Foaming can cost your precision machining shop money by causing loss of fluid, shorten pump life by cavitation, and reduce material removal efficiency  by lowering heat removal and lubricity at the workpiece.
Here is one way (and two tests  to prove) that too much air (as foam) can spoil your metalworking fluids.
Factors that can contribute to foaming in your metal cutting machines include makeup water hardness; fluid type; speed of machining operation; design and maintenance of filtration, return pipes, and sumps; pump parameters; and contaminants. Never underestimate the ability of a fluid in a machine to ‘attract’ contaminants!
Here are a couple tools you can use to evaluate foaming of your metalworking fluid.
Bottle test. Modeled after ASTM D3601, fill a bottle half full and shake at a steady rate for say 45 seconds or so. Stop shaking and immediately measure and record the height of the foam. Count the seconds (use a watch with a second hand) until the foam collapses to an acceptable level. No salt please! Addition of salt to reduce foam is acceptable at the beer garden, but never in the shop!
Blender Test. Using a blender to simulate your machining process is probably more likely to match your process than the ‘arm-strong’ method described above. This method is modeled after ASTM method D3519. Place a  200 milliliter sample of your fluid in the blender and agitate it at 8000 rpm- lid on blender is highly recommended– for 30 seconds. As in the bottle test, measure the foam height immediately after shut off, recording the seconds until the foam collapses to 10 mm in height. If it doesn’t recede to below 10 mm by the end of 5 minutes, note the remaining height.
If you do these tests when you first install fluids, the initial reading will give you a performance benchmark to compare to later periodic tests. A large difference will give you an indication of whether you should  adjust or discard your machining fluid.
We used a version of the blender test to troubleshoot some quenching oil in our laboratory  when I was a lab supervisor. We were getting some really high hardness but sporadic readings on samples quenched at the furnace in one lab, but not the other. After we confirmed furnace temperatures at both labs, hardness tester calibration at both labs, and steel sample analysis we decided to test our quench oil. The oil at one lab reacted differently in our version of the blender test. Further work (and a separatory funnel) revealed water from a leaky roof had contaminated the quench oil at the main lab. Because the water was heavier than oil, it wasn’t visible by other means.
It’s not that we don’t like bubbles.
But they really don’t help us in our machines in the shop.
What have you done to keep control of the foaming of your machine’s metalworking fluids?