes ac-
ported totally and the D values obtained at the
cording to the review by Mackey & Bratchell (1989).
individual treatment temperatures used were
Heating menstruum(-a)/
D** value (s)
few, 1-4. The most heat-resistant strain of the L.
Treatment method(s)
innocua strains investigated was reported by
55°C
60°C
65°C
72°C
Quintavalla & Barbuti (1989). D values deter-
(a) Various/Various
799*
150*
28*
2.7*
mined at 58, 60, 63 and 65 °C using a culture
(b1) Milk/ST
868
132
20
1.4†
medium as heating menstruum were 2.7 to 5.4
(b2) Milk/SF
515‡
109
23
2.6
times greater than the average D values found
** The D value is the time of heat treatment required at a
in the present work for L. monocytogenes at
certain temperature to destroy 90% of the bacterial cells.
these temperatures. Foegeding & Stanley
ST, sealed tubes; SF, slug ?ow heat exchanger.
*, †, ‡ Value calculated using the equation given by the
(1991) tested L. innocua strain ATCC 33091 in
authors for (a), (b1) and (b2) respectively.
buffer and in skim milk at 56, 60 and 66 °C. In
buffer, the D values were lower at 56 and 60 but
resistance of bacteria have been widely dis-
higher at 66 °C than the corresponding average
cussed in recent years and some improvements
values for L. monocytogenes. When L. innocua
or new procedures have been introduced. Fac-
PFEI (strain ATCC 33091 containing a plasmid
tors of this kind may also have contributed to
which did not alter its heat resistance) was
the differences.
tested in skim milk, all D values obtained at
these temperatures were higher, 1.5 to 2.1
Listeria innocua
times, than the values mentioned for L. mono-
The non-pathogenic L. innocua is of special in-
cytogenes. Palumbo et al. (1995) determined D
terest as it has, as mentioned, been proposed to
values for a L. innocua strain isolated from raw
be used as an indicator organism to evaluate
egg. The tests were performed in egg yolk. D
thermal processes for lethality to L. monocyto-
values obtained at 61.1, 63.3 and 64.4 °C were
genes. To function satisfactorily in this respect
2.5 to 2.9 times longer than the corresponding
it is desirable that the indicator has heat resis-
average values for L. monocytogenes. The re-
tance equal to or greater than the average heat
sults reported indicate that L. innocua may have
resistance of L. monocytogenes or, more desir-
greater average heat resistance than L. monocy-
ably, has heat resistance equal to that of the
togenes. However, as mentioned, only few heat
most resistant strains of this species. In the pre-
resistance results are reported for L. innocua
sent work, heat resistance results for L. innocua
and more research on this matter is required.
were found in 5 reports (Quintavalla & Barbuti
1989, Mackey et al. 1990, Foegeding & Stanley
Listeria ivanovii, L. seeligeri and L. welshimeri
1991, Fairchild & Foegeding 1993, Palumbo et
Bradshaw et al. (1991) studied the heat resis-
al. 1995). The equation for the thermal destruc-
tance of L. ivanovii, L. seeligeri and L. wel-
tion line constructed was log
D (D in s) =
shimeri. One strain of each species was tested
10
14.2559 - 0.20077t (r = -0.95519). The average
in milk at 52.2, 57.8, 63.3 and 68.9 °C. The
heat resistance values at 55, 60 and 65 °C cal-
equation for the 3 species taken together is
culated for L. innocua were greater than those
log
D (D in s) = 11.3419 - 0.15713t ; r =
10
for L. monocytogenes (Table 3), but none of
-0.99405. All means of D values obtained for
analysed differences between means of D val-
the 4 treatment temperatures were lower than
ues were statistically signi?cant. As to L. in-
the corresponding means noted in the present
nocua, however, only 36 D values were re-
work for L. monocytogenes. The differences be-
Acta vet. scand. vol. 44 no. 1-2, 2003

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