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Note on Use of Alkaline Water Blanks for Plating Dried Milk Products

Bacteriological Laboratory, Mead Johnson & Co., Evansville, Ind.

ABSTRACT

In the counting of bacterial colonies in plates prepared from dried milk products considerable difficulty is encountered in distinguishing tiny particles of undissolved protein suspended in the agar from small colonies of bacteria, especially in the powdered acid milk products. Due to this difficulty there are times when erroneous results are reported.

Because of this difficulty, a study was made of methods to eliminate the protein particles without materially affecting the results of the plate count of the product. Since milk proteins are dissolved by alkaline solutions, the effects of different, individual alkalies and various combinations of them were tested for their ability to dissolve the protein particles insoluble in distilled water. More than twenty different alkaline solutions, including the sodium phosphates, were tested upon various kinds of dried milk products. The best results were obtained with lithium hydroxide solutions. Further work showed that M/5 and M/10 LiOH solutions were satisfactory for this purpose.