INHIBITORY EFFECT OF LEUCONOSTOC-MESENTEROIDES SUBSP DEXTRANICUM ON PSEUDOMONAS-FLUORESCENS IN MILK STORED AT LOW-TEMPERATURE

The effect of Leuconostoc mesenteroides subsp. dextranicum on the grow th of Gram-negative psychrotrophic bacteria in reconstituted 10% skim milk at 7-degrees-C for 6 days was investigated. Pseudomonas fluoresce ns strains isolated from raw milk were added (4.5x10(5) cfu/mL) to ste rile 10% reconstituted skim milk. Different inoculation levels of L. m esenteroides subsp. dextranicum (10(5)-10(8) cfu/mL) were added to the se milks. Washed and non-washed cells were used. The inhibition was me asured by P. fluorescens counts in a differential medium at different times. Furthermore, inhibition of the P. fluorescens growth in milk ac idified with hydrochloric and lactic acid was studied. The degree of i nhibition of P. fluorescens varied according to the inoculation level of leuconostocs with significant inhibition being achieved with 2.5 x 10(7) cfu/mL. Inhibition of P. fluorescens was observed as a reduction in the growth rate without reduction in the initial Pseudomonas popul ation. The strains of L. mesenteroides subsp. dextranicum used in this study grew in milk at 7-degrees-C. The pH of the milk dropped, especi ally in milks containing the highest leuconostoc inoculation level. Us ing washed cells of Leuconostoc, inhibition of P. fluorescens growth w as maintained but the drop of milk pH was reduced. The milks artificia lly acidified with lactic or hydrochloric acid showed a lower inhibito ry effect than the ones acidified by Leuconostoc cultures. The lactic acid production by leuconostocs could be partially responsible for the inhibitory activity on P. fluorescens.