The effect of high microbial populations on the physical properties of kaolin slurries

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Large populations of microorganisms were shown to have little or no effect on the physical properties of kaolin slurries. The physical properties that were measured were brightness, soluble salts, pH, and viscosity. This was found to be the case when autoclaved samples of kaolin slurries were inoculated with strains of Pseudomonas putida, an organism frequently encountered in kaolin slurries, and incubated for five weeks; or, when uninoculated samples of kaolin slurries, and incubated for five weeks; or, when uninoculated samples of kaolin slurries were incubated for as long as three months to allow the naturally occurring contaminating organisms to grow. In each case, from 10 5 to over 10 7 colony forming units ml −1 developed. In several instances, however, the viscosity increased slightly but within acceptable limits. When kaolin slurries were inoculated with massive numbers of microorganisms in their own spent media, the viscosity increased significantly over a period of five weeks in some, but not all, cases. Brightness, pH and soluble salts content, however, did not change. It was probable that microbial slime material in the spent media plus additional slime material formed owing to the introduction of nutrients from the spent media into the kaolin slurries contributed to the increase in viscosity. The increase in viscosity took place sufficiently slowly, however, that under normal, practical working circumstances the kaolin slurries would have been used before the increase in viscosity became apparent. Moreover, the numbers of microorganisms used in these experiments were more than an order of magnitude greater than that which would develop naturally in kaolin slurries not treated with preservatives. It was concluded from these experiments that under normal conditions high numbers of microorganisms would have little or no effect on the physical properties of kaolin.