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Thesis (M.S.)—University of Nebraska—Lincoln, 1970. Department of Food Science and Technology.
Little is known of the chemistry of the build-up of soil resulting from milk handling operations. This study was developed to find the nature of the presoil and its contribution to the fate of the microflora.
The microenvironment in soil on milk handling equipment is dependent upon available water as influenced by humidity in the atmosphere. The effect of RH (80, 86, 93, and 100%) was, therefore, studied using milk products as the soiling. Varying combinations of whole, raw milk and other milk constituents were layered on 1 cm2 stainless steel surfaces and studied with exposure to controlled humidities.
Varying humidity levels were found to have a selective effect on the microflora of the raw milk film. The lower RH (80%) showed an inhibition of the gram-negative bacteria, while the higher RH (100%) enhanced the growth of gram-negative bacteria.
Stacking and clamping of stainless steel squares, to simulate harborages, produced a microflora similar to that found with a single layer of raw milk using 100% RH. By reducing the surface area of the film exposed to the atmosphere, more water was made available to the bacterial cells of the milk-soil film. A similar retardation in loss of moisture was obtained through presoiling.
Presoiling, to simulate incomplete cleaning plus a layer of raw milk, followed by incubation at 25 C, resulted in a visible yellow film similar to a type of “milkstone” encountered under field conditions. Key constituents were presoiled on stainless steel squares followed by a subsequent film of raw milk. Phospholipids were found to be necessary in the production of the yellow film. Another necessity for the production of the yellow film was growth of bacteria of the genus Pseudomonas. Evaluation of the yellow film produced in the laboratory found its production to be dependent on the following factors: 1) inadequate cleaning to yield a presoil of phospholipids, 2) growth of Pseudomonas sp., 3) high population density, and 4) available water.
Advisor: R. Burt Maxcy
Copyright 1970, the author. Used by permission.