The Role of Enterococcus faecium as a Key Producer and Fermentation Condition as an Influencing Factor in Tyramine Accumulation in Cheonggukjang

The study evaluated the role of Enterococcus faecium in tyramine production and its response to fermentation temperature in a traditional Korean fermented soybean paste, Cheonggukjang. Tyramine content was detected in retail Cheonggukjang products at high concentrations exceeding the recommended limit up to a factor of 14. All retail Cheonggukjang products contained Enterococcus spp. at concentrations of at least 6 Log CFU/g. Upon isolation of Enterococcus strains, approximately 93% (157 strains) produced tyramine at over 100 µg/mL. The strains that produced the highest concentrations of tyramine (301.14–315.29 μg/mL) were identified as E. faecium through 16S rRNA sequencing. The results indicate that E. faecium is one of the major contributing factors to high tyramine content in Cheonggukjang. During fermentation, tyramine content in Cheonggukjang groups co-inoculated with E. faecium strains was highest at 45 °C, followed by 37 °C and 25 °C. The tyramine content of most Cheonggukjang groups continually increased as fermentation progressed, except groups fermented at 25 °C. At 45 °C, the tyramine content occasionally exceeded the recommended limit within 3 days of fermentation. The results suggest that lowering fermentation temperature and shortening duration may reduce the tyramine content of Cheonggukjang, thereby reducing the safety risks that may arise when consuming food with high tyramine concentrations.

Decarboxylase activity test of the genus Enterococcus isolated from goat milk and cheese

Biogenic amines are aliphatic, aromatic or heterocyclic alkalic substances with a biological impact on live organisms. They may cause serious problems to sensitive persons in combination with some medicaments or in case of higher intake. They are present in non-fermented food, usually coming from contaminating microflora, and especially in fermented food where biogenic amines might be produced by microbiota used for procedure. The genus Enterococcus spp. can occur in cheese because their resistance to pasteurizing temperatures is much higher compared to other mesophilic microorganisms. Previous studies have targeted the occurrence and problems of enterococci isolated from cow and sheep milk. The aim of this study was to detect decarboxylase activity of enterococci isolated from goat milk and cheese and to see how the particular temperatures involve decarboxylase activity using a rapid and inexpensive screening method. In this study, bacteria Enterococcus faecium, E. mundtii, E. durans were isolated from 9 samples of goat milk and cheeses. Colonies of bacteria were inoculated on diagnostic medium fortified with amino acids (lysine, arginine, phenylalanine, histidine, tyrosine and tryptophan) and acidity indicator. Changes in colour detected decarboxylase activity of enterococci. The only positive reactions were determined in samples containing arginine and tyrosine. Cultivation of bacteria was confirmed by PCR. All of the tested microorganisms showed significant activity of tyrosindecarboxylase and arginindecarboxylase which was regulated by temperature and influenced by duration of cultivation. The test of decarboxylase activity using colour changes is suitable for a relatively rapid and inexpensive detection of microorganisms that are able to produce biogenic amines.