International Journal of Aquaculture, 2016, Vol.6, No.19, 1
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was replaced every one to two days, earlier reported by Howgate (2006). Lower levels of TVBN at the beginning
could therefore be due to lower levels of endogenous ammonia as a result of reduced microbial activity during the
first 12 days of storage of the fish in ice (Okeyo et al., 2009). This agrees with the assertion that volatile amines
especially TMA-N does not present a reliable method for determining fresh fish spoilage because they are only
detected when spoilage of fish has already progressed (Okoro et al., 2010). Mhongole (2009) also observed that
sensory and microbiological methods were more reliable for quality assessment of fresh fish than chemical
methods.
4.4 Physico-chemical analysis
pH is directly correlated to microbial growth (Huss, 1995), hence important in influencing post-mortem changes
in the freshness of fish. Decreasing pH values of the fish with storage time in ice for this study have been
previously reported (Obemeata et al., 2011). Generally, pH decreases during anaerobic formation of lactic acids
during the first hours after death (Huss, 1995). Decrease in pH level may be due to the decomposition of
carbohydrate of the fish by proteolytic microbes that produce acids thereby increasing the acid level of the
medium (Eyo, 1993). This is well supported by the observed slow growth of bacteria until sensory rejection – an
indication of the importance of ice in retarding microbial growth (Huss, 1995). Consequently, TVB-N and
TMA-N started to increase sharply after day 12 which also coincided with increase in total viable bacteria counts
on the fish indicating as associated between pH levels and microbial activity (Huss, 1995). Most gram negative
spoilage bacteria cannot survive at low pH (acidic) hence lower pH eventually entails lower microbial activity
(Huss, 1995). It has been suggested that tropical fish species reach a very low pH explaining why they have a
longer shelf life (Huss, 1995). Increased pH values after sensory rejection could possibly be an indication of
accumulation of alkaline compounds as well as volatile bases such as ammonia and trimethylamine produced by
autolytic activities and metabolism of spoilage bacteria (Liu et al., 2010). Thus, increase in pH after day 16 could
have been associated with rapid spoilage of the fish. Similarly, advanced gaping of the muscle was observed
concurrent to the increase in pH around the time of sensory rejection (Figure 3) suggesting that temperature
influences post-mortem pH (Huss, 1995). It is reported that even though changes in pH are generally rather small,
they have great technological importance (Huss, 1995).
5 Conclusions
Results of different methods (sensory, microbiological, biochemical and physico-chemical) of assessing freshness
and quality of Lake Malawi Tilapia have been discussed. Increasing bacteria numbers, increasing TVB-N and
TMA-N, changes in pH against declining sensory quality suggest a strong correlation in fish spoilage. The general
trend nevertheless, show that results from a well trained and experienced sensory evaluation panel can still be
reliable in the absence of the other methods. pH can also be used as a quick freshness indicator with an
understanding that muscle pH for live fish is generally neutral and increases as deterioration of the quality of the
fish progresses in storage. Relatively low values of TMA-N suggest that this may not be a reliable method for
assessing freshness quality of freshwater fishes such as the Lake Malawi Tilapia.
The strength of the study is that use of combined methods in freshness and quality assessment of fresh fish
appears to be the first in Malawi and hence, could create more interest in similar studies in future. Probably, the
main challenge or weakness was the complexity of analysing TVB-N and TMA-N using the modified Conway
Diffusion Method. Further, analysis of TMA-N in freshwater fish such as tilapia appears to be a challenge.
6 Recommendations
Rejection of fish samples before attainment of unacceptable microbial limits underpins the need for using more
than one method for accurate freshness and quality assessments of fresh fish. Such study should be conducted on
fish at different storage conditions other than ice only.
