Title: Influence of thawing and post thawing storage on microbial succession and biogenic amine development on frozen Atlantic mackerel

At abused temperature handling, scombroid fish including mackerel develops toxic biogenic amines commonly associated with scrombotoxicosis. The present study examined the influence of two thawing methods: fast (FT) and slow (ST) thawing on bacterial diversity and formation of biogenic amines on frozen mackerel stored up to 36 hours and 12 days at ambient temperature (30°C) and refrigerated condition (2-5°C) accordingly. The FT was achieved by immersing frozen mackerel in tap water initially at 18°C for 2 hours, while in ST, polystyrene boxes containing frozen mackerel were kept at 30°C for 12 hours. Total viable counts (TVC), hydrogen sulphide (H2S) producing bacteria and pseudomonads counts were enumerated on culture method while their identification and diversity was performed by sequencing purified 16S rRNA amplicons obtained from extracted DNA from the samples. Formed biogenic amines were quantified using HPLC. FT mackerel exhibited higher (p<0.0001) bacterial growth at 30oC than ST mackerel but the difference was not apparent at 2-5°C. The levels of the biogenic amines were insignificantly influenced by thawing methods though their formation during ambient storage was realized at a lower TVC of log 4 in ST and log 6 for FT after 12 hours. At 2-5 °C they were first quantified on the 8th day for ST and delayed to 10th day for FT at a much higher total viable log counts. Moreaxellaceace group of bacteria dominated the bacteria in both treatments with low relative abundance of higher BA producers save for Enterorbactericeae at 30oC than 2-5°C. Because of the low amine levels even when the fish was spoiled, their use as spoilage indicators may not be dependable.