Which instrument is used with plastic beads of varying density to estimate nonfat solids in milk rapidly?

The instrument that utilizes plastic beads of varying density to quickly estimate nonfat solids in milk is indeed the hydrometer. A hydrometer operates based on the principle of buoyancy. When placed in a liquid, it floats at a level that corresponds to the density of that liquid. In the case of milk, the addition of plastic beads of different densities allows for more precise readings. These beads enable users to gauge nonfat solids content by observing how the hydrometer's level changes with the adjustment of the beads.

By using the hydrometer in this way, producers can rapidly monitor the quality of the milk based on its nonfat solids percentage, which is critical for determining its overall value and suitability for processing. This application is particularly important in the dairy industry because nonfat solids are vital for making various dairy products and understanding milk composition.

The other options, while instruments related to milk testing, do not serve this specific purpose effectively. Refractometers measure total soluble solids, volumeters are used for measuring volume, and calorimeters determine heat content, none of which directly correlate with estimating nonfat solids using the method described.