Cell Growth Density and Nisin A Activity of the Indigenous Lactococcus lactis subsp. cremoris M78 Costarter Depend Strongly on Inoculation Levels of a Commercial Streptococcus thermophilus Starter in Milk: Practical Aspects for Traditional Greek Cheese Processors

Abstract

ABSTRACT Mixed thermophilic and mesophilic commercial starter cultures (CSCs), particularly those including Streptococcus thermophilus as a primary milk acidifier, have been found to reduce growth and counteract in situ nisin A (NisA+) antilisterial effects by the novel, indigenous Lactococcus lactis subsp. cremoris M78 costarter in traditional Graviera thermized milk cheese curds. Therefore, this model challenge study evaluated growth and in situ NisA+ activity of strain M78 in coculture with S. thermophilus ST1 singly in sterilized raw milk (SRM). Strain ST1, derived from a CSC for cheese, was challenged at two inoculation levels (5 and 7 log CFU/mL) in SRM against 6 and 3 log CFU/mL of strain M78 and Listeria monocytogenes, respectively. Pure cultures of each strain and cocultures of strain ST1 with the CSC L. lactis LL2, in replacement of strain M78, served as controls. At the high (7-log) inoculation level, the rapid, competitive growth (>9.3 log CFU/mL) of S. thermophilus ST1 reduced growth of both L. lactis by at least 10-fold; the industrial strain LL2 retained slightly higher relative population densities (7.4 to 9.1%) than the wild NisA+ strain M78 (3.8 to 5.6%) after 6 h at 37°C, followed by an additional 66 h of incubation at 22°C. In full contrast, at the low (5-log) inoculation level, S. thermophilus ST1 failed to predominate in SRM at 6 h; thus, the starter lactic acid bacteria populations were reversed in favor of L. lactis. Notably, strain M78 retained higher relative population densities (83.0 to 90.1%) than the CSC strain LL2 (80.3 to 85.2%) at 22°C. Moreover, at the 5-log ST1 level, the direct and deferred in situ NisA+ activities of strain M78 were at similar levels with its pure culture with L. monocytogenes in SRM, whereas at the 7-log ST1 level, the respective NisA+ effects were counteracted. Hence, 10- to 100-fold lowered inoculation levels of CSC S. thermophilus are required to enhance the performance of the M78 costarter in traditional Greek cheese technologies. HIGHLIGHTS