Predicting fruit quality

Chris Willett, ENZA

Tree fruit producers now have tools that can help them adjust their practices in order to deliver optimum quality fruit.Dr. Stefano Musacchi, pomologist with Washington State University, says that dry matter content of fruit can be measured before or at harvest to predict its quality after storage and producers can use this information to adjust growing practices.

A second tool, the DA-meter, can be used to ensure that fruit is picked at the correct maturity. “It’s a new frontier,” Musacchi said. “It can drive us in a more precise way to manage our orchards. The implications of these tools are amazing.”

Dry matter is what’s left of the fruit after the moisture has been removed. It includes starches, soluble solids, and acids, and is composed mainly of carbohydrates.

Musacchi emphasized that dry matter has nothing to do with the maturity of the fruit. It is an indicator of storage quality, and it is already being used in the kiwi and avocado industries. Studies have shown that consumers like avocados with the highest levels of dry matter.

“Dry matter concentration needs to be used before or at harvest as a predictor of the sensory potential of the fruit,” Musacchi told members of the International Fruit Tree Association at their annual meeting in Kelowna, British Columbia, in Canada.

New Zealand

This is not a new concept. Researchers in New Zealand have been studying dry matter since the 1980s, and dry matter analysis is a common practice in New Zealand, said Chris Willett, quality control and packing manager with ENZA, a large variety management and exporting company in New Zealand,

Dry matter is calculated by placing a slice of apple in a hydrator for 24 to 36 hours until it is completely dry. Dry matter differs on the sun-exposed and shaded sides of the apple, so a cross-sectional slice is used. The dry matter content is the dry weight divided by the initial weight, expressed as a percentage. “In New Zealand, we’re analyzing every block and every variety for dry matter, and I presume our competitors are as well,” Willett said.

The relationship between dry matter content and total soluble solids at harvest is not great, Willett said, but the correlation improves after the fruit has been stored for several months, which is why it is a good predictor of storage quality and consumer satisfaction. “Dry matter at harvest gives a good indication of where the sugars will be after 12 weeks in storage,” he said.

Willett said dry matter forecasting models can help in making decisions. For example, growers use the information to make adjustments in the orchard to improve the dry matter content. Warehouses use it to identify the best storage regime for the fruit, and salespeople use it to market the fruit at the appropriate time.

In Washington State, three packing companies are licensed to produce ENZA varieties. Willett said last year they began analyzing dry matter content of the apples to establish a baseline index on a block-by-block basis to give growers some feedback.

Willett said he does not expect to see regional differences in dry matter content. Block-to-block variations due to different horticultural practices are more likely.

Musacchi said research in New Zealand has shown that growers can increase the dry matter concentration by reducing the crop load and minimizing shading.

Willett said dry matter is highest in trees on dwarfing rootstocks, and in trees that have a balanced crop. For the ENZA varieties Pacific Rose, Jazz, and Envy, apples from overcropped trees have much lower dry matter.
“When we do consumer panels to evaluate fruit, fruit from overcropped trees consistently have less consumer appeal than fruit from trees with a balanced crop load,” he reported.

Fruit with high dry matter content comes from orchards that have good light exposure in the canopy and good water management. Fruit from earlier picks also has higher dry matter. Dry matter content is not a measure of when to pick the fruit, and if the fruit is not harvested at the correct maturity, the quality prediction will not be valid, Willett stressed.

“Dry matter can be used to determine the quality potential of the fruit, but good harvest management practices are essential.”

Dry matter analysis requires minimal investment, but is labor intensive, particularly for large operators who need to process a lot of samples in a short time. Drawbacks are that it is destructive, so that the same fruit cannot be monitored over time, and there’s a 24- to 36-hour delay in receiving the results. ENZA hopes to use near-infrared technology to overcome some of these difficulties.