Good Fruit Grower Magazine Features the Mohr Digi-Test

Warner, G. "Inside Scoop". Good Fruit Grower Magazine, October 2007

A new tool for measuring the firmness of fresh apples provides more information about internal fruit quality than the standard penetrometer, reports Dr. Jim Mattheis, postharvest physiologist with the U.S. Department of Agriculture in Wenatchee, Washington.

The penetrometer, the standard tool used by the apple industry, measures only the outer 0.32 inches of the flesh. In contrast, the Digi-Test, an instrument made by Mohr and Associates of Richland, Washington, can measure the firmness of the fruit all the way to the core.

Speaking at Washington State University's recent Tree Fruit Quality School, Mattheis said the penetrometer assumes that the firmness of the outside of the fruit is representative of the entire apple, whereas, in fact, softening is not always uniform throughout, and there may be other textural changes in the flesh during storage that it would be useful to know about.

In experiments, Mattheis compared two Golden Delicious apples, one of which was harvested on September 18 and the other on October 26. At harvest, both measured 15.5 pounds firmness with a penetrometer. However, Digi-Test reading showed that the interior of the later-harvested apple was much softer than the other.

Similarly, he measured the firmness of two Gala apples that had been stored for two months, one in regular atmosphere and the other in controlled atmosphere storage. Though the penetrometer gave the same readings for both apples, the Digi-Test showed that the one that had been stored in regular atmosphere had softer flesh.

Mattheis said the Digi-Test readings seem to be a good indicator of how crisp people perceive the apples to be when they eat them.

The Digi-Test is commercially available and is being used in the industry. Mattheis suggested to packers that they might want to try using the instrument as another means to assess fruit firmness, particularly when the apples come out of storage to provide more information about the internal quality of the fruit as it goes into the market stream. However, a penetrometer can still be useful at harvest for indicating storability or lot-to-lot variability, he said. — G. Warner