Transcript Document

Development and validation of a microbial spoilage model for aerobic stored ground meat
A. Stamatioua,a,c , K. Koutsoumanisb, M. R. Adamsc and G.J. Nychasa,
aLab.
Microbiology & Biotechnology of Foods, Dept. Food Science &Technology, Agricultural University of Athens, Iera odos 75, Athens, 11855, Tel.-Fax.: 210-5294693, e-mail: [email protected]
bAristotle University of Thessaloniki, Faculty of Agriculture, Dept. Of Food Science and Technology, Lab of Food Microbiology and Hygiene, Thessaloniki,54124, tel.: 2310-471467, Fax: 2310-471257,
e-mail: [email protected]
cSchool of Biological Sciences, University of Surrey, Guildford, Surrey Gu2 5XH, UK tel.: +44-1483-300-800, Fax: +44-1483-300-374, e-mail: [email protected]
Abstract
The growth of the different measured constituents of ground pork natural microflora was monitored at different isothermal temperatures conditions from 0 to 20 oC. At all temperatures, growth of
Pseudomonads which is well established spoilage indexes for air stored chilled meat, followed closely the decrease of sensory quality. The kinetic parameters of pseudomonads (maximum specific growth rate) was
modelled as a function of temperature using the Arrhenius model. The model was further validated at non-isothermal conditions using different fluctuating temperature scenarios. The results showed that the kinetic
model developed in the present study can accurately predict growth of pseudomonads and shelf life of ground meat at both isothermal and dynamic temperature conditions. Such models can be useful tools for quality
optimisation in quality management systems of chilled meat.
Introduction
Figure 2
Figure 1
Results and Discussion
Representative experimental data for growth of the different measured constituents of ground pork natural microflora at 5
and 15 oC are shown in Figures 1 and 2. Pseudomonads were the dominant organisms at all temperature testing following
by Br. thermosphacta, lactic acid bacteria and Enterobacteriaceae. At all temperatures, growth of Pseudomonads which is
well established spoilage indexes for air stored chilled meat, followed closely the decrease of sensory quality and end of
shelf life coincided with a pseudomonads level of 9.0 log10 cfu/g. The growth data of spoilage bacteria from four individual
replicated experiments with ground pork stored at different isothermal conditions (0, 5, 10, 15 and 20 oC) were modelled as a
function of time using the Baranyi model and the kinetic parameters (max, Lag phase and Nmax) were estimated. The
results showed that the storage temperature did not affect the maximum concentration (Nmax) which was found to be
constant (for pseudomonas an average value of 9.6 log10 cfu/g. Further, the temperature dependence of the kinetic parameters
was modelled using the Arrhenius equation (Figure 3). The estimated values and statistics of the Arrhenius model
parameters for the different spoilage bacteria are shown in Table 1. The activation energies were 69.1, 67.6, 98.2 and 96.0
kj/mol for pseudomonads, Br. thermosphacta, lactic acid bacteria and Enterobacteriaceae respectively.
The model for pseudomonads growth was validated at non-isothermal conditions using different fluctuating temperature
scenarios (figures 4-6). Furthermore, the predicted shelf life (e.g time required for pseudomonads to growth from their initial
level N0 to the spoilage level Ns=109 cfu/g) was compared to the observed shelf life estimated by the sensory analysis (Table
2).
8
VRBG
7
6
STAA
9
MRS
8
VRBG
7
6
5
5
4
4
3
3
2
2
100
200
300
400
CFC
10
Log CFU/g
MRS
500
0
30
90
Time (hours)
Figure 3
Table 1
Pseudomonads
Br. thermosphacta
lactic acid bacteria
Enterobacteriaceae
-1
-1.5
-2
-2.5
-3
-3.5
-4
0.0035
0.0036
120
 =ref x exp(-EA/R*(1/T ref-1/T))
Pseudomonads
EA (kj/mol)
69.1
ref
0.056
r2
0.990
Br. thermosphacta
EA (kj/mol)
67.6
ref
0.044
2
r
0.981
L. acid bacteria
EA (kj/mol)
98.2
ref
0.021
r2
0.981
Enterobacteriaceae
EA (kj/mol)
96.0
ref
0.025
r2
0.954
-0.5
-4.5
0.0034
60
Time (hours)
0.0037
1/T
Figure 5
12
30
25
10
25
20
8
15
6
10
4
5
2
0
0
0
30
60
90
-5
150
120
cell conc. (l og cells/ml)
10
30
temperature (C)
12
20
8
15
6
10
4
5
2
0
0
-5
150
0
30
60
time (h)
90
120
time (h)
Figure 6
12
temperature (C)
Figure 4
Table 2
30
25
10
20
8
15
6
10
4
temperature (C)
where T is the absolute temperature, μref is the growth rate at a reference temperature Tref, EA is the activation energy and R
the universal gas constant.
A limited number of samples were freeze-stored to serve as controls during sensory evaluation of colour and odour.
Microbiological analysis. Samples (25g) of minced meat were aseptically weighed, added to ¼ strength Ringer's solution
(225ml), and homogenized in a stomacher (Lab Blender 400, Seward Medical, London) for 60s at room temperature. Decimal
dilutions in quarter strength Ringer's solution were prepared and duplicate 1 ml or 0.1 ml samples of appropriate dilutions
were poured or spread on the following media: Plate Count Agar (PCA; Merck, 1.05463, Darmstadt, Germany) for total
viable count (TVC), incubated at 25o C for 72h; Brochothrix thermosphacta on STAA medium supplemented with
streptomycin sulfate, thallous acetate and cycloheximide (actidione); this medium was made from basic ingredients in the
laboratory, and incubated at 25o C for 72h; Lactic acid bacteria on MRS (Merck, 1.10660, Darmstadt, Germany) overlaid with
the same medium and incubated at 25oC for 96h under anaerobic conditions; Pseudomonas spp. on cetrimide- fucidincephaloridine (CFC) agar (Oxoid, CM559 supplemented with selective supplement SR 103E, Basingstoke, UK) incubated at
25o C for 48h; Enterobacteriaceae on Violet Red Bile Dextrose Agar (Merck, 1.10275, Darmstadt, Germany) overlaid with
the same medium and incubated at 37o C for 24h.
Sensory analysis. Sensory evaluation of minced pork samples was performed during storage by a five member sensory panel
composed of staff from the laboratory. The same trained persons were used in each evaluation, and all were blinded to which
product was being tested. The sensory evaluation was carried out in artificial light and the temperature of packed product
approximated the ambient temperature. Special attention was given to the colour. Organoleptic evaluation: product cooked in
aluminum foil at 180oC for 20min. Each attribute was scored on a 3-point hedonic scale where: 1=acceptable; 2=rejection
point; and 3=unacceptable.
9
cell conc. (log cells/ml)




STAA
cell conc. (log cells/ml)
 1
1


T
T ref

10
PCA
11
CFC
0
ln(mumax)
 max =  ref
-E
A
exp 
 R
PCA
11
Materials and Methods
Preparation of samples. Ground pork divided into portions of 50g and placed on plastic trains and simply enclosed into
permeable polyethylene plastic film transported from open market to the laboratory within 24h at 5oC after their production.
The samples stored under controlled isothermal conditions (0, 5, 10, 15 and 20 oC) in high-precision (0.2oC) lowtemperature incubators (model MIR 153; Sanyo Electric Co., Ora-Gun, Gunma, Japan). Samples were taken at appropriate
time intervals to allow for efficient kinetic analysis of microbial growth. The models developed from the studies at isothermal
conditions were validated at dynamic temperature conditions. Four different temperature scenarios were tested: T1: 24 h at
0oC-24 h at 10oC, T2: 12 h at 0oC-6 h at 10oC-6 h at 15oC, T2: 6 h at 0oC-6 h at 10oC-6 h at 20oC, T2: 10 h at 5oC-6 h at 20oC.
Τhe temperature dependence of the kinetic parameters was modelled using the Arrhenius equation
12
12
Log CFU/g
The current philosophy for food quality assurance is steadily decreasing the focus on end product testing and verification,
traditionally the cornerstones of quality and regulatory control. The efforts of producers and legislation is concentrating on
the development and application of structured quality assurance systems based on prevention through monitoring, controlling
and recording of critical parameters through the entire product’s life cycle extending from production to final use. Effective
application of this approach requires systematic study and modelling of the temperature dependence of shelf life. Ideally this
would mean establishing a time correlation between measured microbiological activity and sensory value for the conditions
of interest. The objective of the present study was to study the relation between microbial growth and organoleptic
characteristics of ground meat and develop and validate a microbial spoilage model for accurate predictions of fresh meat
shelf life.
Temperature
SL
SL pred. Temp. Difference
profile
observed
model
%
T1
85.3
76.5
10.3
5
2
0
0
T2
98.0
62.8
35.9
T3
68.8
50.5
26.6
T4
71.5
67.7
5.3
-5
0
30
60
90
time (h)
References
1.Koutsoumanis, Κ., M. Giannakourou, P. S. Taoukis, and G.J.E. Nychas (2000) Application of SLDS (Shelf life Decision system) to marine cultured fish
quality. Int. Journal of Food Microbiol. 73, 375-382
2.Koutsoumanis, K. P., Taoukis, P., Drosinos, E. H, and Nychas G-J.E. (2000) Applicability of an Arrhenius model for the combined effect of temperature and
CO2 packaging on the spoilage microflora of fish. Applied Environemental Microbiology, 66, 3528-3534
3.Koutsoumanis, K. and Nychas G-J. (2000) Application of a systematic experimental procedure to develop a microbial model for rapid fish shelf-life
prediction. International Journal of Food Microbiology, 60, 171-184