GROWTH OF LACTOCOCCUS-LACTIS SUBSP LACTIS IN RECONSTITUTED IRRADIATED MILK POWDER

Low heat milk powder was submitted to gamma-ray irradiation in order to sterilize it and then tested for growth of L lactis subsp lactis strains CNRZ 1076 (prt+) and its non-proteolytic-variant, CNRZ 1075 (prt-). Inhibition was observed when reconstituted milk was inoculated at a level of 10(4) but not 10(6) cfu/ml. This inhibition was partly relieved by addition of superoxide dismutase (SOD) suggesting a correlation with the production of O2-. The maximum stationary phase populations of CNRZ 1076 (prt+) in reconstituted milk from irradiated powder and in reference milk were identical while the maximum stationary phase population of CNRZ 1075 (prt-) was significantly higher in reconstituted milk made from irradiated powder. Amino acids analysis of the non-protein fraction revealed that irradiation led to a release of peptides, which likely enhanced final yields of prt-. Consequently, irradiated milk acidification by L lactis CNRZ 1076 (prt+) was delayed, while that of CNRZ 1075 (prt-) was greatly stimulated.