Capacitance microbiology as a means of determining the quantity of spoilage bacteria on fish fillets

Abstract

An experiment was conducted to determine if a method for enumeration of Pseudomonas fluorescens in less than 11 h could be used to predict potential spoilage of fresh fish of four species. In each of three separate replications (Rep), five boneless fillets from each species of fish, including rainbow trout (RT), Atlantic salmon (AS), red grouper (RG), and tilapia (T) were obtained fresh from a retail outlet. For each species, six 25-g samples of fish flesh were asceptically removed from each fillet, placed into a polyethylene bag, and stored at 3°C for 0, 1, 2, 3, 4, or 5 days. After storage, samples were analyzed for psychrotrophic plate count (PPC), Pseudomonas fluorescens plate counts (PFPC), and P. fluorescens capacitance detection times (PDT) and subjectively evaluated for odor (ODOR). PPC gradually increased on all fish species as storage time increased. In most cases, PFPC decreased slightly and then progressively increased as storage time increased. In Reps 1 and 2, PDT decreased gradually (indicating an increase in bacteria); however, in Rep 3, PDT were erratic and difficult to interpret. Odor increased gradually throughout the storage period for all fish species. Linear correlations (R2 > 0.80) were observed between PPC and day of storage (DAY) for all fish species and Reps except for RT and RG in Rep 3. PFPC correlated (R2 > 0.70) to DAY for all fish except RT in Rep 3 and RG in Rep 2. PDT was negatively correlated to DAY for RT and T in Rep 1 and for all fish in Rep 2. Odor scores were highly correlated (R2 ≤ 0.84) to DAY for all fish tested. PPC and PDT were negatively correlated for RT in Reps 1 and 2, AS in Rep 2, and T in Reps 1 and 2. Because results can be obtained in <12 h, the capacitance procedure with further refinement may provide an excellent alternative to conducting PPC as a means of predicting potential spoilage of fish such that the fillets of inferior quality (i.e., those that will spoil rapidly) may be sent to distribution outlets that are known to move fish products quickly and are able to sell the fish before it spoils.