New test helps select best time to pick apples

British Columbia researcher finds a way to measure chlorophyll levels in apple peels, which is an indicator of ripeness

KELOWNA, B.C. — A British Columbia researcher is developing better testing to help apple growers know the best time to harvest their crop. 


Peter Toivonen of Agriculture Canada’s research centre in Summerland, is working with an Italian LED sensor, the DA meter. The instrument is portable, easy to use, reliable and an excellent predictor of how well an apple performs during long-term storage.


“That’s really what the packing house wants to know,” said Toivonen. 


“Are these apples going to hold up to storage?”


They don’t want to find out in February that the apples are starting to break down.


Controlled atmosphere storage is an airtight building with an atmosphere that differs from the air humans breath. A combination of low oxygen, high nitrogen, higher levels of carbon dioxide and temperatures just above freezing create an environment that delays the ripening of the apple crop.


Apples must be placed in controlled atmosphere storage at just the right level of maturity, Toivonen said, and the DA meter helps with this by determining the physiological age of the fruit. 


“It is an excellent predictor of how the fruit will hold up,” he said. 


“It relates to how much firmness will be retained over time.” 


LED lights shine into the apple, and a sensor measures how much light reflects back out. This is called delta absorbance (DA), which measures the level of chlorophyll in the apple peel. 


“Chlorophyll concentration is an indicator of how ripe the apple is inside,” said Toivonen. 


“It doesn’t matter what color the apple is, the DA meter only measures the chlorophyll.”


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The industry’s standard test of maturity measures the level of starch in an apple. 


Five to 10 apples are taken from an orchard, labelled and brought to a central location, such as a branch of the packing house. The apples are sliced in half and soaked in an iodine solution. The starch in the apple absorbs the iodine, while sugars, which have converted from the starch as the apple ripens, do not take up the iodine. The coloration is compared to a standard chart after several minutes, and the less coloured the apple, the riper it is.


Starch tests can take several days, and apples are destroyed in the process. As well, it can be a subjective measure. 


“After a while, it can be like looking at Rorschach blots,” said Toivonen, referring to the ink smudge test. 


“Each person sees a different thing, while the DA meter gives you a number.” 


The starch test is also not always accurate. 


“In our studies in the last four years, with Ambrosia specifically, we are finding that some years the starch value may not be a good indicator of storage quality,” he said. 


The DA meter was originally developed in Italy to test the ripeness of peaches, and work has also been done with pears.


However, collecting good data for apples has been problematic, and some researchers gave up. 


Fortunately, Toivonen did his PhD using light based instrumentation. 


“I could get a handle on what the issues were quite quickly after I started,” he said.


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Toivonen has installed a shroud that blocks out surrounding light for a more accurate reading, developed in-field protocols for testing and collected data to validate the instrument’s accuracy over the last six years. 


“I don’t want to say to growers, ‘use this,’ until we know that it works every time,” said Toivonen. 


Industry staff are now testing the DA meter in the field. 


The units cost $4000 each, which means field men will likely be the main operators.


Toivonen has been researching the Ambrosia, an apple that was discovered and developed in British Columbia, while a colleague in Kentville, N.S., studies Honey Crisp. 


Optimum storage is essential for these high value varieties. 


“It is important to have those superlative qualities. That’s why people are buying those apples,” said Toivonen. 


“They are not commodity apples. They are premium apples.”


Ambrosia has an average picking window of 10 days, but some years that can be as short as five days. 


“You have to be fast on your feet, and the DA meter allows you to make decisions faster,” said Toivonen. 


“You can sample multiple trees across your orchard. Eventually we hope to be able to map the orchard and help the grower decide where he can start harvesting.”. 


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