Screening Non-Conventional Yeasts for Biogenic Amines

To confirm that the non-conventional yeast strains shortlisted here (Table 1) are safe to use, we checked that none produced any biogenic amines (BAs). Production of BAs was determined using an adapted version of the method explained by Joosten and Northholt [28 (link), 29 (link)]. Briefly, yeast strains (10⁶ cells per ml) were inoculated onto YPD agar plates supplemented with bromocresol purple (Sigma Aldrich) 0.006% and an amino acid mix with a total mass concentration of 1% (MP Biochemicals, LLC). The added amino acids are tyrosine, histidine, phenylalanine, leucine, tryptophan, arginine and lysine at equal ratios. Subsequently, the plates were incubated at 30°C for 7 days and the growth and changes in the color of the medium was monitored daily to test for the presence of BAs. In strains with no BA production, the growth area was surrounded by a yellow halo caused by glucose fermentation, followed by a pH reduction that causes the medium to turn purple after a period that depends on the growth rate of the strain. By contrast, when BAs are produced, amino acid decarboxylation resulted in a purple halo from the very beginning, which grew bigger and darker as a function of time.