3.1. Chemical quality
3.1.1. Changes in pH value
The effect of organic acid salts and storage time on the pH of salmon slices during storage at 1 °C is shown in . The initial pH value of the control slices (6.45) was significantly (P
< 0.05) higher than that of samples treated with NaA (6.32) or NaL (6.36), but not NaC (6.41). These findings are consistent with those of Kim, Hearnsberger, Vickery, White, and Marshall (1995)
, who reported an initial decrease in pH value of catfish fillets treated by tumbling with 0.5% sodium acetate when compared with untreated control fillets, and also with Williams, Rodrick, and West (1995)
, who reported a decrease in the initial pH of NaL-treated catfish fillets (6.32 versus 6.5 for the control). On the other hand, Zhuang et al. (1996)
found no differences (P
> 0.05) in the initial pH values between control catfish fillets and those treated with 2% NaA or NaL.
Effects of organic acid salt treatments and storage time on the pH of sliced salmon during refrigerated storage (1 °C)
Storage time had a significant (P
< 0.05) effect on the pH values, which tends to increase for each of the control as well as NaA- and NaC-treated salmon slices and, by the end of the storage period (day 15), significant difference was observed in the pH values between control (7.1) and all of the other treated samples, which were 0.50–0.63 U lower than the control. On the other hand, González-Fandos, Villarino-Rodríguez, García-Linares, García-Arias, and García-Fernández (2005)
found no significant change in the pH of salmon slices throughout a storage period of 45 days at 2 and 10 °C.
NaL has been revealed to stabilize the pH during storage of meat products (Maca et al., 1997
; Sallam & Samejima, 2004
). Our results also verified an almost constant pH (6.34–6.37) for NaL-treated salmon slices throughout the storage time, while the pH of control or NaA- and NaC- treated samples were significantly increased.
3.1.2. ATP breakdown products
The initial quality loss in fish is primarily caused by post-mortem autolytic changes and is unrelated to the microbiological activities. Of particular importance in this respect is the degradation of adenosine nucleotides (ATP-related compounds) (Gram & Huss, 2000
). The rate and pattern of nucleotide degradation in fish varies with species, body location (dark or white muscle), fish maturity, stress during capture, handling, season and storage conditions (Erikson, Beyer, & Sigholt, 1997
; Huss, 1995
; Luong, Male, Masson, & Nguyen, 1992
). Degradation of ATP goes through the intermediate products ADP, AMP, IMP, Ino and Hx. Most of the adenosine nucleotides disappeared quickly since they degraded to IMP within 1–3 days after fish capture, and as the degradation continues, Ino and then Hx will be produced. The autolytic changes contribute to spoilage, mainly by making catabolites available for bacterial growth (Huss, 1995
). Traditionally, the degradation of ATP to IMP has been attributed to muscle endogenous autolytic enzymes (Gram & Huss, 1996
), meanwhile the degradation of IMP to Ino and Hx has also been connected with the growth of bacteria. Hx has a slight bitter taste and is regarded as a contributor to off-flavours (Dalgaard, 2000
; Gram & Huss, 2000
), whereas IMP is desirable as a flavour component enhancer and is strongly associated with acceptability in fresh fish (Fletcher & Statham, 1988
Hypoxanthine (Hx) accumulation in fish muscle is shown in . The levels of Hx were significantly (P
< 0.05) increased from initial values of 0.62–0.68 μmol/g of salmon to final values of 3.58, 2.13, 2.43, and 2.7 μmol/g for control, NaA-, NaL- and NaC- dipped samples, respectively, by the end of the storage period. Significant increase in Hx concentration with the increase of storage time has also been reported in various studies for many seafoods (Alasalvar et al., 2001
; Greene, Babbitt, & Reppond, 1990
; Özogul, Özogul, & Gökbulut, 2006
Fig. 1 Changes in hypoxanthine (Hx) concentrations of sliced salmon treated by dipping in sodium acetate (NaA), sodium lactate (NaL) and sodium citrate (NaC) solutions during storage at 1 °C. Values represent means ± SE of three replicates; LSD (more ...)
It has been reported that bacterial growth has a direct correlation with Hx production, and the rate of bacterial production of Hx is higher than the production rate by the autolytic activities (Gram & Huss, 1996
). Treatment of salmon slices with the sodium salts of organic acids induced significant (P
< 0.05) reduction in Hx values when compared with the non-treated control slices. This might be related to the inhibitory effect of these salts on the growth of the various bacterial groups (Maca et al., 1997
; Sallam & Samejima, 2004
; Zhuang et al., 1996
) with consequent reduction in Hx formation.
Both rates of Hx production and Hx concentration vary substantially between fish species. Nevertheless, Hx concentration has been useful for shelf life determination in specific seafoods (Dalgaard, 2000
). Our result also revealed that hypoxanthine (Hx) concentrations can be a useful index for freshness of sliced salmon during refrigerated storage.
value index, calculated from ATP degradation products, is the percent of the sum of Ino and Hx divided by the sum of ATP, ADP, AMP, IMP, Ino, and Hx. The K
value has been reported as a good indicator of fish freshness in various species of fish (Ehira & Uchiyama, 1987
; Özogul et al., 2006
). Many factors affect the K
value of fish, including fish species (Hattula & Kiesvaara, 1992
), type of muscle (Murata & Sakaguchi, 1986
), stress of fish during capture (Erikson et al., 1997
), and storage temperature (Guizani, Al-Busaidy, Al-Belushi, Mothershaw, & Rahman, 2005
summarizes the mean K
values obtained for salmon slices analyzed during refrigerated storage at 1 °C. At the beginning of the storage, the K
value ranged from 13.8% to 14.4%, and then increased to a high level of 72.3% in the control samples at day 15 of the storage. On the other hand, significantly (P
< 0.05) lower K
values of 41.1%, 47.5%, and 51.2% were detected, in comparison with the control, by the end of the storage period for NaA-, NaL-, and NaC-treated samples, respectively. K
value development during cold storage of salmon has also been reported in various studies (Einen & Thomassen, 1998
; Erikson et al., 1997
Fig. 2 Changes in K value of sliced salmon treated by dipping in sodium acetate (NaA), sodium lactate (NaL) and sodium citrate (NaC) solutions during storage at 1 °C. Values represent means ± SE of three replicates; LSD is defined at P < (more ...)
In this study, the pattern of increase of K
value for control samples occurred linearly at a relatively high rate of 6% per day during the first 6 days, and then increased with a slow rate of 2.44% per day during the following 9 days, indicating that the K
value is considerably affected by the early autolytic activity. In treated samples, however, the K
value increased at moderate rates of 2.81%, 3.27%, and 3.84% per day for samples dipped in NaA, NaL, and NaC, respectively during the first 6 days of storage, and then it increased with much slower rates of 1.41%, 1.57%, and 1.53% in the same samples, respectively, during the next nine days. The increase of the K
value primarily resulted from the sharp decline of IMP in the fish flesh during the first week of storage (data not shown). This finding is in agreement with those of Özogul et al. (2004)
and Guizani et al. (2005)
, who showed that the K
value was strongly affected by the fast depletion of IMP.
For both the control and treated samples, the K
value significantly (P
< 0.05) increased with the storage time. In addition, it was found that, as the K
value increased, the sensory quality of sliced salmon decreased, and when the sliced salmon of control was considered unacceptable (days 7–8), based on the sensory evaluation, a K
value of about 55% was estimated; nevertheless the K
value for all of the treated samples did not reach such a value even by day 15 at the end of the storage period. Wide variations in the K
value have been reported among the different species on the day of fish rejection. Özogul et al. (2004)
detected a K
value of 80% for vacuum-packed sardine stored at 4 °C on the rejection day (day 9), while a lower K
value of 39% was determined on the day of rejection (day 17) for sea bream stored at 2 ± 2 °C (Alasalvar et al., 2001
In some species, the K
value increases faster than the observed sensory changes. Greene et al. (1990)
found that the K
value of flattened sole was 80% after only 1 day of storage in ice. Likewise, Luong et al. (1992)
revealed that the K
value of Pacific cod, stored on ice (0–4 °C), approached 100% after 2 days; yet the fish were judged suitable for consumption up to 10 days later.
value and Hx concentration determined in this study were found to be almost associated with the freshness quality in both the control and treated sliced salmon during the refrigerated storage. Generally, the determination of ATP breakdown products is most reliable and suitable for the evaluation of fish freshness, when a high demand of freshness, as in Japan, is proposed for sashimigrade seafood (Ehira & Uchiyama, 1987
). Sashimi is raw fish or shellfish served sliced, “Sashimi”, or as a finger-size piece of raw fish on a bed of a small rice ball “Sushi”. A maximum K
value of 20% has been set for sashimi (Saito et al., 1959
). It is not, however, relevant to establish any concentration limit of ATP breakdown products as a limit acceptable for fish, because there are variations in breakdown products between fish species and even between the individuals within the same fish species.
3.1.3. Total volatile base nitrogen
TVB-N and TMA are most useful indices for spoilage in fresh and lightly preserved seafood (Dalgaard, 2000
). In the current study, the initial TVB-N values (mg N/100 g muscle) in sliced salmon analyzed ranged from 8.69 in NaC-treated samples to 9.32 in NaL-treated samples (). Slight increase in the TVB-N value was then observed throughout the first days of storage, reaching a value of 13.6, 11.3, 12.8, and 12.8 mg N/100 g by day 6 in each of the control, NaA-, NaL- and NaC- treated samples, respectively. By day 9 and afterwards, however, a sharp rise of TVB-N value was noticed in the control, which reached a value of 22.7 mg N/100 g muscle on day 9, while the sliced salmon dipped in NaA, NaL, or NaC showed lower values of 14.6, 15.8, and 17.8 mg N/100 g muscle, respectively, on the corresponding day of storage. A similar pattern of the increase in TVB-N values for the control has been reported by Hozbor et al. (2006)
during cold storage of sea salmon.
Fig. 3 Effects of sodium acetate (NaA), sodium lactate (NaL) and sodium citrate (NaC) treatments on total volatile base nitrogen (TVB-N) content in sliced salmon during storage at 1 °C. Values represent means ± SE of three replicates; LSD is (more ...)
At the time of fish rejection (days 8, 12, 12, and 15 for control, NaC-, NaL-, and NaA-treated samples, respectively), a TVB-N value of about 20 mg N/100 g muscle was detected. Similarly, Özogul, Özyurt, Özogul, Kuley, and Polat (2005)
reported a TVB-N value of 22.6 mg/100 g at time of spoilage in European eel stored in boxes at 3 ± 1 °C, although they recorded a higher value of 103 mg TVB-N per 100 g flesh by the end of the storage period (12 days). It has been suggested that the TVB-N value is affected by species, season, harvesting area, age and sex of fish (Kilinc & Cakli, 2005
As for many fish species, the formation of TVB-N increased with the time of storage, and by the end of the storage period (day 15), a significantly (P
< 0.05) higher value of 34.5 mg/100 g was detected for TVB-N in control when compared with those in the different treated samples, which attained much lower values of 19.3, 22.6, and 24.4 mg N/100 g muscle for samples dipped in NaA, NaL, and NaC, respectively. Nevertheless, the TVB-N values in the different samples analyzed, throughout the entire storage period, were all below the maximum value of 35 mg N/100 g flesh specified by the EC guidelines (Commission Decision 95/149/EC, 1995
) for different species of raw fish. Based on the results obtained from this study, TVB-N value could be useful in assessing the degree of salmon deterioration more than in evaluating the changes occurring during the first stages of storage.
The changes in TMA content in fresh sliced salmon stored under refrigeration at 1 °C are shown in . The initial TMA value ranged from 0.65 to 0.73 mg/100 g muscle, which then increased very slowly during the first 6 days of storage, reaching low values of 1.63, 1.08, 1.18, and 1.36 mg/100 g for each of the control, NaA-, NaL-, and NaC-dipped samples, respectively. By the day 9 of storage and thereafter, however, the TMA value of control samples increased steadily, attaining a final value of 6.57 mg/100 g flesh by the end of the storage period (day 15), whereas significantly (P < 0.05) lower values of 3.41, 4.37, and 4.58 mg/100 g flesh were detected for sliced salmon dipped in NaA, NaL, and NaC, respectively.
Fig. 4 Effects of sodium acetate (NaA), sodium lactate (NaL) and sodium citrate (NaC) treatments on trimethylamine (TMA) concentration in sliced salmon during storage at 1 °C. Values represent means ± SE of three replicates; LSD is defined at (more ...)
The relatively small increase in TMA over the storage period in this study reflects the low level of trimethylamine oxide (TMAO) in the flesh of sliced salmon. Salmon has been reported to have a low level of TMAO, ranging from 11 to 14 mg/100 g flesh (Emborg, Laursen, Rathjen, & Dalgaard, 2002
). In addition, it has been indicated that the concentrations of TMA-N in numerous fatty fish never reached the limit of 5 mg TMA-N/100 g (Özogul et al., 2004
), although the rejection limit in fish flesh is usually more than this limit.
The level of TMA found in fresh fish rejected by sensory panels varies between species, but is typically around 10–15 mg TMA-N/100 g in aerobically-stored fish (Dalgaard, Gram, & Huss, 1993
). In the present study, the TMA value detected at the time of fish rejection, as estimated by the sensory attributes, was <4 mg TMA-N/100 g for both control and treated salmon samples. Such a value is about 3-fold lower than the proposed value for spoiled fish. However, Hozbor et al. (2006)
, determined a higher concentration of 15.8 mg nitrogen/100 g for TMA at the point of spoilage (day 10) in sea salmon during aerobic storage in ice at 0 °C.
TVB-N and TMA are directly related to the microbial spoilage in various species of fish during their storage under refrigerated conditions (Dalgaard, 2000
). The amount of TVB-N and TMA were low during the edible storage period and increased only rapidly when the fish was near to rejection. Therefore, TVB-N and TMA are considered unreliable to estimate the degree of freshness in the early stages of storage of sliced salmon; they only reflect the degree of spoilage in the later stages.