Several techniques detect apple watercore

By Melissa Hansen

"The x-ray may be adaptable." Dr. Ralph Cavalieri

Several promising technologies to detect and sort fruit with watercore at the packing house are under research; the major challenge is in finding a method that is compatible with existing fruit warehouse operations.

Watercore is a troubling, physiological disorder occurring unpredictably in susceptible apple varieties, such as Red Delicious and Fuji. Through years of study, researchers have learned that preharvest high and low temperatures, poor calcium nutrition, maturity status at harvest, and cropping levels can lead to its development, said Dr. John Fellman of Washington State University's (WSU) postharvest physiology program in Pullman.

Because of difficulty in controlling and predicting the disorder, the biological systems engineering department at WSU has searched for a detection method that could be utilized on sorting and packing lines. According to Dr. Ralph Cavalieri, chair of the department, there are four methods that have potential for watercore detection.

The optical density concept has been around since the 1960s, and works by shining a bright light through the apple's core, with the calyx end down. The light can read internal browning, and the method has about a 91% accuracy rate.

Researchers at Cornell University in Geneva, New York, have experimented with a broad spectrum, incandescent light to view the core, posting a 95% accuracy rate for slightly affected and moderately affected fruit.

Optical density can be fairly accurate and may be a useful concept, but one of the difficulties is in properly orienting the calyx on the packing line.

Another technique that has been used in the potato industry, and holds possibility for apples, is the use of x-ray absorption equipment. Similar to x-rays used in the medical field, watercore tissue absorbs more x-ray energy than normal tissue, and can be visually detected when looking at an x-ray of the fruit.

Cavalieri said the accuracy of the x-ray method decreases when an object is moving during the picture-taking process. Results of a digital processed line scan used for apple watercore detection averaged only 64% accuracy, being 75% accurate when detecting no or slight watercore, but only 40% accurate in detecting moderate watercore.

"The x-ray may be adaptable," said Cavalieri, adding that its strength is in separating fruit with no watercore.

Sorting the fruit by mass density separation may hold potential. Watercored apples, which are denser than normal apples, tend to sink during the wash process. Density of the fruit is related to watercore for most varieties.

In a study using three sizes of Red Delicious--72, 90, and 100--air bubbles were added to the water, with a water baffle installed at the bottom of the wash line, to separate those apples with watercore. All of the larger apples in the moderate to high incidence of watercore were correctly sorted, while those with slight watercore were sorted with 80 to 90% accuracy.

Magnetic resonance imaging (MRI), a technique used in medicine, has been used in New Zealand to follow the disappearance of watercore in apples during storage.

"It is expensive," said Cavalieri, "but it may have potential for varieties like Fuji, in which the mass density is not well correlated with watercore density."

Of all the promising watercore detection techniques, the density separation is the least expensive and easiest to adapt to packing lines, he said. The x-ray technique should be suitable, but it has a low accuracy rate; MRI needs additional research and development before its use is widespread; and, optical density has potential, but he was not aware of any current research exploring its use.