Reduction of tyramine accumulation in Thai fermented shrimp (kung-som) by nisin Z-producing Lactococcus lactis KTH0-1S as starter culture

Abstract

Tyramine production of 20 strains of lactic acid bacteria and coagulase-negative staphylococci isolated from Thai traditional fermented shrimp (kung-som) was investigated by using chromatographic and molecular methods. Among 20 strains tested, Enterococcus faecalis 9Y, Enterococcus faecalis D0KS13 and Weissella cibaria D0KS11 were able to produce tyramine and also harbor the tyrosine decarboxylase (tdc) gene. This indicated a positive correlation existing between the presence of tdc gene and production of tyramine. The effectiveness of nisin Z-producing Lactococcus lactis KTH0-1S against tyraminogenic strains was evaluated in co-cultivation experiments. Both co-culture of E. faecalis D0KS13 and W. cibaria D0KS11 with L. lactis KTH0-1S significantly decreased the growth rate and the maximal cell concentration, depending on the initial inoculum level of tyraminogenic strain. The reduction of tyramine accumulation decreased strongly when tyraminogenic strain was inoculated at 4.0 log cfu/mL and, to a lesser extent, at 5.0 and 6.0 log cfu/mL. Additionally, the inoculated kung-som with L. lactis KTH0-1S as starter culture showed significantly lower (p < 0.05) of Enterobacteriaceae, Staphylococcus aureus and total viable bacteria count than those of spontaneous kung-som. The addition of L. lactis KTH0-1S as a starter culture in kung-som was markedly reducing accumulation of tyramine by about 100 mg/kg in comparison with naturally fermented kung-som. Hence, the use of bacteriocinogenic amine-negative strain as a starter culture is advisable for production of safer kung-som with minimalized pathogen contaminants and lowering the tyramine content.