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Studies on the Relative Bacterial Cleanability of Milk-Contact Surfaces

Department of Dairy Industry, Cornell University, Ithaca, New York

ABSTRACT

These studies are concerned with the removal of bacteria, deposited with various soils, on milk-contact surfaces. The bacteria were labeled with Phosphorus³², incorporated into test soils, dried down with the soils upon various milk-contact surfaces under highly reproducible conditions, removed by a standard cleaning procedure, and assayed by the agar-submersion, Geiger-counting, and autoradiographic techniques. The data so derived were tested for significance by an Analysis of Variance, and grouped by the Duncan Multiple Range Test. The results show statistically significant differences in cleanability, between different surface finishes applied to a given material and between different materials having comparable finishes. Organisms suspended in a homogenized whole milk soil were much more easily removed from the surfaces studied than organisms suspended in an 0.85% saline solution. Multiple soiling and washing of a surface results in a general increase in the number of bacteria retained by a surface. Brushing was found to be effective in cleaning smooth polished, ground, and molded surfaces, and relatively ineffective in removing bacteria entrapped in the surface features of cold rolled, abraded, blasted, and porous surfaces.

On the basis of all the tests performed, the surface finishes which displayed the greatest ease of cleanability were the highly polished, nonporous surfaces. The finely ground and smoothly molded finishes followed close behind, and the cold rolled, abraded, blasted, and porous finishes were the most difficult to clean. Under the conditions of testing utilized in these experiments, the direct agar-submersion technique proved to be ineffective as a means of accurately determining the relative cleanability of different surface finishes and of different materials. The radiological counting procedure proved to have higher reproducibility than agar-submersion methods for estimating the numbers of bacteria remaining on a cleaned surface.