THE EFFECT OF SELECTED PLATING PROCEDURES ON THE BACTERIAL COUNT OF INSTANT NONFAT DRY MILK

Abstract

SummaryIt was estimated that the addition of 10.5g of instant non-fat dry milk to 99 ml of water more nearly approximated a 1:10 dilution than did the currently recommended addition of 11 g of powder. It was estimated that the addition of a gram of powder to 99 ml of water increased the volume 0.5798 ml. Incubation of plates for 3 days produced counts 71% higher than those obtained using 2 days incubation. The use of dilution blanks tempered at 45 C resulted in a 27% increase over the count obtained using dilution blanks at room temperature. The interaction between incubation time and water blank temperature indicated that the warmed dilution blanks exerted a stimulatory effect upon the microorganisms in the powder. Counts obtained using overlayed plates could be more accurately replicated but non-overlayed plates yielded counts 15% greater. The use of Standard Methods Agar produced counts 10% higher than those obtained using Milk Protein Hydrolysate Agar. Shaking dilution bottles 50 times produced counts no larger than those obtained by shaking the bottles 25 times. A pooled estimate of the variance between duplicate plates was found to be 0.0087.