Fish sauce (nước mắm, nam pla, kecap ikan, patis) is an important condiment in Southeast Asian countries. It is made by naturally fermenting a mixture of fresh small fish and salt in a ratio of approx. 3 to 1 for 12 months or longer. Fish sauce has a pungent smell, a characteristic flavour and is considered a good source of protein, amino acids, and minerals (Lopetcharat et al., 2001, Zaman et al., 2010). However, the presence of high concentrations of biological amines in fish sauce, and especially histamine, can pose a risk to human health. During fish sauce fermentation, histidine from fish protein is converted to histamine by histidine decarboxylase, which is formed by certain bacteria (Lopetcharat et al., 2001, Sanceda et al., 1996, Zaman et al., 2010). Commercial fish sauce products were reported to contain histamine concentrations ranging from 100 to 1000 mg/kg (Brillantes and Samosorn, 2001). The maximum level of histamine in fish sauce regulated by the Codex Alimentarius Commission is 400 mg/kg (Codex-stand:302-2011), which approximately corresponds to 3.6 mM. It is thus obvious that the histamine content in some commercial products exceeds the Codex standard by far. Several methods have been suggested to minimize histamine formation in food products including irradiation, high hydrostatic pressure, modified atmosphere packaging, addition of preservatives, and antibacterial agents. However, histamine is a stable compound, so except for irradiation, other processing methods cannot degrade this compound. Furthermore, these methods could create undesirable effects on the quality and safety of food. The presence of histamine-degrading enzymes has been described for various microorganisms, e.g., Rhizobium sp. (Bakke et al., 2005), Nocardioides simplex (Siddiqui et al., 2000), Natrinema gari (Tapingkae et al., 2010a), Bacillus amyloliquefaciens, Bacillus subtilis (Lee et al., 2015), Staphylococcus xylosus (Martuscelli et al., 2000), Staphylococcus carnosus (Zaman et al., 2014), Arthrobacter crystallopoietes (Sekiguchi et al., 2004), or Brevibacterium linens (Leuschner et al., 1998). These organisms contain either a histamine oxidase or a histamine dehydrogenase, which catalyze histamine degradation via oxidative deamination (Lee et al., 2015, Tapingkae et al., 2010a, Zaman et al., 2011). In addition to their histamine-degrading ability, these species isolated from fish sauce fermentations were often found to be resistant to high salt concentrations. The extremely halophilic archaeon Natrinema gari isolated from Thai fish sauce is tolerant to salt concentrations of 20–30% (Tapingkae et al., 2010a).

The application of microbial cells containing specific enzymes can have certain advantages over the use of isolated enzymes in biocatalytic applications, including the elimination of the separation and purification procedures, increased enzyme stability, or saving time (Junter and Jouenne, 2004, Stolarzewicz et al., 2011). This is also true for the employment of immobilized microbial cells, which have been widely applied in industrial production, the medical field, or environmental applications (Mrudula and Shyam, 2012, Park and Chang, 2000). Immobilization of cells was also shown to be advantageous in various food processes, including enhanced fermentation productivity, cell stability and reduced downstream processing costs (Groboillot et al., 1994).

In this study, halophilic bacteria, isolated at different stages of Vietnamese fish sauce fermentation, were screened for their histamine-degrading capability under high salt conditions. Virgibacillus campisalis TT8.5 was selected for further studies, and the efficiency of immobilized V. campisalis TT8.5 cells in reducing histamine in fish sauce products was evaluated.