Biology Reference
In-Depth Information
Various types of salts are used for salting and fermentation of fi sh. They
include solar salt, rock salt and vacuum salt and have their own microfl ora.
Solar salt, which is the most widely used in fi sh curing, has been found to
contain the largest amount of microorganisms. The general bacterial fl ora
of solar salt mostly comprises
Bacillus
types (75%) with the remainder
being
Micrococcus
and
Sarcina
types. The most important spoilage
organisms always present in solar salt are the red halophilic bacteria.
Krieg and Holt (1984) noted that the red halophiles belong to two genera
of bacteria, namely
Halobacterium
and
Holococcus
.
Halobacterium
consists
of rod-shaped bacteria and requires at least 10-15% salt concentration for
growth while
Halococcus
can thrive at 5-10% salt concentration.
Both genera are strictly aerobic and grow optimally at about 37°C and
also produce red carotenoid pigments. In situations where brine is reused
a number of times, the chemical composition of the salt solution is altered.
Signifi cant amounts of organic material are introduced and the bacterial
load of the brine becomes extremely high, especially the red halophiles and
the osmophilic moulds. Two common defects of salted fermented fi shery
products called pink and dun are the result of spoilage by red halophilic
bacteria and a highly osmophilic fungus respectively. The red halophilic
bacteria grows in brine solutions at temperatures ranging from 15°C to
55°C. Table 6.5 shows that only a few pathogenic organisms can proliferate
at salt concentrations higher than 10%. However, it is known that many of
these organisms survive in saturated salt solutions. For instance
,
Typhus
bacteria can survive in saturated salt solutions for 3 to 6 mon, salmonella
in 10% salt solutions for 1 to 3 mon.
Escherichia coli
and
Staphylococcus
aureus
can survive for many weeks in salted fi sh (ICMSF, 1980b).
In general, the types of microorganisms identifi ed in fermented fi sh
products are:
1) Gram-negative aerobic halophilic rods (70%)
2) Gram-positive aerobic halotolerant cocci (20%)
3) Yeasts (3%).
Microorganisms in Fish Sauce and Paste
Fish sauces contain high concentrations of salt (25-30% NaCl, w/v), thus
microorganisms found during sauce production are generally classifi ed
as halotolerant or halophilic bacteria (Tanasupawat et al., 2009). Earlier
studies have indentifi ed the following microorganisms from samples of
nam-pla
; mostly
Bacillus
spp.;
Bacillus cereus
,
B. circulans
,
B. licheniformis
,
B.megaterium
,
B. pumilus
and
B. subtitils
(Crisan and Sands, 1975). The
important roles of bacteria in fi sh sauce/paste are protein degradation
and fl avour-aroma development. Bacteria involved in fi sh sauce and paste
can be classifi ed into two major groups (Lopetcharat et al., 2001).
