173
regions were surrounded by residential houses and fishponds (Sorio and Peralta
2018). It has been also reported that the microflora of the oysters contains
Pseudomonas, Vibrio, Serratia, Clostridium, Proteus, Bacillus, Escherichia,
Lactobacillus, Flavobacterium, Enterobacter and Micrococcus (Jay 2000). Cao
et al. (2009a) detected gram-negative bacteria as dominant in oyster (C. gigas), and
they reported that 22% and 20% of these were Pseudomonas and Vibrionaceae,
respectively. Shewanella, Alcaligenes, Enterobacteriaceae, Moraxella,
Acinetobacter, Flavobacterium, Corynebacterium, Staphylococcus, Micrococcus,
Lactic acid bacteria and Bacillus were also detected as minor organisms. After
death, the microbial flora of oysters may increase depending on environmental conditions. Indeed, some researchers have reported increases in the total viable count
(TVC) (Cruz-Romero et al. 2008; Hu et al. 2008; Cao et al. 2009a). In oyster samples collected from seafood commercial establishments and the cultivation area,
Vibrio cholerae, V. parahaemolyticus and Salmonella spp. were not detected. The
counts of coliforms at 35 and 45 °C indicated that samples obtained from both the
cultivation area and place of sale were contaminated. E. coli was detected in 4 (9%)
samples collected in the cultivation area and in 16 (35.5%) samples obtained from
commercial establishments. It was commented that inadequate storage of oysters in
the period between collection and sale may contribute to reaching high numbers
(Pereira et al. 2006).
A measure of pH decrease is considered a better test of spoilage in oysters and
other molluscan shellfish than volatile nitrogen bases (Jay 2000). Although pH
decreases due to lactic acid formation from glycogen after death, pH also increases
with the increase of basic compounds in the later period. When pH drops, the net
surface charge on muscle proteins is reduced. In this case, loss in water holding
capacity is seen with partial denaturation. This also causes toughening in the texture
(Songsaeng 2010).
During spoilage of oysters, total volatile basic nitrogen (TVB-N) including trimethylamine (TMA), dimethylamine (DMA), ammonia and other volatile basic
nitrogen increase, and these compounds are considered as spoilage indicators.
Studies have reported increases in TVB-N contents during storage of oysters
(Balasundari et al. 1997; Lopez-Caballero et al. 2000; Cao et al. 2009b; Songsaeng
2010; Songsaeng et al. 2010). It is reported that total volatile basic nitrogen (TVBN)
can be used as freshness indicator for raw Eastern oysters, with the acceptability of
11 mg/100 g (Zhang et al. 2017).
Due to their high content of unsaturated fatty acids, oysters have post-mortem
changes in lipids. The hydrolysis and oxidation events in lipids decrease the quality
of oyster. Changes in oyster lipids can be observed during refrigerated or frozen
storage. Indeed, some researchers have reported increases in peroxide (PV) and
thiborbutiric acid reactive substances (TBARs) during storage of oysters
(Balasundari et al. 1997; Jeong et al. 1990).
3.1 Bivalves
regions were surrounded by residential houses and fishponds (Sorio and Peralta
2018). It has been also reported that the microflora of the oysters contains
Pseudomonas, Vibrio, Serratia, Clostridium, Proteus, Bacillus, Escherichia,
Lactobacillus, Flavobacterium, Enterobacter and Micrococcus (Jay 2000). Cao
et al. (2009a) detected gram-negative bacteria as dominant in oyster (C. gigas), and
they reported that 22% and 20% of these were Pseudomonas and Vibrionaceae,
respectively. Shewanella, Alcaligenes, Enterobacteriaceae, Moraxella,
Acinetobacter, Flavobacterium, Corynebacterium, Staphylococcus, Micrococcus,
Lactic acid bacteria and Bacillus were also detected as minor organisms. After
death, the microbial flora of oysters may increase depending on environmental conditions. Indeed, some researchers have reported increases in the total viable count
(TVC) (Cruz-Romero et al. 2008; Hu et al. 2008; Cao et al. 2009a). In oyster samples collected from seafood commercial establishments and the cultivation area,
Vibrio cholerae, V. parahaemolyticus and Salmonella spp. were not detected. The
counts of coliforms at 35 and 45 °C indicated that samples obtained from both the
cultivation area and place of sale were contaminated. E. coli was detected in 4 (9%)
samples collected in the cultivation area and in 16 (35.5%) samples obtained from
commercial establishments. It was commented that inadequate storage of oysters in
the period between collection and sale may contribute to reaching high numbers
(Pereira et al. 2006).
A measure of pH decrease is considered a better test of spoilage in oysters and
other molluscan shellfish than volatile nitrogen bases (Jay 2000). Although pH
decreases due to lactic acid formation from glycogen after death, pH also increases
with the increase of basic compounds in the later period. When pH drops, the net
surface charge on muscle proteins is reduced. In this case, loss in water holding
capacity is seen with partial denaturation. This also causes toughening in the texture
(Songsaeng 2010).
During spoilage of oysters, total volatile basic nitrogen (TVB-N) including trimethylamine (TMA), dimethylamine (DMA), ammonia and other volatile basic
nitrogen increase, and these compounds are considered as spoilage indicators.
Studies have reported increases in TVB-N contents during storage of oysters
(Balasundari et al. 1997; Lopez-Caballero et al. 2000; Cao et al. 2009b; Songsaeng
2010; Songsaeng et al. 2010). It is reported that total volatile basic nitrogen (TVBN)
can be used as freshness indicator for raw Eastern oysters, with the acceptability of
11 mg/100 g (Zhang et al. 2017).
Due to their high content of unsaturated fatty acids, oysters have post-mortem
changes in lipids. The hydrolysis and oxidation events in lipids decrease the quality
of oyster. Changes in oyster lipids can be observed during refrigerated or frozen
storage. Indeed, some researchers have reported increases in peroxide (PV) and
thiborbutiric acid reactive substances (TBARs) during storage of oysters
(Balasundari et al. 1997; Jeong et al. 1990).
3.1 Bivalves