Research Update DA meter reliability put to the test

The aimis to determine how well it compares to traditional apple starch indexmeasurements.

By Susan McIver P

eter Toivonen is taking the next step in determining the reliability of the DA (Delta Absorbance) meter for use in determining the optimum harvest time for apples. Toivonen is a research scientist at the Summerland Research and Development Centre specializing in postharvest physiology.

“Will nitrogen affect the reliability of DA values? That is the big question,” Toivonen said.

In order to answer that question, Toivonen has initiated studies on the relationship between nitrogen levels in fruit and DA meter and traditional starch index measurements.

“We now have an understanding of how nitrogen content affects starch clearing more than the DA meter values,” Toivonen said.

He is also continuing to evaluate post- storage firmness when using the DA meter to measure maturity compared to the use of starch clearance. Previous work done by Toivonen, primarily with Ambrosia, and John DeLong at Kentville, Nova Scotia, with Honeycrisp, has shown the meter to be an effective tool for determining the optimum harvest time for best performance in storage.

To date, growers have used the imprecise method of cutting apples open and spraying them with iodine, which detects starch. When an apple is ripe, most of the starch has been converted to sugar and the iodine creates only a light stain.

The handheld DA meter measures the differential between two wave bands of LED light directed into the fruit. The results, expressed as a simple numerical reading, are a measure of the amount of chlorophyll, specifically chlorophyll-a, in an apple’s peel as the fruit matures. The measurements are independent of peel colour. “This takes the subjectivity out of determining maturity,” Toivonen said.


Peter Toivonen, left, and Changwen Lu are studying the reliability of the Delta Absorbance meter for use in determining optimum harvest time in apples. Lu shows how the meter is held against an apple.

Another advantage of the meter is that it allows growers to test fruit without destroying it.

In his current work, Toivonen, who holds a PhD from Simon Fraser University, is being assisted by Changwen Lu, a biologist at SRDC with a PhD from the agricultural university in Beijing. Toivonen and Lu worked in five Cawston orchards within 10 km of each other.

“We recorded orchard temperatures and took samples to determine nitrogen levels from the middle of the apple flesh.” The samples were analyzed at A & L Canada Laboratories in London, Ontario, a facility recommended by the B.C. Fruit Growers’ Association.

In tests on the effect of fruitlet nitrogen on maturity indices in 2015 and 2016, Toivonen and Lu found a correlation between the starch index and nitrogen content, whereas there was no correlation between DA value and starch clearing.

The higher the nitrogen level, the greener the fruit. This affects the DA values because the meter measures only chlorophyll, which decreases with ripening, Toivonen explained. “Three factors go into measuring the fruit with starch clearance: nitrogen level, differential between day and night temperatures and ripening itself.” The orchard temperature profile had a minor effect.

Toivonen and Lu also found that when the DA meter was used to determine maximum ripeness there was less loss of firmness during CA storage than when the starch index was used. This work builds on previously mentioned studies yielding the same results.

“The goal is to pick fruit as close to maximum ripeness as possible.” Why are DA values and the starch index not closely linked?

In general terms, the answer relates to British Columbia FRUIT GROWER • Summer 2017

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