Comparative Evaluation on the Determination of Coliforms and Enterococci in Frozen Seafood Products

Abstract

Fifty samples of commercial frozen raw and prepared seafood products were tested for total bacterial counts, coliforms, enterococci and Escherichia coli. For the determination of coliforms desoxycholate agar plate method and lactose broth-BGLB Most Probable Number (MPN) method were used, while Azide Esculin (AE) agar plate method and ACAC MPN method according to Horie, et al. were used for enterococcus counts.The values obtained with the plate method and the MPN method were fairly related both in the coliform and the enterococcus test. Relationships between total bacterial counts, coliform counts obtained by desoxycholate agar and enterococcus counts by AE agar were investigated. Enterococcus counts were more closely related to total counts than to coliform counts, while coliform counts were not so related to both the other two counts. On the other hand, E. coli was not detected except in only 3 samples.Bacterial suspensions of coliforms and enterococci in 10% skim milk were frozen and storaged at -21°C for long period. Viable cell counts of coliforms decreased significantly during freezing and freezing strorage, furthermore, majorities of the survived population had lost the ability of colony-formation in desoxycholate agar. Conversely, enterococcus counts determined by AE agar remained relatively constant during the same storage period. In general, lower values of coliform counts of frozen food samples, as compared with enterococcus counts, may be considered to be caused by not only rapid death but also loss of the growth ability of coliform organisms in desoxycholate agar during freezing storage.The result suggests that entrococci can be used advantageously in preferrence to coliforms as indicator of bacterial contamination before freezing of frozen seafood products.