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The firmness of pears is used as a harvest indicator and to help track ripening of the fruit in wholesale and retail operations. Firmness is measured with a penetrometer that measures the force required to push an 8 mm diameter convex probe into the flesh of the fruit. The industry would like to use a nondestructive instrument that could replace the penetrometer.

We compared three new methods of measuring Bartlett pear firmness with the industry standard penetrometer method.

Two companies now manufacture bench-top devices (in addition to on-line versions of the technology) for measuring fruit firmness nondestructively. During the 2005 harvest, they were each leased for $2,000 per season.

The device from Sinclair Systems International taps the fruit and measures the deceleration of the impactor as it hits the fruit. The Aweta-Autoline Company manufactures a similar device that measures impactor deceleration and acoustic transmission of the flesh. These units require 110-volt electricity and usually require that fruit be brought to the laboratory for measurement.

The third device, called a durometer, is significantly less destructive than the traditional penetrometer method. It has been used for many years to measure the firmness of materials such as plastic and rubber foams. It measures the relative distance a spring-loaded probe with a maximum force of 1.8 pounds can be pushed into a soft material. The deflection of the probe is quite small—around 0.1 of an inch—so it usually does not cause noticeable damage to the flesh of firm fruit. The device requires no power, is operated by hand, and can be taken to the orchard to measure firmness of fruit on the tree. It costs about $500 to $700.

Results

The durometer correlated well with penetrometer firmness, and the relationship was linear at penetrometer firmnesses between 10 and 22 pounds (see figure at left below). In that range,
the penetrometer firmness equaled durometer firmness minus 71 units.

Operations that ripen pears are interested in firmnesses below 10 pounds and would need a calibration table to correlate measurements with a penetrometer, although a better approach would be to convert ripening instructions directly into durometer numbers and eliminate the use of penetrometer terminology.

Actually, the durometer provides better resolution at low fruit firmnesses and would be more useful than the penetrometer in tracking firmness changes of ripe fruit. It causes no damage to firm fruit, but may cause a small circular bruise to ripe fruit. As with a penetrometer, repeatability would likely be better when the durometer is used in a lever-operated press, as we did in our testing. The durometer is compact, easy to use, and costs about the same as a good penetrometer. It is a good alternative to a penetrometer for use in the field and for managing ripening.

Firmness measured by the Sinclair unit correlated well with penetrometer firmness at penetrometer firmnesses greater than 14 pounds (see figure at right below). At lower firmness, the Sinclair unit had very poor resolution and would not be suitable for measuring the firmness of ripening or ripe fruit. The unit is not particularly convenient for measuring firmness in the orchard because it is too large to take to the field. Fruit would need to be picked in the orchard and brought to the lab.

The Aweta unit measured firmness with an impactor and with an acoustic sensor. The Aweta impact firmness score and acoustic firmness score were well correlated with the Sinclair impact firmness score, but the Aweta impact score showed more variability with penetrometer firmness than the Sinclair impact firmness score.

Fruit firmness is influenced by flesh temperature and moisture status of the fruit. Future work is needed to determine if either of these factors influence the correlation between penetrometer firmness and the alternative methods.