Occurrence of histamine and histamine-forming bacteria in salted mackerel in Taiwan

Thirty-three samples of salted mackerel sold in retail markets and supermarkets in Taiwan were tested to determine the occurrence of histamine and histamine-forming bacteria. The numbers of aerobic plate count (APC) in all samples were below the Taiwanese regulatory level of 6.47 log cfu/g. The levels of pH, salt content, and total coliform in all samples ranged from 5.7 to 6.4, 5.0 to 18.1%, and <3 to 60 most probable number (MPN)/g, respectively. None of these samples contained Escherichia coli. However, eight of the 33 samples (24.2%) had unacceptable levels of total volatile basic nitrogen (TVBN), based on Taiwanese standard for TVBN in seafood products. Although the average content for each of the nine biogenic amines in all samples was less than 3 mg/100 g (30 ppm), two of the 18 samples collected from southern Taiwan contained 70.1 and 120.2 ppm of histamine, which are more than the 50 ppm allowable limit suggested by the US Food and Drug Administration. Of the 40 presumtive histamine-forming bacterial colonies isolated on the differential agar plates for the test samples, 4 strains produced histamine, ranging from 18.3 to 21.0 ppm, in TSBH broth supplemented with 2% L-histidine. These histamine-producing bacteria were identified as Pantoea sp. (2 strains), Pantoea agglomerans (1 strain), and Enterobacter cloacae (1 strain).