A study on the relationship between microbial growth, histamine development and organoleptic changes in retailed fresh sprangled emperor and big eye tuna

The safety of fresh fish is often compromised due to contamination by bacterial human pathogens and histamine-forming bacteria. In addition, the quality of fish is also adversely affected during proliferation of spoilage microorganisms thus reducing their shelf life. The purpose of the study was to comparatively assess the safety and quality of non-scombroid fish ‘Sprangled Emperor’ (Lethrinus nebulosus) and scombroid fish ‘Big Eye Tuna’ (Thunnus obesus) purchased from market and supermarket using sensorial, microbiological, histamine and molecular analyses. Fish sold in market and supermarket were sampled at two time points: T0 (immediately upon display) and T8 (eight hours after display). Microbial load of fish samples was determined by enumeration of Total Viable Counts (TVC), Vibrio spp., Clostridium perfringens, Pseudomonas spp., marine organisms and spoilage yeasts and molds. Fish were also subjected to histamine testing by the AOAC method. Sensorial evaluation of fish samples for organoleptic traits such as overall appearance, color, odor and texture was also carried out at both time points. An increase in the microbial load of ‘Sprangled Emperor’ (SE) and ‘Big Eye Tuna’ (BET) purchased from both market and supermarket was generally observed after eight hours of display; TVC increase was higher in SE (Δ = 0.7 log cfu/g) compared to BET (Δ = 0.3 log cfu/g) although the difference was not significant (P>0.05). The level of marine organisms, C. perfringens and Pseudomonas spp. were significantly higher (P<0.05) after eight hours of display. Initial sensory scores of fish purchased from market were significantly higher than those of supermarket (P<0.05) and marked sensorial quality deterioration was noted after eight hours of display. Histamine testing revealed a lower level of histamine for SE (<2.3 ppm) compared to BET (1-8 ppm) at both time-points. Findings of this study showed that displaying fish for ≥ eight hours at retail can considerably compromise the safety and quality of scombroid fish such as tuna.