Working with Penetrometers

The days of touchy-feely production are giving way to taking measurements for fruit firmness and soluble solids. Such measurements are sometimes required for export and may soon be required for domestic sales. If the tools you use to measure these vital statistics aren't used properly, you're wasting some of the most valuable information at your disposal.

There are three basic types of penetrometers: the Magness-Taylor, the Effigi and the electronic EPT. The Effigi tester is the only one sold at this moment by Frostproof Growers Supply. The Magnus-Taylor tester has a long plunger that is encased in a metal barrel. The more expensive EPT uses a probe on the end of a pivoting arm to measure fruit firmness. The Effigi is a smaller and somewhat easier to use instrument. A needle on a circular dial indicates the maximum force required to insert the plunger into the fruit.

When using your penetrometer you should check it daily before each use. You can loosen the springs by working the plunger a few times. You can calibrate the penetrometer by slowly pressing down on the pan of a weighing scale (not a spring scale) until the scale measures close to what you expect fruit firmness in subsequent tests to be. Make sure the reading of the penetrometer is the same as the scale reading.

Selecting the right fruit for testing is as important as making sure your penetrometer is calibrated. Such points to remember when selecting fruit are:

• Avoid testing undersized or oversized fruit.
• Trees you take the fruit from should be representative of the entire block's age and vigor.
• Use ten or more fruit from many different trees.
• Take comparative readings from fruit that is the same temperature.

Because of the many variables involved in taking penetrometer readings by hand, it's best to have one well-trained person do the testing for consistency.

Reprinted in part with permission from American Fruit Grower