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Food Control 21 (2010) 125–131
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Food Control
journal homepage: www.elsevier.com/locate/foodcont
Radio frequency heating of comminuted meats – Considerations in relation
to microbial challenge studies
B. Byrne a,*, J.G. Lyng b, G. Dunne a, D.J. Bolton a
a
b
Ashtown Food Research Centre, Teagasc, Ashtown, Dublin 15, Ireland
School of Agriculture, Food Science and Veterinary Medicine, UCD Dublin, Belﬁeld, Dublin 4, Ireland
a r t i c l e
i n f o
Article history:
Received 19 June 2008
Received in revised form 23 February 2009
Accepted 2 March 2009
Keywords:
Radio frequency cooking
Bacillus cereus
Clostridium perfringens
Dielectric and thermal properties
a b s t r a c t
Bacillus cereus and Clostridium perfringens vegetative cell and spore cocktails in maximum recovery diluent (MRD) were inoculated into pork luncheon meat to challenge a previously developed radio frequency
(RF) cooking protocol. After RF cooking and cooling microbial enumeration results showed a reduction in
B. cereus vegetative cell and spores of 5.4 and 1.8 log10 cfu g1, respectively while the corresponding
reduction for C. perfringens vegetative cells and spores were 6.8 and 4.1 log10 cfu g1, respectively. However, post cooking temperatures within the product were lower than anticipated. Subsequent analysis of
product thermal and dielectric properties indicated that MRD addition and compositional variations
within meat ingredients altered thermal and dielectric properties which in turn contributed to reduced
and less uniform temperatures. The study shows that for RF microbial challenge studies, adjustment of
product formulation prior to MRD addition is critical to ensure a similar composition to the normal product and a true picture of microbial inactivation.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Traditionally, meat products are cooked by immersion in hot
water or in a steam oven. With both of these methods, heat is
transferred by conduction from the outer surface of the product
to its interior which in turn can lead to overheating of the surface
while waiting for the interior to reach an appropriate temperature.
In contrast, RF cooks products volumetrically with the product
forming a dielectric between two electrodes acting as capacitor
plates. These electrodes are alternatively charged from positive
to negative several million times in a second (e.g. 27 MHz) and
as a result the polar molecules in the product are constantly realigned causing internal friction to occur and leading to the production of heat. RF heating achieves quicker cooking times than
conventional cooking and all parts of the product heat at the same
rate (Brunton et al., 2005; Laycock, Piyasena, & Mittal, 2003;
Zhang, Lyng, & Brunton, 2004b; Zhang, Lyng, Brunton, Morgan, &
Mc Kenna, 2004a).
Best practice for safe production of heat chill processed foods
requires the development and implementation of validated and
veriﬁed heating and chilling processes. This will ensure that all
products receive adequate heating and chilling to ensure production of a safe food. RF heating has been reported to inactivate
Escherichia coli O157:H7 (Awuah, Ramaswamy, Economides, &
* Corresponding author. Tel.: +353 1 8059985; fax: +353 1 8059550.
E-mail address: [email protected] (B. Byrne).
0956-7135/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.foodcont.2009.03.003
Mallikarjunan, 2005; Guo, Piyasena, Mittal, Si, & Gong, 2006),
Salmonella (Nelson, Lu, Beuchat, & Harrison, 2002), Listeria, generic
E. coli in milk (Awuah et al., 2005) and increase the shelf life of food
(Orsat, Bai, Raghavan, & Smith, 2004). From a microbial inactivation perspective, it has also been proposed that RF heating differs
from conventional heating as in addition to heat inactivation of
microorganisms, non thermal inactivation or ‘‘cold pasteurisation”
has been claimed though differing reports of this theory can be
found in the literature. Non-thermal inactivation has been attributed to selective heating, electroporation, cell membrane rupture,
and magnetic ﬁeld coupling (Geveke & Brunkhorst, 2008; Geveke
& Sun, 2005). This paper does not speciﬁcally attempt to determine
whether cold pasteurisation occurs during RF heating as such a
study would involved the application of RF under non-thermal
conditions. However, it does challenge the ability of RF heating
to inactivate B. cereus and C. perfringens vegetative cells and spores
in inoculated meat emulsion samples. In a previous study, Zhang,
Lyng, and Brunton (2004) validated RF heat processes for emulsion
meat products using published microbial inactivation kinetic data
(i.e. D and z values) for Listeria monocytogenes. A similar meat
emulsion recipe was used in the present study but as this kinetic
information is composition dependent, the D and z values for B.
cereus and C. perfringens vegetative cells and spores were determined for the actual meat batter formulation used in the present
study (Byrne, Dunne, & Bolton, 2006). The paper also explores
how the RF temperature proﬁle of a meat batter can be inﬂuenced
by changes in composition, density and also thermal and dielectric
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B. Byrne et al. / Food Control 21 (2010) 125–131
properties as an understanding of the inﬂuence of these factors on
heating is important to individuals charged with responsibility for
validating RF heating protocols.
2. Materials and methods
2.1. Bacterial cultures
Three B. cereus cultures were used in this study. Two B. cereus
strains, DSM 4313 (isolated from a food poisoning incident) and
DSM 626 (extensively used in sporulation and germination
studies) were obtained from DSMZ (Deutsche Sammlung von
Mikroorganismen und Zellkulturen, Braunschweig, Germany).
The third strain, NCTC 07464 (reference culture) was obtained
from the Health Laboratory Services (HLS, London, UK). The three
C. perfringens cultures used in this study included DSM 11784 (food
poisoning incident isolate), NCTC 10614 (food poisoning incident
isolate) and NCTC 08237 (reference culture). All cultures were
stored on cryo-protective beads (Technical Service Consultants
Ltd., Lancaster, UK) at – 20 °C.
2.2. Sample preparation and inoculation
Cocktails were used to represent the probability of more than
one strain of B. cereus or C. perfringens growing in the meat. One
protective bead of each B. cereus and C. perfringens strain was inoculated into 30 ml of brain heart infusion broth (BHI, Oxoid,
Basingstoke, Hampshire, England) and 30 ml of ﬂuid thioglycolate
(FTG, Oxoid, Basingstoke, Hampshire, England), respectively. The
former was incubated aerobically at 30 °C for 24 h. The latter was
anaerobically incubated in a anaerobic incubator (Don Whitley Scientiﬁc Limited, 14 Otley Road, Shipley, West Yorkshire, UK) at
37 °C for 24 h (Labbe, Downes, & Ito, 2001). Each inoculum was
centrifuged using an Eppendorf Centrifuge 5403 (Davidson &
Hardy Ltd., Belfast, N Ireland) at 3000g for 10 min at 4 °C. The
recovered pellets were then washed three times with, and resuspended in 10 ml maximum recovery diluent (MRD, Oxoid, Basingstoke, Hampshire, UK) (1 g l1 peptone + 8.5 g l1 sodium chloride).
The acridine orange direct count technique (Walls & Sheridan,
1989) was used to establish the concentration of each of the vegetative cell suspensions. Phase contrast microscopy (PCM) (Olympus BX61, Japan) was carried out on each individual strain to
conﬁrm that each strain had not sporulated (González, López,
Martínez, Bernardo, & González, 1999; Sarker, Shivers, Sparks,
Juneja, & Mc Clane, 2000). The concentration of each strain was adjusted by diluting with MRD, to obtain a uniform cell concentration
of 9.0 log10 cfu ml1 for each vegetative cell strain. The three strains
of B. cereus were then mixed together to produce a B. cereus vegetative cell cocktail and the three C. perfringens stains were mixed to
prepare the C. perfringens vegetative cell cocktail. The cocktail concentrations were conﬁrmed using a plating technique as follows. B.
cereus and C. perfringens were plated onto the non selective media
tryptic soya agar (TSA, Oxoid) and aerobically incubated at 30 °C
for 48 h (Mazas, González, López, González, & Sarmiento, 1995)
and anaerobically incubated at 37 °C for 48 h (de Jong et al.,
2003), respectively.
B. cereus spores were obtained using the technique of Leguerinel
and Mafart (2001). C. perfringens spores were obtained using the
method of Duncan and Strong (1968). Brieﬂy, starter vegetative
cultures (30 ml) of each of B. cereus and C. perfringens isolates were
aerobically incubated at 30 °C in brain heart infusion broth (BHI,
Oxoid) (Coroller, Leguérinel, & Mafart, 2001), and anaerobically
incubated overnight at 37 °C in ﬂuid thioglycolate (FTG, Oxoid)
medium (Sarker et al., 2000), respectively. The B. cereus inoculum
was used to inoculate the surface of nutrient agar (NA, Oxoid) containing manganese sulphate (MnSO4; 40 mg l1, Sigma) and cal-
cium chloride (CaCl2; 100 mg l1, Sigma). These plates were
incubated at 30 °C for 5 d. The spores were harvested by pouring
10 ml of sterile distilled water onto each plate and scraping the
surface of the agar with a sterile hockey stick. The resultant spore
suspension was removed aseptically using a sterile syringe and
centrifuged at 3000g for 10 min at 4 °C. The recovered pellets were
then washed 3 times with MRD and suspended in a 10 ml of a 50%
(v v1) ethanol (Sigma) solution and stored at 4 °C for 12 h in order
to reduce the number of vegetative non-sporulated bacteria. After
refrigeration, the spore suspensions were washed a further three
times with MRD, the ﬁnal suspension was stored in eppendorf
microtubes at 4 °C.
C. perfringens spore solutions were prepared by inoculating
0.2 ml of a starter FTG culture into 10 ml of Duncan–Strong (D–S,
Sigma) sporulation broth, incubated anaerobically at 37 °C for
24 h. The inocula were stored at 4 °C in D–S broth (de Jong et al.,
2003; Duncan & Strong, 1968) and washed again three times by
centrifugation. The spore suspensions were checked at each stage
using PCM (González et al., 1999; Sarker et al., 2000).
The concentration of each B. cereus strain was established by serial diluting each spore suspension and plating onto TSA which was
incubated aerobically at 30 °C for 24 h. The concentration of each C.
perfringens strain was established by serial diluting each spore suspension and plating onto reinforced clostridrial agar (RCA, Oxoid)
and incubating anaerobically at 37 °C for 24 h. The spore concentration of each culture was adjusted with MRD to contain
9.0 log10 cfu ml1. The three strains of B. cereus were mixed
together to produce a B. cereus spore cocktail and the three
C. perfringens stains were mixed to prepare a C. perfringens spore
cocktail.
Pork luncheon roll was prepared according to the protocol of
Zhang, Lyng, and Brunton (2004). Pork meat (Local supplier), seasoning (Blakes Ingredients, Dublin, Ireland) and cure solution
(water (240 g), salt (50 g), NaNO2 (0.2 g) and NaNO3 (0.2 g)) were
used in the preparation of the pork luncheon roll. The meat was divided into 1.1 kg samples, which were placed into polyethylene
bags, vacuum-sealed and frozen at 20 °C (Zhang, Lyng, & Brunton,
2004). Prior to inoculation, these 1.1 kg samples of luncheon meat
were thawed overnight in a coldroom (4 °C) before being inoculated with a 10% inoculum of the B. cereus or C. perfringens cocktails, and blended (Kenwood food processor FP110, Hampshire,
UK) for 5 min to ensure that an even distribution of the cells or
spores in the meat samples. After inoculation, 1 kg of each meat
sample was vacuum packed, and inserted in a casing (to ensure
no leakage of meat samples during cooking), a third meat sample
was prepared by substituting the inoculum with MRD as a control.
2.3. Radio frequency cooking of pork luncheon roll
Samples were RF cooked (500 W) under circulating water
(80 °C) for 33 min using the RF oven and polyethylene cell described by Zhang, Lyng, and Brunton (2004). After 33 min the RF
power was turned off and the samples were held in the circulating
water at 80 °C for a further 2 min. This cooking protocol was repeated for all samples. Following RF cooking all samples (excluding
those destined for post cooking temperature distribution measurement) were immediately placed in ice water for 40 min, until the
sample temperature was at 4 °C.
To determine post cooking temperature distribution, three
samples with 10% added MRD (no bacterial cells or spores) were
cooked as per inoculated (MRD plus bacterial cells or spores)
samples following which they were immediately placed in a thermocouple jig (described by Zhang et al. 2004b) to determine post
cooking temperature distribution. A Grant Squirrel Meter/logger
1600 series (Grant Cambridge, UK) was used to measure and record the temperature of the samples (Zhang et al. 2004b). From
127
B. Byrne et al. / Food Control 21 (2010) 125–131
the resultant data an average, maximum and minimum post
cooking temperature for the RF pork luncheon roll was
calculated.
(i) Temperature proﬁle during RF cooking and holding: The
start temperature of samples immediately pre-cooking was
measured in triplicate samples using a hand held thermocouple probe and was 5.5 °C. As all samples were equilibrated in the same incubator for similar times it was
assumed that this start temperature was common to all
samples. As temperature validation is always carried out
on the ‘‘cold spot” within a product (i.e. the area in the product deemed to have the lowest temperature), the lowest
internal post cooking product temperature recorded by the
thermocouple jig was used as the temperature at the end
of the heat hold cycle. At the end of the heat hold period
the thermocouple jig always showed temperatures of
<80 °C in all areas of the sample. Therefore as the surrounding water was at a higher temperature (80 °C) and heat
always transfers from a region of higher to lower temperature it was assumed that heat was still passing into the sample during the holding period and that a temperature rise of
1 °C occurred within the sample in this 2 min period. This
1 °C was deducted from the temperatures measured at the
end of the heat hold period to get a predicted temperature
at the end of RF heating. A linear temperature proﬁle was
then constructed from the 5.5 °C start temperature to this
predicted temperature across the 30 min duration of the
RF cook cycle.
(ii) Temperature proﬁle during cooling: An average cooling
curve was calculated by recording time temperature proﬁles
from the geometric centre of triplicate rolls while samples
were cooled using the procedure described in Section 2.3.
2.4. Bacterial enumeration
The cooked pork luncheon meat was aseptically divided into
twenty sections, as per Fig. 1. From each section, 25 g of meat were
removed, 225 ml of MRD were added and this mixture was stomached for 1 min at medium speed. The stomacher ﬂuids were serially diluted and stored at 4 °C prior to plating. The vegetative cells
were recovered by plating 0.1 ml of stomacher ﬂuids directly onto
Bacillus cereus agar (BCA, Oxoid) for B. cereus and plating on to tryptose–sulﬁte–cycloserine (TSC, Oxoid) (with TSC overlay) for C. perfringens. To recover vegetative cells that were injured but not
destroyed, 0.1 ml of the resulting stomacher ﬂuids was spread plated onto TSA, incubated for 2 h at 25 °C and then over-poured with
the relevant selective media (BCA for B. cereus and TSC for C. perfringens) (Juneja, Snyder, & Marmer, 1997). The plates were incubated aerobically at 30 °C for 24 h for B. cereus and incubated
anaerobically at 37 °C for 24 h for C. perfringens. Surviving spores
were enumerated using a similar method but with the following
modiﬁcations. For B. cereus samples, the dilutions were plated onto
NA and BCA and incubated at 30 °C for 48 h. C. perfringens samples
were plated onto RCA and TSC (with TSC overlay) and incubated
anaerobically at 37 °C for 48 h prior to counting colonies (Sarker
et al., 2000). The colony forming units (cfu’s) were recorded and
converted into log10 cfu g1. The RF cooking protocol was repeated
3 times and the mean log10 cfu g1 were calculated.
2.5. Prediction of temperature proﬁles and calculation of
pasteurisation units
Fibre optic probes allow sample temperature measurements
during RF cooking but most commercially available systems have
1–4 probes which is insufﬁcient for a comprehensive measure of
sample temperature distribution in a large 1 kg sample such as
that found in the present study. However, it is accepted that RF
heating is a volumetric form of heating and this has been illustrated by ﬁbre optic temperature proﬁles for a range of meats
cooked using the current setup by Zhang, Lyng, and Brunton
(2004), Zhang, Lyng, and Brunton (2006) and Brunton et al.
(2005). These proﬁles show linear temperature increases during
RF cooking followed by a plateau in the 2 min holding period while
samples were held in 80 °C circulating water. Model temperature
proﬁles were constructed by dividing the temperature proﬁle into
two regions (i) RF cooking followed by holding in circulating water
at 80 °C (ii) Cooling by immersion in iced water. Temperature proﬁles for these regions were estimated as follows.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
Sections
From the predicted time temperature proﬁle constructed in (i),
pasteurisation units (PU) for B. cereus (vegetative and spores) and
C. perfringens (vegetative and spores) were calculated using z value
and reference temperature (h) data presented in Table 1 and the
following equation
Z
tp
Th
z
dt 10
¼ Pasteurisation units ðPUÞ
ð1Þ
0
where, T is the predicted temperature (°C) at the coldest point in the
product at any time during the heat process, dt is the duration of
time at a particular temperature (min) and tp is the time at the
end of the heating process (min).
2.6. Proximate analysis
Moisture (M) and fat (F) contents of meat batter was determined by an automated, integrated microwave moisture and
methylene chloride fat extraction method (Bostian, Fish, Webb, &
Arey, 1985). This was carried out using a CEMÒ microwave moisture/solids analyser system (Model No. AVC-80, CEM Corporation,
3100 Smith Farm Road, Matthews, NC 28105, USA) and a CEMÒ
(Model No. FES-80) automatic fat extraction system. Protein (P)
analysis was carried out using a LecoÒ (Model No. FP-428 LecoÒ
Corporation, 3000 Lakeview Ave., St. Joseph, MI 49086 - 2396,
Table 1
D and z valuesa for B. cereus and C. perfringens vegetative cells and spores in the pork
luncheon roll used in the calculation of pasteurisation units in the current study.
Fig. 1. Diagram illustrating the division of the luncheon roll for microbial analysis
following RF cooking.
Microorganism
h (°C)
Dhb value (min)
z Value (°C)
B.
B.
C.
C.
60
95
60
95
1
2
8.5
9.7
6.6
8.6
7.7
8.3
cereus cells
cereus spores
perfringens cells
perfringens spores
a
b
Adapted from Byrne et al. (2006).
Dh: D value at a reference temperature h.
128
B. Byrne et al. / Food Control 21 (2010) 125–131
USA) Nitrogen Protein Analyser using the method of Sweeney and
Rexroad (1987). Ash (A) was determined by incinerating samples
in a mufﬂe furnace, using the method of Kolar (1992) while carbohydrate (C) content was calculated by difference. Composition of
raw luncheon roll meat batter was measured before and after
MRD addition.
ð1aÞ
and fat meat was used and also the fact that rolls were vacuum
packaged during cooking in the present study. These differences
would appear to be minor, though when post cooking temperatures were compared, the samples cooked by Zhang et al.
(2004b) reached higher average, minimum and maximum temperatures than the samples in the present study (i.e. average: 80.6 °C
vs. 72.2 °C; minimum: 77.4 °C vs. 66.9 °C; maximum: 83.3 °C vs.
78.6 °C respectively). In addition, using data supplied by Zhang
et al. (personal communication) a comparison was made between
the average of the 6 thermocouples reading 1.5 cm under the product surface (‘‘outer” thermocouples) vs. the 6 thermocouples reading 3 cm under the product surface (‘‘mid” thermocouples) vs. the
3 thermocouples reading 4.5 cm under the product surface (‘‘centre” thermocouples) (Table 2). These results suggest the samples
in the present study showed much greater heating towards the
outer regions of the samples and a much colder interior, while
the samples of Zhang et al. (2004b) were much more uniform
when outer, mid and core temperatures were compared. This study
observed DT (i.e. maximum–minimum temperatures) values of
11.7 °C which were larger than the DT values of 5.9 °C reported
by Zhang, indicating less uniformity in the current investigation.
Temperature differentials and ‘‘cold spots” are a major concern in
the validation of heat process. The following section attempts to
explain why these quite different temperature results were found
when two relatively similar studies were compared.
ð1bÞ
3.2. Factors inﬂuencing RF heating in the present study
2.7. Dielectric property measurement
Dielectric properties were measured on uncooked batters with
and without MRD addition and before and after vacuum packaging.
Samples were equilibrated in an air-conditioned laboratory at
23 °C (±1 °C) and measurements at 27.12 MHz were subsequently
carried out using the methods described by Zhang, Lyng, and Brunton. (2004).
2.8. Thermal property and density prediction
The thermal properties heat capacity (c) (J kg1 C1) and thermal conductivity (k) (W m1 C1) and also density (q) (kg m3)
were predicted from proximate composition data using Equations
below given in (Bufﬂer, 1993).
c ¼ ð4190 MÞ þ ð1780 PÞ þ ð1980 FÞ þ ð1420 CÞ þ ð950 AÞ
k ¼ ð0:6 MÞ þ ð0:2 PÞ þ ð0:18 FÞ þ ð0:2 CÞ þ ð0:14 AÞ
q ¼ ð1000 MÞ þ ð1290 PÞ þ ð920 FÞ þ ð1930 CÞ þ ð1740 AÞ
ð1cÞ
where M is the moisture (% wet weight), P is the protein (% wet
weight), F is the fat (% wet weight), C is the carbohydrate (% wet
weight), and A is the ash (% wet weight).Thermal diffusivity (a) (expressed as m2 s1 107) values were calculated using the predicted values for cp, q and k and Eq. (2) provided by (Dickerson,
1965).
a¼
k
qc
ð2Þ
2.9. Statistical analysis
Analysis of variance (ANOVA) was used to test the effect of MRD
addition, and vacuum packaging on the composition and thermophysical properties measured using the SAS software package (Version 8.2, Statistical Analysis Systems, Cary, NC, USA).
3. Results and discussion
3.1. Post-cooking temperature proﬁles
The equipment, basic luncheon roll recipe and cooking procedure used in the present study were identical to those used by
Zhang et al. (2004b), though there were some differences between
the two studies including the addition of 10% MRD to the batter in
the present study and also the fact that a different batch of lean
The RF oven and cooking conditions used in this study were
identical to those used in the study of Zhang, Lyng, and Brunton
(2004). The star diagram (Fig. 2) highlights the signiﬁcant differences between the composition, q, thermal and dielectric properties of the batter used in these studies. This diagram presents the
results for each proximate, thermal and dielectric attribute on separate radii. Different scales are used on each radius with the centre
of the circle representing 0 and the value adjacent to the circumference indicating the maximum on the scale for that particular attribute. When the composition of samples in the present study was
compared to those of Zhang, Lyng, and Brunton (2004), the batter
used in the present study had higher moisture (P < 0.001) and
ash (P < 0.01) and lower protein (P < 0.001) and carbohydrate
(P < 0.05) contents. There was no signiﬁcant difference between
the fat content of the batters in both studies. The higher moisture
in the batter used in the present study is most certainly due to the
addition of MRD while the higher average fat content may be due
to differences in the composition of the raw meat used between
the studies. The signiﬁcantly greater ash levels measured in the
batter prepared in the current study can most likely be attributed
to the MRD. These differences in composition inﬂuence the thermal and dielectric properties of the material as well as its q. For
example MRD used in the current study was found to have a
dielectric loss factor (e0 0 ) of 890 (and a dielectric constant (e0 ) of
75.1). This e0 0 translates to the MRD having the equivalent e00 value
of a 0.9% NaCl solution, based on a regression performed using data
provided by Lyng, Zhang, and Brunton (2005). Differences in
composition can also affect q and Fig. 2 shows signiﬁcantly higher
Table 2
A comparison of temperatures recorded by the outer, mid and centre thermocouples of samples in the current study vs. Zhang et al. (2004).
Distance under product surface (n = number of thermocouples)
Descriptor
Present study (°C)
Zhang et al.a (°C)
1.5 cm (n = 6)
3 cm (n = 6)
4.5 cm (n = 3)
Outer
Mid
Centre
76.0
73.7
67.6
80.4
80.8
80.4
a
Zhang et al. (personal communication).
129
B. Byrne et al. / Food Control 21 (2010) 125–131
e00 of comminuted meats at both RF and MW frequencies (Zhang,
**d p(cm)
% Fat NS
25
0.7
15 % Protein ***
15
* Tan δ 20
5 % Ash **
** ε ’’ 1000
15 % Carbohydrate *
** ε’ 60
4000 c ***
0.6 k ***
-71.5
*** α x 10
1500
ρ*
Current study
Zhang et al (2004 b)
Fig. 2. A comparison of the composition, density dielectric and thermal properties
of luncheon roll batter used in the current study vs. that used by Zhang et al. (2004).
predicted q than the batter used by Zhang, Lyng, and Brunton
(2004) and the current study (P < 0.05). However, it is very important to note that this difference in predicted q is purely based on
Eq. (1c). This equation uses proximate composition of each batter
to calculate q and does not take into account q differences imposed
by vacuum packaging. In the present study, the samples were vacuum packaged following batter preparation and were subsequently frozen, thawed and MRD was mixed in before further
vacuum packaging pre-cooking. In contrast the batter of Zhang,
Lyng, and Brunton (2004) was not vacuum packaged between batter manufacture and cooking. Therefore it is possible that the q of
the batter prepared in the present study was actually increased by
vacuum packaging. Fig. 2 also shows higher calculated a in the batter used in the present study compared to that of Zhang, Lyng, and
Brunton (2004) (P < 0.001) which would suggest that the batter in
the current study should heat faster. However, as q is used in the
calculation of a (which in turn gives an indication of the speed of
heating), an increase in q (due to vacuum packaging) could also reduce a. This in turn would be expected to reduce heating rate
which would be more in line with actual observations in the present study. Overall these results on q and a prediction suggest a certain amount of caution should be used when interpreting the
results from composition based thermal property prediction formulae. Another important contributing factor to the lower temperature rise in the batter prepared in the current study is the c value.
This value is lower (P < 0.001) in the batter prepared by Zhang,
Lyng, and Brunton (2004), (Fig. 2) which meant that the batter prepared for the current study required greater amounts of energy input for a given temperature increase.
The higher e0 in the batter in the current study (Fig. 2) means a
greater proportion of the RF power approaching the surface of the
product will be reﬂected away (i.e. higher Pr (i.e. Fig. 2)). This also
may have had some inﬂuence on the temperature rise in the batter.
However, once the RF energy enters the product the higher e0 in the
batter used in the current study (Fig. 2) (most likely due to higher
levels of ions introduced by the 10% added MRD and the major
inﬂuence of ionic compounds on dielectric properties at
27.12 MHz (Lyng et al., 2005) means that a greater proportion of
the power will be absorbed in the outer regions of the product,
which contributed to a lower dp (Fig. 2). The removal of air by
vacuum packaging has been shown to signiﬁcantly increase the
Lyng, & Brunton, 2007) as it is generally accepted that the presence
of air lowers dielectric properties. Therefore in this study, vacuum
packaging of the product may have also contributed to the increase
in observed e00 increase compared to that of Zhang, Lyng, and
Brunton (2004).
This increase in e00 effect has also manifested itself in Section 3.1
where a comparison between outer mid and core temperatures
show greater temperature rises in the outer regions of the samples.
This in turn could also partially account for the overall lower temperature rise in the batter in the current study as less power would
be available to heat the inner regions of the product due to greater
levels of absorption in the outside of the product.
Overall, compared to the pork luncheon roll of Zhang, Lyng, and
Brunton (2004), some changes in the thermal and dielectric properties of the batter (and its temperature rise on RF heating) may
have been induced by differences in raw meat composition and
vacuum packaging. However, many of the observed modiﬁcations
can be attributed to the addition of MRD. This suggests that a more
appropriate approach when performing microbial challenge tests
on RF (and indeed microwave and ohmically heated meats) is to
adjust the formulation of the batter (e.g. by reducing water and salt
addition) to such an extent that when MRD is added the products
composition is in line with the composition of conventional samples to which no MRD has been added. Alternatively this effect
could be avoided by utilising an inoculation system which allowed
the introduction of challenge microorganisms while avoiding the
use of a disruptive additive such as MRD as the validity of the model may be reduced by changing the (input) product to compensate
for the impact of MRD.
3.3. Predicted temperature proﬁles and pasteurisation units
Predicted temperature proﬁles for the current study are presented in Fig. 3 and are compared to temperature proﬁles for the
luncheon roll batter of Zhang, Lyng, and Brunton, (2004). RF protocols show the lower maximum temperatures attained in the current study. In addition, cooling proﬁles also differ as samples in
the current study were cooled in iced water while Zhang, Lyng,
and Brunton (2004) cooled samples in circulating tap water for
40 min according to the method of Boccard et al. (1981) before
transferring samples to refrigerated storage.
Target pasteurisation units required to eliminate the inoculated
levels of each microorganism type were calculated (Table 3). In this
study, the approach taken towards calculating this target was to
80
Current Study
Zhang et al (2004b)
70
60
o
* Pr
Temperature ( C)
***
% Moisture
100
50
40
30
20
10
0
0
10
20
30
40
50
60
70
80
Time (min)
Fig. 3. A comparison of time temperature proﬁles from the current study
(predicted) vs. Zhang et al. (2004).
130
B. Byrne et al. / Food Control 21 (2010) 125–131
Table 3
Pasteurisation units (PU) calculated for B. cereus and C. perfringens vegetative cells and spores from predicted time temperature proﬁles from cold spot of pork luncheon roll used
in the current study.
Microorganism
B.
B.
C.
C.
a
cereus (V )
cereus (Sa)
perfringens (V)
perfringens (S)
a
b
c
d
e
Innoculation level (IL) (log10 cfu g1)
Dhb value (min)
Target PU (min) for sterility
7.5
7.6
8.5
9.7
D60
D95
D60
D95
7.5
15.2
58.65
74.69
1
2
8.5
9.7
Actual PU (min)d
Zhange PU (min)
6.426
0.001
6.591
0.0004
1593
0.041
758
0.034
V: Vegetative cells; S: Spores.
Dh: D value at a reference temperature h.
Actual PU (min) was calculated by multiplying the number of log inoculated by Dh to get the number of min at h required to sterilised the product.
Calculated using Eq. (1).
Zhang et al. (2004).
multiply the number of log10 cycles of each organism type inoculated by the reference Dh value (min) to give a target time (min)
at a reference temperature h required to render the product sterile
with respect to the inoculated organism. Generally for a product to
be sterile with respect to an organism the actual PU should at a
very minimum be P target PU.
up times during RF cooking of PLM in the current study compared
to those in Byrne et al. (2006) may have thermally damaged the
spores thereby giving an underestimate of the actual numbers
present. However, a separate dedicated study would be required
to determine the actual cause of these differences.
4. Conclusions
3.4. Challenge study
Table 3 shows that the PU’s produced by the predicted heating
protocol used in the current study (Fig. 3) were not sufﬁcient to
render the product sterile with respect to the inoculated levels of
each organism type examined (i.e. Actual PU < Target PU). However, Table 4 shows that the actual microbial reductions obtained
for vegetative C. perfringens cells were smaller than the detection
level for this organism, which on initial inspection seems surprising given the greater heat resistance of this organism (Table 3).
However, this apparent decrease in the thermal resistance of C.
perfringens vegetative cell could possibly be attributed to the terminal inﬂuence of chilling on C. perfringens vegetative cells (Danler
et al., 2003). However, similar reports of B. cereus inactivation on
chilling have not been found in the literature. Interestingly the actual spore reduction was signiﬁcantly higher than the predicted
spore reduction and this was especially evident for C. perfringens
spore cocktails. In general spores are not inactivated by pasteurisation temperatures. Therefore, factors present in the current study,
that were not present in the study by Byrne et al. (2006) may have
attributed to the higher reduction of spores. These factors could include [1] the RF energy having a non-thermal effect on the inactivation of the spores (Geveke & Brunkhorst, 2003; Kozempel,
Annous, Cook, Scullen, & Whiting, 1998; Ponne, Balk, Hancioglu,
& Gorris, 1996; Geveke & Brunkhorst 2004); [2] the longer heating
Table 4
Summary of surviving B. cereus and C. perfringens vegetative cells and spores in the
pork luncheon roll following the heat hold cool protocol described in Sections 2.4 and
2.5.
B. cereus
C. perfringens
Vegetative cells
Spores
Vegetative cells
Spores
Pre-cook
Average (log10 cfu g1)
7.5
7.6
6.9
7.7
Post-cook
Average (log10 cfu g1)
Standard deviation
N
Maximum
Minimum
2.2
0.593
20
3
0.5
5.8
0.216
20
6.1
5.3
<1 cfu in 10
0
20
0
0
3.6
0.86
20
5.7
1.7
Reduction pre-Posta
Average (log10 cfu g1)
5.3
1.8
6.9
4.1
a
c
Predicted reduction imposed by the heat treatments was calculated by dividing
the PU for each microorganism by its reference Dh value.
In conclusion the results show that in addition to natural composition variations which occur in meat the addition of a dielectrically active ingredient such as MRD to a meat batter for an RF
pasteurisation challenge study can alter thermal and dielectric
properties of a product. These changes can inﬂuence RF interaction
with the product, temperature proﬁles and DT’s within a product
and also can potentially give a false picture of microbial inactivation compared to the normal product with no MRD. A more appropriate approach would be the adjustment of product formulation
prior to MRD addition to such an extent that the addition of
MRD produces a product with identical composition to the conventional product.
Acknowledgement
This research has been partly funded by grant aid under the
Food Institutional Research Measure, which is administered by
the Department of Agriculture, Fisheries and Food, Ireland.
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