Abstract
A laboratory system using 1 cm2 pieces of stainless steel with controlled temperature, humidity, and inocula was used to study the fate of the microflora of dairy origin on various films. Metal squares were treated to simulate incomplete cleaning, then inoculated with raw milk. After incubation for 16 hr at 80, 86, 93, or 100% RH, the microflora was recovered in sterile phosphate buffer and evaluated using the standard plate count method. Isolates from countable plates were observed for general characterization. Humidity of the ecosystem was a major factor in rate of water loss from a film. Available water, and indirectly the solute concentration, influenced the microenvironment and the resulting population density of microorganisms. Likewise, the microenvironment had a selective effect on outgrowth of bacteria, because of varying sensitivity of bacteria to drying and solute concentration. The gram-negative bacteria were inhibited at a lower humidity (80% RH), while the higher humidities (93 or 100% RH) provided sufficient water for abundant outgrowth. Reduction of area of a film exposed to air reduced water loss and provided protection for harborage of microorganisms. Pre-soiling prior to addition of a film of milk decreased the rate of water loss from the film and increased bacterial growth. Induced lipolysis within a film of milk influenced outgrowth of bacteria with an inhibitory effect on gram-positive types. Results indicate that modem closed systems and extensive mechanical handling of milk may be favoring growth of undesirable gram-negative bacteria, e.g., coliforms and psychrotrophs.

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