Tyramine production in Dutch-type semi-hard cheese from two different producers

Tyramine content and counts of lactic acid bacteria (LAB) and enterococci were measured (including tyrosine-decarboxylase activity testing, and testing of the presence of the tyrosine-decarboxylase gene sequence, tyrdc, by PCR) during ripening (0, 26, 54, 88, 119, 146, and 176 days) in the core (C)- and edge (E)-samples of Dutch-type semi-hard cheese produced from pasteurized milk by two dairies (R, H) with two levels of fat content (30 and 45%) using two different starter cultures (Y, L), respectively. Tyramine content (y, mgkg(-1)) increased (P<0.001) with increasing time of ripening (x, days) in the cheeses of both producers (R: y=0.88x-31.4, R(2)=0.30; H: y=0.50x-6.3, R(2)=0.18), and its content was higher (P<0.01) in E-samples in comparison with C-samples. Time of ripening, part of the cheese and starter culture accounted for 67%, 28%, and 4% of explained variability of tyramine content in the cheese, respectively. After 26 days of ripening, using decarboxylase screening medium (DCM), tyrosine-decarboxylase positive LAB isolates constituted 7-27% and 6-32% of the square root of total countable colonies of LAB isolates of the producer R and H, respectively; tyrosine-decarboxylase positive enterococci were present only in R-cheeses (4-26% of the square root of total countable colonies). Tyrdc was confirmed only in 13% and 42% of the tyrosine-decarboxylase positive LAB and enterococci isolates, respectively (presumably due to the tendency of DCM to give false-positive results). Lactobacillus curvatus subsp. curvatus and Enterococcus durans, Enterococcus faecalis, and Enterococcus casseliflavus were identified as tyrdc-positive LAB and enterococci in the cheeses, respectively.