Analyses were made at the Institute chemistry laboratory over 2 dairy seasons of 900 composite milk samples prepared for routine fat testing at 2 dairy factories and held at ambient temperature. The composites, which were preserved with mercuric chloride, frequently had high free fat acidities and high bacterial and mould counts.

At one factory, where the composites were sampled at 85−90 °F, the samples contained incompletely dispersed fat. Werner Schmid fat values for 83 milks pipetted immediately after factory sampling were lower on average by 0·084±0·089% than values obtained for the same samples after high-speed mixing at 115°F. At the other factory, where the samples were mixed at 98–103°F, Werner Schmid tests on 56 milks showed a corresponding difference of only 0·033±0·047%.

For 273 factory composites, mixed at high speed, results by the Babcock test and by the Gerber test averaged, respectively, 0·015±0·019% and 0·059±0·054% higher than those by the Werner Schmid test.

High bacterial and mould counts in factory composites were attributed to contamination from unsterilized bottles and stoppers and to failure of factory staff to mix the contents of each bottle after the daily addition of milk. Low bacterial and mould counts and negligible increases in free fat acidity were found in composites made up from factory milk in the Institute laboratory in sterilized bottles, kept at ambient temperature and swirled daily. Composites made up from fresh farm milks and held at 38–40°F remained in good condition for sampling after warming to 98–103°F but showed wide variation in free fat acidity.

Rises in free fat acidity occurred only in composites preserved with mercuric chloride. Other preservatives, however, were less effective in preventing bacterial growth at ambient temperatures.

Increases in free fat acidity up to 10 (ml N alkali/100 g fat) had no effect on results in the Babcock tests but caused a slight rise with the Gerber tests. Acidities over 20 caused a decrease in Babcock and a further rise in Gerber values.

It is suggested that fat destabilization in composite samples could be minimized by avoiding rises in temperature during transport of fresh daily samples from farm to factory, by holding composites at 55–60°F and by mixing after each daily addition of milk.