Biomedical Engineering Reference
In-Depth Information
acted synergistically to inactivate
L. innocua
cells and total mesophilic microorgan-
isms. No surviving
L. innocua
were recovered in the caviars after application of the
nisin-RF combined treatments at 65 °C. The come-up times in the RF-heated
product were signifi cantly lower compared with the water bath-heated caviar at all
treatment temperatures. The visual quality of the caviar products treated by RF with
or without nisin was comparable to the untreated control. The effect of nisin, chemi-
cal antimicrobials or moderate heat (singly or in combination) on inactivation of
L. monocytogenes
in sturgeon caviar was further investigated. Treating caviar with
500 or 750 IU/ml nisin initially reduced
L. monocytogenes
by 2-2.5 log units
(Al-Holy et al.
2005
). Nisin in combination with 2 % lactic acid plus 134 ppm chlo-
rous acid reduced viable counts of
L. monocytogenes
below detectable levels at
several points during storage at 4 °C, and it also reduced total mesophilic viable
counts. However, best results were obtained for the combinations of nisin and mild
heat (60 °C for 3 min). Mild heating in combination with nisin synergistically
reduced viable counts of
L. monocytogenes
and total mesophiles. No
L. monocyto-
genes
cells were recovered from caviar treated with heat and nisin (750 IU/ml) after
a storage period of 28 days at 4 °C.
L. monocytogenes
can be highly prevalent in minced tuna and fi sh (salmon and
cod) roe, where it can multiply more rapidly under temperature-abuse conditions
(Takahashi et al.
2011
). Such seafood products are among the most popular sushi
ingredients for consumers of all age groups. Since the complete elimination of
L. monocytogenes
from the processing environments in which minced tuna and fi sh
roe products are prepared considered to be very diffi cult, it is necessary to apply
other preservation methods that, at the same time, do not negatively affect the taste
of these seafood products (Takahashi et al.
2011
). So in one study, nisin in the form
of Nisaplin and other antimicrobials (lysozyme, e-polylysine, and chitosan) were
tested for inhibition of
L. monocytogenes
in the seafoods stored at 10 °C (Takahashi
et al.
2011
). Nisaplin effectively inhibited
L. monocytogenes
growth in minced tuna
at 500 ppm and in salmon roe at 250 ppm within their standard shelf lives.
Consequently, 500 ppm of Nisaplin, (which is the legal standard for cheese and
meat products in many countries), was considered an appropriate and safe concen-
tration for seafood.
Karashi-mentaiko is red-pepper seasoned cod roe. However,
L. monocytogenes
has been isolated from Karashi-mentaiko, and since there is no heat treatment in the
manufacturing process of Karashi-mentaiko, the control of bacteria is very impor-
tant (Hara et al.
2009
). Nisin can effectively inhibit growth of
L. monocytogenes
in
Karashi-mentaiko (Hiwaki et al.
2007
). In tests carried out independently on eight
different strains, the number of
L. monocytogenes
in Karashi-mentaiko stored at
4 °C was decreased by Nisaplin added at 60 and 600
μ
g/g (Hara et al.
2009
). In the
samples containing 60
g/g Nisaplin, most of the isolates were undetected (except
for two strains) through the whole storage period (28 days), while at 600
μ
g/g
Nisaplin none of the strains were detected. When samples were stored at 15 °C, both
concentrations of Nisaplin (60 and 600
μ
g/g) brought all strains undetectable during
the whole storage period. Interestingly, the MICs for Nisaplin obtained in Karashi-
mentaiko were lower compared to BHI broth, suggesting that ingredients of Karashi-
mentaiko, storage temperature and a
w
infl uenced the effi cacy of Nisaplin.
μ
