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A yellow film of milkstone developed on a laboratory system that simulated unclean stainless steel surfaces. Cursory washing or presoiling with phospholipids and subsequent growth of Pseudomonas sp. in a milk film produced the yellow color in 1 to 2 days. Sequence of treatments, microenvironmental conditions, and microbial growth contributing to the formation of the yellow film were determined, Pseudomonas sp. was isolated from laboratory developed yellow films and from milk soil deposits on farm equipment. The overall requirements for the production of yellow films were: a) inadequate cleaning or a presoil of phospholipids, b) growth of Pseudomonas sp., c) high bacterial population, and d) available water.