FLEISCHWIRTSCHAFT international 1/2017

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Fleischwirtschaft international 1_2017
63
Research &Development
D-value; pH’ is the pH of the recovery medium; pH’opt is the optimal pH value
of the recovery medium; z’pH is the distance of pH from pH’ of the recovery
medium which leads to aten-fold reduction of the D-value; a’w is aw of the
recovery medium; a’wopt is the optimal aw of the recovery medium; z’aw is the
distance of aw from the a’w of the recovery medium which leads to atenfold
reduction of the D-value.
Taking into consideration the correlation between the time of decimal
reduction and the degree of lethality L:
D=
D*
L
anew calculation model of the Lvalue was elaborated:
T–T* IpH–pH*I a w–1
a´w–1
pH´–pH´opt
( ) 2
z´aw
L =10
( ) 2
z T z pH z aw z´pH
+ + ++
Explanations as in the equation above.
It replaces the well-known model elaborated by BIGELOW (1921).
The application of the above-presented model requires the determination
of appropriate coefficients (zpH,zaw,z’pH,z’aw)for the indicator microorganism.
These coefficients were determined for bacteria from the
Bacillus and Clostridium genera, in other words, sporulating bacteria
which should be taken into consideration during the sterilisation process
(LEGUÉRINEL et al., 2000, 2005; GAILLARD et al., 1998; COROLLER,etal., 2001;
MAFART et al., 2001).Following the process of sterilisation of canned meat
products of the SSP type, spores of bacteria are no longer capable of
germinating because of unfavorable environmental conditions (reduced
aw and pH values, presence of NaNO2)and, therefore, when assessing the
effectiveness of the heating process, it is necessary to take into account
the survivability of thermoresistant non-sporulating bacteria. Enterococci
are considered to be most thermoresistant among the vegetative bacteria
(GIRAFFA,2002; FRANZ et al., 2003; HUGAS et al., 2003). These bacteria
occur in many foods (meat, dairy and vegetable origin) and play an important
role in the production of fermented meat products and cheese (PAVIA
et al., 2000). The surface of pig carcasses contain 10 4 to 10 8 enterococci
per 100cm 2 .The predominant isolated species are E. faecium and E.
faecalis (KNUDTSON and HARTMAN,1993). Enterococci are used as starter
cultures and their bacteriocins are usuallyactive towards such
pathogens like Listeria and Clostridium (GIRAFF et al., 1995, 1997; AYMERICH
et al., 2000). They are also used as human probiotics however they are
important nosocomial pathogens, that cause bacteraemia, endocarditis
and other infections. The role of enterococci in diseases call into question
their safety for the usage in foods or as probiotics. The presence of
enterococci in the gastrointestinal tract of animals leads to ahigh potential
for contamination of meat at the time of slaughter (FRANZ et al., 2003).
In the case of these microorganisms, the impact of water activity on
changes in their thermoresistance and possibilities of regeneration after
pasteurisation was determined, in other words, zaw and z’aw values were
determined which –for Enterococcus faecium PCM 1859 –amount to 0.14
and 0.44, respectively(DANYLUK et al., 2013). On the other hand, no information
is available with respect to the influence of pH on the behavior of
these microorganisms during pasteurization and their possibilities of
regeneration during astorage period, i.e. zpH and z’pH values were not
determined.
The aim of this paper was to ascertain thermoresistance and regeneration
possibilities of thermallydamaged Enterococcus faecium PCM 1859
cells depending on the pH value of the medium during heating and incubation
following the thermal process and to determine zpH and z’pH values. In
combination with the results published earlier (DANYLUK et al., 2013), they
allow the determination of anew formula for the calculation of the degree
of lethality which constitutes the basis for the determination of the pasteurization
value Pduring heating of canned meat products. The subjectmatter
of the article is very complex and depends on anumber of factors
mentioned in the manuscript and it constitutes amodel system which
provides abasis for further investigations in future.
Materials and methods
Preparation of samples
The bacterial strain used in the described experiments was that of Enterococcus
faecium PCM 1859 derived from the Strain Collection of the Polish
Academy of Sciences in Wrocław. Experimental bacteria were cultured on
Slanetzand Bartey medium with differing pH values. The substrate pH value
was reduced with by HCl and the following variants were obtained:
r basic medium (optimal) containing: pepton 20.0 g, dipotassium phosphate
4.0 g, yeast extract 5.0 g, glucose 2.0 g, sodium azide 0.4 g,
TTC0.1 g, agar 15.0 g, distilled water 1L;pHofthe ready medium 7.2,
aw=1.0 –A
r basic medium +0.8 mL 1NHCl; aw=1.0, pH= 7.0–B
r basic medium +2.7 mL 1NHCl; aw=1.0, pH= 6.8 –C
r basic medium +5.0 mL 1NHCl; aw=1.0, pH= 6.5 –D
Following inoculation, the samples were incubated for 48 hatatemperature
of 37 °C. Bacteria collected from media AtoDwere placed in the test
tubes (Ø= 16 mm) with aphysiological fluid containing the same quantity of
HCl as during culturing. Their initial concentration amounted to 10 6 to
10 8 cfu/mL. Next, 10 mL suspension was collected from each flask, transferred
to three test tubes and heated in awater bath at atemperature of
55 °C, respectively, for 10,20and 30 min. The same procedures were followed
when bacteria were heated at atemperature of 60 °C for 1, 3and
5min and at 65 °C for 1, 2and 3min. Each experiment was repeated three
times. After the appropriate time of heating, the bacteria were inoculated
Heat resistance
Tab. 1: Heat resistance of Enterococcus faecium cultured, heated and recovered in medium with different pH.
Decimal reduction time
pH value of medium during
Variant
Heating Recovery D55 (min.) D60 (min.) D65 (min.)
7.2 7.2 A 10.82 c ±0.45 1.51 c ±0.01 0.91 e ±0.10
7.0 7.2 B 10.17 c ±0.22 1.43 c ±0.26 0.84 de ±0.07
7.0 7.0 B’ 9.78 bc ±1.22 1.46 c ±0.07 0.69 bcd ±0.06
6.8 7.2 C 9.92 c ±0.15 1.38 bc ±0.11 0,80 bcde ±0.11
6.8 6.8 C’ 8.97 abc ±0.11 1.15 abc ±0.05 0.67 bc ±0.08
6.5 7.2 D 7.83 ab ±0.85 1.04 ab ±0.17 0.61 ab ±0.00
6.5 6.5 D’ 7.26 a ±0.45 0.95 a ±0.14 0.51 a ±0.02
The same letters in columns denote not significant difference for means at p ≤ 0.05 (n= 6; mean ±standard deviation)
Source: DANYLUK and STANGIERSKI FLEISCHWIRTSCHAFT international 1_2017