Effects of high-pressure treatment on viability and autolysis of starter bacteria and proteolysis in Cheddar cheese

Abstract

Abstract Cheese starters (Lactococcus lactis strains 303, 223, 227 and AM2) were subjected to high pressure (HP) in the range 100–400 MPa at 25°C for 20 min either in 0.1 m citrate buffer (pH 5.3), the same buffer containing 4.5% NaCl, or in cheese manufactured using each strain individually as a starter. Inactivation (total viable counts) and cell lysis (release of lactate dehydrogenase, LDH) of each strain were examined post-pressurisation. Starter bacteria were more pressure tolerant in cheese than in buffer. No evidence of release of LDH, and thus autolysis, from the starter cells resulting from HP treatment was observed either in buffer or in cheese. Primary proteolysis, studied by urea-PAGE analysis, increased as a result of the HP treatment. However, while cheeses made with different starter strains varied with respect to levels of pH 4.6 soluble nitrogen expressed as % total N and free amino acids, no further increase in these parameters were observed due to HP treatment. Thus, HP treatment for 20 min inactivated starter bacteria in Cheddar cheese (2–5 log cycle reduction at 400 MPa), but did not induce autolysis.