COMPARISON OF CONDUCTIMETRIC AND TRADITIONAL PLATING TECHNIQUES FOR DETECTING YEASTS IN FRUIT JUICES

Frozen fruit juice concentrates containing an average microbial popula tion of log(10) 1.54 cfu ml(-1) were examined by traditional plating t echniques and direct and indirect conductimetry. The initial populatio ns in diluted (1 : 4) concentrates increased to an average of log(10) 3.82 cfu ml(-1) during incubation at 25 degrees C for 24 h. Incubation before plating and subjecting to conductimetric tests also facilitate d the resuscitation of cells that may have been freeze-injured. Yeasts were recovered in equal numbers on acidified (pH 3.5) potato dextrose agar and dichloran rose bengal chloramphenicol agar (pH 5.6). Yeasts and bacteria were recovered on orange serum agar. Detection times dete rmined by indirect conductimetry correlated fairly well (r = - 0.73) w ith populations (cfu ml(-1)) detected on traditional plating media. Po pulations in diluted concentrates which were not incubated before exam ination were detected conductimetrically in an average of 48.9 h, wher eas detection times for diluted concentrates incubated for 24 h at 25 degrees C before testing were reduced to an average of 14.1 h. Examina tion by conventional (direct) conductimetry required an additional 10- 20 h to reach changes in conductance of 5 mu S h(-1).