The falling Number (FN) Test For Wheat Gives an Indication of the Amount of Sprout Damage That has Occurred in a Wheat Sample.
With the weather turning more favorable for grain harvesting, much of the wheat that has not yet been harvested will be taken off. Perhaps the wheat was physiologically mature, and weather conditions did not allow for a timely harvest. Or, perhaps the grain had more moisture than you would like, so you have let it in the field to dry down naturally.
However, there are some concerns with leaving the grain in the field after it has fully matured. When it rains just before harvest, the grain may start to germinate, or sprout, in the head. The germination process causes an increase in alpha amylase, an enzyme that breaks down starch. The longer the grain sprouts, the greater the amount of alpha amylase is formed. This enzyme is a problem, because it reduces the quality of flour, and products made from the flour. It can cause sticky dough, and affect loaf volume and shelf life. In pasta, sprouting can reduce shelf life, increase cooking loss, and result in softer cooked pasta.
The falling number (FN) test gives an indication of the amount of sprout damage that has occurred in a wheat sample. It measures the effect of the enzyme on wheat quality. The test calculates the amount of time it takes for a plunger to fall to the bottom of a glass tube that is filled with a heated paste of wheat meal and water. The time taken for the plunger to fall is known as the falling number, and is reported in seconds. High quality wheat makes a very thick paste. The greater the sprout damage, the less sticky the paste, and will result in a much lower FN.
So, what is an acceptable falling number? According to the USDA Wheat Quality Lab, if the FN is greater than 300, there is no sprout damage. If the FN is 200 to 300, there is some sprouting. If the FN is less than 200, severe sprout damage is present. In some cases, good quality wheat can be blended to bring the FN up to a required level. If the quality required for milling cannot be improved by blending, it will have to be utilized for other uses (most likely feed wheat), often resulting in a substantial price deduction.
(Source – http://www.farms.com/news/wheat-quality-and-the-falling-number-test-64965.aspx)