FLEISCHWIRTSCHAFT international 1/2017

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Fleischwirtschaft international 1_2017
Research &Development Thermoresistance and regeneration of heat-damaged E. faecium PCM 1859 ...
Source: DANYLUK and STANGIERSKI FLEISCHWIRTSCHAFT international 1_2017
Fig. 1: Change of the D-value (%) during heating of E. faecium in the environment
with reduced pH value.
a, b–different letters for agiven temperature indicate statisticallysignificant
differences at p≤0.05 (n= 6)
in order to ascertain the number of survived microorganisms. Bacteria
cultured on medium Awere flushed after heating with medium A; bacteria
collected from the media of reduced pH value were incubated on medium A
(optimal) and, simultaneously, on the medium characterised by the same
parameters as during heating. The Asurvival curve was plotted for agiven
heating temperature from which the time of the decimal reduction Dwas
determined for both the bacteria incubated on the basic medium and on
the modified medium (of reduced pH value) and then from the curve of
log D-pH dependence, zpH and z’pH coefficients were determined. When
plotting the log D-pH curve, alinear course was assumed (R 2 =0.7634 for
D55;R 2 =0.6326 for D60;R 2 =0.8048 for D65).
Measurements of pH
Measurements of pH value were conducted employing aSchott Geräte
pH-meter type CG 840 (Mainz, Germany) equipped with aglass electrode.
Statistical analysis
All the determinations were performed in two replications and the results
were subjected to statistical analysis. One-factorial analysis of variance
and post hoc Tukey’stest were applied for multiple comparison of mean
value. The level of significance was p≤0.05. All computations were performed
using Statistica PL v. 10 software by StatSoft.
Results and discussion
Table 1presents the impact of the medium pH during heating on the D-
values (variants A, B, Cand D-regeneration on optimal medium). The obtained
research results indicate that the sensitivity of the assessed bacteria
to heating differed depending on the pH of the environment during the
heating process.
Figure 1presents differences in the D-values during heating at agiven
temperature depending on the pH of the medium. The D-value determined
during sample heating on the substrate of the optimal pH of 7.2was assumed
as 100%. During heating at the temperature of 55 and 60 °C, the
reduction in the D-value was similar and amounted to, respectively, 6and
5.3% at pH 7.0and to 8.3 and 8.6% at pH 6.8. Heating at the temperature of
65 °C led to the greatest reduction in the value of D, i.e. by 7.7% at pH 7.0
and by 12.1% at pH 6.8. The lowest thermal resistance of the strain was
recorded when samples were heated in the environment where the pH
amounted to 6.5; the D-values were reduced by 27.6% (55 °C), 31.1% (60 °C)
and 33.0% (65 °C) in comparison with the values determined during the
heating at agiven temperature at optimal pH (7.2). However, astatistically
significant impact of the medium pH on E. faecium’s thermoresistance
expressed by the D–value was demonstrated onlyinthe case of heating of
the bacteria in the environment of pH 6.5 irrespective of the temperature
of heating. On the other hand, experiments on the thermal resistance of
Bacillus stearothermophillus carried out by LÓPEZ et al., (1996) demonstrated
that the effect of any given pH is depended on the treatment
temperature. At low treatment temperatures (115 °C), amarked reduction
of the D-values was observed when pH was lowered from 7.0to4.0. This
reduction was significantlylower at 125°C. At higher temperatures
(135°C), the D-values obtained in pH 6.0 and 7.0 did not show significant
differences.
The available literature data indicate that pH reduction enhances the
preservation effect of food products. This may be caused by the growth
inhibition of microorganisms, whose growth depends on levels of free H +
ions and concentrations of undissociated, weaker acids which, in turn,
is dependent on pH. Anions of some weaker acids are metabolised in the
bacterial cell in such away that H + ions are liberated by acidifying the
interior of the cell to the level of inhibiting growth (SABATAKOU et al., 2001).
The effect of preservation can also be the result of diminished thermoresistance
of microorganisms in the environment with reduced pH. In the
case of spores, the mechanism of this phenomenon can be explained by
the fact that, during heating in acid environment, hydrogen ions replace
calcium ions associated with the cell and form the so called H-sporeions
characterised by lower thermoresistance. All calcium ions which
constitute approximately2%ofspore dry matter can be removed in this
way.This is areversible phenomenon. When pH increases, hydrogen ions
are again substituted by calcium found in food products. However, this
process is so slow that during heating of food products spores continue
to be sensitive to heat (GOULD,1996).
Reduced thermoresistance at lower pH was demonstrated for Salmonella
enteritidis and Escherichia coli indicating simultaneouslythat the effect
depended on the type of the applied acid: lactic acid and acetic acid exhibited
asimilar or even greater lethal effect than HCl, whereas the application
of citric acid decreased this effect (BLACKBURN et al., 1997). Reduced
thermoresistance under the influence of lactic and acetic acids was also
reported in the course of heating of S. faecium (HOUBEN,1980; 1982). It was
demonstrated that bacteria were more sensitive to pH changes than
yeasts and molds (SABATAKOU et al., 2001).
The calculation of D-values determined in the case of E. faecium
heating in environments characterised by different pH allowed determination
of log D-pH dependence as well as the zpH coefficient, i.e. the
difference in the pH value which causes tenfold reduction of D. The zpH
values presented in Table 2confirm that, together with the increase of
heating temperature, the sensitivity of the examined strain to the
medium acidity also increases: zpH decreases from 5.12 (55 °C) to 4.09
(65 °C).
If during the heating process the conditions (aw and pH) in the can are
not optimal for bacteria, then also during storage the possibility of regeneration
of microorganisms will be reduced. The results collated in Table 1
indicate that heating and regeneration of E. faecium bacteria on the
medium with areduced pH value (variant B`, C` and D`) –incomparison
with the control (variant A) –caused astatisticallysignificant diminishment
of the decimal reduction times D55 and D60 onlythen, when the pH
value was lowered to 6.5 (variant G`). On the other hand, heating at the
temperature of 65 °C caused astatisticallysignificant change of the
decimal reduction time already at pH ≤7.0 indicating that the D65 value for
B`, C` and D` variants differed statisticallysignificantlyfrom the D65 value
determined for the control sample (variant A). From the dependence log D-
pH of the medium during regeneration, the z’pH coefficient was determined
for E. faecium on amodified medium of differing pH. The obtained results
are collated in Table 2. The z’pH coefficient declined together with the
increase of heating temperature and fluctuated from 4.08 (55 °C) to 2.98
(65 °C). These values were by 20.3 to 27.4% lower in comparison with the
zpH value which means that the thermoresistance of the examined bacteria
was significantlysmaller than those determined on optimal media.