Influence of frozen storage time and thawing methods on the microflora of thawed Nile tilapia fillets

Abstract

This study was aimed to find out the influence of frozen storage time and thawing methods on the counts of coliforms, E. coli, Pseudomonas spp., and total viable counts (TVC) of thawed de-skinned Nile tilapia (Oreochromis niloticus) fillets. Individual quick frozen fillets of size 120-170 g, packed in polyamide packs (2 fillets/pack), were stored at -18 ± 2 °C. Samples were taken for thawing experiments every month (months 0, 1, 2, … and 8). Four different defreezing environments, namely cold air (7 ± 1 °C), air at ambient temperature (30 ± 1 °C), cold water (8 ± 1 °C), and water at normal temperature (25 ± 1 °C), were investigated. All experiments were triplicated. No E. coli was detected in all thawed samples. No differences (p > 0.05) were observed in coliforms, E. coli, Pseudomonas spp., and TVC of the newly stored samples (month 0) after thawing by the four studied methods. No significant effects (p > 0.05) of storage time (months 1, 2, and 3) and thawing methods on the coliforms and Pseudomonas quantity of tilapia fillets were found. Thawing by cold air showed to be the most suitable mean, which resulted in significantly lower TVC (p < 0.05) compared to defrosting in ambient air or in water at normal temperature. Meanwhile, air at ambient temperature showed to be the worst medium, giving the highest TVC in the fillets after thawing. The results supported a slow thawing method (e.g. by cold air) for this fillet product or similar ones, which are processed and frozen pre-rigor mortis.