Enumeration of Psychrotrophic Bacteria in Raw Milk Inoculated with Lactic Acid Bacteria by the Agar Droplet Method

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Abstract
Selected cultures of lactic acid bacteria (LAB) can be added to raw milk with the aim of inhibiting growth of pyschrotrophic bacteria. We compared the standard pour plate method (7°C/10 d) and a rapid surface plate method (18°C/45 h) with the agar droplet method for enumeration of psychrotrophs in raw milks seeded with LAB. The LAB did not interfere with the standard method or the 7°C/5 d agar droplet method, but interfered with the surface plate method. High correlations were obtained between the standard method and the agar droplet method (7°C) in uninoculated raw milk. High correlations were also found when LAB were added, but only when agar droplet plates were read after 4 or 5 d of incubation (7°C); sharp decreases in correlations were observed upon further incubation. Correlations between standard method and agar droplet method (7°C) were influenced by the LAB culture added as well as their level of inoculation. Inoculation levels greater than 106 per mL produced a decrease in correlation.

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