Methods to detect freeze damage in citrus show promise
Methods to detect freeze damage in citrus show promise
ORANGE COVE, CA -- A damaging freeze occurs in the citrus-growing areas of California about once every seven to 10 years, but when it does happen one of the bigger challenges facing the industry is trying to determine which fruit has been damaged by the freeze and cannot be packed, and which is good for sale.
Sometimes the damage is evident, but often a damaged orange or tangerine looks fine even to a trained observer -- until the fruit is cut open.
The California Department of Food & Agriculture has a standardized protocol for cutting fruit to evaluate freeze damage, but it has limitations. One is the fact that it is a destructive test and is, therefore, suitable as a sampling method only.
Another limitation, according to David Slaughter, a professor at the University of California at Davis, is that it is "a subjective method," and since citrus- damaging freezes occur so infrequently, when there is an extensive freeze such as occurred in mid-January this year, "you don't have a large pool of possible inspectors that have a lot of experience in evaluating freeze damage."
For about the last six years, Mr. Slaughter, along with Jim Thompson, an agricultural engineering specialist at UC Davis, Mary Lu Arpaia, an extension specialist in subtropical horticulture at UC Riverside, David Obenland of the U.S. Department of Agriculture's Agricultural Research Service and others have been taking a proactive approach to finding better methods of detecting freeze-damaged fruit. The research has been funded by the Citrus Research Board.
Of "a dozen or so different concepts" that have been under evaluation, according to Mr. Slaughter, at least three of them show promise, and two were demonstrated to growers at citrus-packing plants and citrus groves in Exeter and Orange Cove on Feb. 6-7.
The two methods demonstrated involve the use of ultraviolet or black light and the use of a Breathalyzer, an only slightly modified version of the same device used by the California Highway Patrol to determine whether a driver has been drinking.
A third method employs nuclear magnetic resonance technology.
After six years of evaluating the various methods in the laboratory, researchers have been waiting for an actual freeze in order to test the validity of the methods in the real world.
This year's freeze gave them that opportunity, according to Ms. Arpaia. "We are trying to get the field validation," she said. So far, the methods do not appear to be foolproof, but they are more tools in the tool box "to make sure that we give good fruit to the consumers."
The methods are still at the experimental stage, but one of them -- the black light method -- could have some immediate application.
The principle involved is very simple. When the fruit is exposed to ultraviolet light of the appropriate wavelength (365 nanometers) and intensity, "the outside of the peel often fluoresces with bright yellow pinpoint dots," according to Mr. Thompson.
The method is non-destructive and can be used on every piece of fruit. It can be applied in the field to inspect the fruit on the trees at night to help assess whether there is sufficient undamaged fruit on a tree to justify harvesting and packing, and it can be used in the packinghouse to sort out freeze-damaged fruit before it is packed.
Researchers caution that the method is invalid for packed fruit because the finished product may show some fluorescence under black light even though it may have absolutely no freeze damage.
In the packing process, the black-light sorting must take place at the front end of the packingline, "right after the fruit is dumped from a bin," said Mr. Thompson.
Fortunately, California citrus packinghouses already have black-light rooms through which they run their fruit as it enters the packinghouse in order to detect and sort out clear rot, Mr. Thompson said. That damage shows up as "a large, bright-yellow area" roughly the size of a quarter. Freeze damage "is very different, because it [results in] pinpoint marks on the fruit, so we would have to train people to look at that, but we are using the exact same equipment."
In the black-light rooms, the fruit is run through on roller conveyors that allows inspectors to look at all sides of the fruit and manually remove any that shows signs of damage.
Potentially, the visual inspection could be replaced with an automated UV sorting system.
In the orchard, the extent of freeze damage on a tree can be assessed using black-ight flashlights. The appropriate flashlights cost from $250 to $600. According to Mr. Thompson, "growers will need to test their fruit with the UV light and cut fruit to determine the amount of fluorescence corresponding to damaged fruit for their individual groves. But once they understand the calibration, this tool will be helpful in surveying a grove to learn where the better fruit is located."
The black-light method, while still in the experimental stage, may have some benefit this season in helping growers to find more packable fruit and in helping to prevent freeze-damaged fruit from reaching the marketplace. The Breathalyzer method uses a standard breath alcohol analyzer to detect the presence of ethanol in a plastic bag into which a piece of fruit has been placed. According to Mr. Slaughter, "During a freeze, there is damage to the tissue in the peel. It appears that the peel becomes more permeable to ethanol," so ethanol escapes from the fruit and into the bag. The breath analyzer is fitted with a sensor that can be inserted into the bag to read the ethanol content. The fruit is left in the bag for about 30 minutes at 68 degrees before the measurement is taken.
The method would not be used for sorting fruit, but would be used only for sampling, as a non-subjective instrumental alternative to the current USDA cutting method, according to Mr. Slaughter. It will not be implemented this season except on a test basis.
The method's efficacy is currently being evaluated with naturally frozen fruit, but laboratory trials showed a promising degree of correlation with the cutting method.
Nuclear magnetic resonance equipment was not demonstrated at the events, but according to Mr. Thompson, "a new generation of simple, relatively inexpensive nuclear magnetic resonance equipment can also detect damaged fruit," and it has "the potential of operating at packingline speeds," so that the technology could potentially be built into packinglines.
Lab tests have shown "a good correlation with fruit identified as damaged by this method and the standard manual inspection techniques," he said.
Sometimes the damage is evident, but often a damaged orange or tangerine looks fine even to a trained observer -- until the fruit is cut open.
The California Department of Food & Agriculture has a standardized protocol for cutting fruit to evaluate freeze damage, but it has limitations. One is the fact that it is a destructive test and is, therefore, suitable as a sampling method only.
Another limitation, according to David Slaughter, a professor at the University of California at Davis, is that it is "a subjective method," and since citrus- damaging freezes occur so infrequently, when there is an extensive freeze such as occurred in mid-January this year, "you don't have a large pool of possible inspectors that have a lot of experience in evaluating freeze damage."
For about the last six years, Mr. Slaughter, along with Jim Thompson, an agricultural engineering specialist at UC Davis, Mary Lu Arpaia, an extension specialist in subtropical horticulture at UC Riverside, David Obenland of the U.S. Department of Agriculture's Agricultural Research Service and others have been taking a proactive approach to finding better methods of detecting freeze-damaged fruit. The research has been funded by the Citrus Research Board.
Of "a dozen or so different concepts" that have been under evaluation, according to Mr. Slaughter, at least three of them show promise, and two were demonstrated to growers at citrus-packing plants and citrus groves in Exeter and Orange Cove on Feb. 6-7.
The two methods demonstrated involve the use of ultraviolet or black light and the use of a Breathalyzer, an only slightly modified version of the same device used by the California Highway Patrol to determine whether a driver has been drinking.
A third method employs nuclear magnetic resonance technology.
After six years of evaluating the various methods in the laboratory, researchers have been waiting for an actual freeze in order to test the validity of the methods in the real world.
This year's freeze gave them that opportunity, according to Ms. Arpaia. "We are trying to get the field validation," she said. So far, the methods do not appear to be foolproof, but they are more tools in the tool box "to make sure that we give good fruit to the consumers."
The methods are still at the experimental stage, but one of them -- the black light method -- could have some immediate application.
The principle involved is very simple. When the fruit is exposed to ultraviolet light of the appropriate wavelength (365 nanometers) and intensity, "the outside of the peel often fluoresces with bright yellow pinpoint dots," according to Mr. Thompson.
The method is non-destructive and can be used on every piece of fruit. It can be applied in the field to inspect the fruit on the trees at night to help assess whether there is sufficient undamaged fruit on a tree to justify harvesting and packing, and it can be used in the packinghouse to sort out freeze-damaged fruit before it is packed.
Researchers caution that the method is invalid for packed fruit because the finished product may show some fluorescence under black light even though it may have absolutely no freeze damage.
In the packing process, the black-light sorting must take place at the front end of the packingline, "right after the fruit is dumped from a bin," said Mr. Thompson.
Fortunately, California citrus packinghouses already have black-light rooms through which they run their fruit as it enters the packinghouse in order to detect and sort out clear rot, Mr. Thompson said. That damage shows up as "a large, bright-yellow area" roughly the size of a quarter. Freeze damage "is very different, because it [results in] pinpoint marks on the fruit, so we would have to train people to look at that, but we are using the exact same equipment."
In the black-light rooms, the fruit is run through on roller conveyors that allows inspectors to look at all sides of the fruit and manually remove any that shows signs of damage.
Potentially, the visual inspection could be replaced with an automated UV sorting system.
In the orchard, the extent of freeze damage on a tree can be assessed using black-ight flashlights. The appropriate flashlights cost from $250 to $600. According to Mr. Thompson, "growers will need to test their fruit with the UV light and cut fruit to determine the amount of fluorescence corresponding to damaged fruit for their individual groves. But once they understand the calibration, this tool will be helpful in surveying a grove to learn where the better fruit is located."
The black-light method, while still in the experimental stage, may have some benefit this season in helping growers to find more packable fruit and in helping to prevent freeze-damaged fruit from reaching the marketplace. The Breathalyzer method uses a standard breath alcohol analyzer to detect the presence of ethanol in a plastic bag into which a piece of fruit has been placed. According to Mr. Slaughter, "During a freeze, there is damage to the tissue in the peel. It appears that the peel becomes more permeable to ethanol," so ethanol escapes from the fruit and into the bag. The breath analyzer is fitted with a sensor that can be inserted into the bag to read the ethanol content. The fruit is left in the bag for about 30 minutes at 68 degrees before the measurement is taken.
The method would not be used for sorting fruit, but would be used only for sampling, as a non-subjective instrumental alternative to the current USDA cutting method, according to Mr. Slaughter. It will not be implemented this season except on a test basis.
The method's efficacy is currently being evaluated with naturally frozen fruit, but laboratory trials showed a promising degree of correlation with the cutting method.
Nuclear magnetic resonance equipment was not demonstrated at the events, but according to Mr. Thompson, "a new generation of simple, relatively inexpensive nuclear magnetic resonance equipment can also detect damaged fruit," and it has "the potential of operating at packingline speeds," so that the technology could potentially be built into packinglines.
Lab tests have shown "a good correlation with fruit identified as damaged by this method and the standard manual inspection techniques," he said.
