An area of milk quality that is at times overlooked is emptied milk tankers that haven’t been cleaned. The pasteurized milk ordinance dictates that tankers be cleaned every 24 hours, but it does not prescribe how long tankers can sit empty without cleaning.
The scientific concern behind empty tankers is the buildup of bacteria on the interior of the tank that could contaminate newly loaded milk.
A recent study at Oregon State University analyzed downtime of uncleaned milk tankers. In a small pilot study utilizing 5-gallon milk cans, raw milk was filled into a container then emptied. The containers then sat empty for 3, 6, 10, or 20 hours at 86°F (30°C). Following the incubation period, commercially pasteurized whole milk was loaded into the cans and cross-contamination was measured.
They found the whole milk to be of lower quality with each of the incubation times.
The scientists then scaled up the experiment to a commercial tanker vehicle. They loaded raw milk from a herd known to have lower quality milk, emptied the tanker, and then let it sit uncleaned for 0 hours or 6 hours. Then, they reloaded it with milk from a herd known to have higher quality milk.
In the second experiment, milk quality in the second load was not compromised at either time point. They concluded that microbiological populations and enzyme activity in subsequent loads of milk are of high enough quality to not merit changing the 24-hour cleaning practice.