Raw milk is stored on dairy farms and combined dairy/agricultural farms and the like in milk holding tanks until the milk is collected and transported by truck to a dairy plant. Typically, and for the purposes of this invention, the milk holding tank has an agitator, e.g., an electric-motor-driven impeller, for agitating and making homogeneous the milk in the holding tank.
Raw milk purchase price, inter alia, depends on its total volume and on the butterfat content of the raw milk. As is well known, raw milk stratifies into an upper, cream layer which has a much higher butterfat content and lower proportionate volume than does the milk's lower layer. It is therefore important to both the dairy farmer seller and the dairy plant purchaser to ascertain at the time of collection of the raw milk both its total volume (e.g., by a flow meter or by measuring the depth of the raw milk in the holding tank before pumping is begun) and its average butterfat content.
Most, if not all, states have regulations calling for determination of average butterfat content of raw milk at the time of collection by requiring that the holding tank agitator be operated for a minimum, specified time, e.g., five to ten minutes, and then taking a sample of milk from the tank. Typically, the milk hauler takes such sample with a long handle dipper and pours it into a marked receptacle. The marked receptacle, and the total volume of raw milk collected from each tank, are given to the dairy plant for analysis of butterfat content per unit volume and calculation from volume and average butterfat content of the purchase price of the raw milk from each holding tank and/or seller. In a typical day, a milk hauler will make several collections from different sellers of raw milk before he returns to the dairy plant. The vehicle's tank contains a mixture of raw milk from the different farm sites.
The time of agitation of the raw milk is usually at the control and option of the milk hauler. If he is hurried or is careless, he may agitate for less than the minimum time specified in a state's regulations with the result that the raw milk has not been agitated enough to attain homogenity of the differing butterfat contents in cream and milk layers in the tank. If the hauler samples such inadequately agitated raw milk, his sample more than likely will not be representative of the true, average butterfat content of the raw milk in the tank. This works to the disadvantage of the seller or the purchaser--depending on whether the sample contained a disproportionately higher or disproportionately low butterfat content, vis a vis the true, average butterfat content.