Fruit cherries
Cherries are a commodity so growers are always doing the intricate dance that balances cost against freshness and flavour. Air transport is the quickest way to get fresh fruit to overseas or long-distance consumers, but it is an expensive option getting more costly every year.
The alternative, sending the fruit by sea, breaches the time limit meaning quality deteriorates, but Dr. Peter Toivonen, a researcher in postharvest physiology at the federal government’s Summerland Research Station is examining a possible solution.
Toivonen believes Chilean cherry growers solve the problem by harvesting cherries when they are immature and “rock hard”, thereby sacrificing flavour for firmness and longer shipping life.
In contrast, B.C. growers tend to harvest later, so the flavour is better, but the fruit is more sensitive in the handling process.
Now Toivonen is working on a method he hopes will literally go the distance, delivering both freshness and flavour.
The Challenges
Sent by ship, cherries from the Okanagan can reach European markets in two to three weeks or China in four weeks. Chilean cherries reach Canada in about five weeks, which is why they are forced to send immature fruit.
Under ideal test conditions fully ripened cherries, carefully picked, shaded from the sun, quickly hydro-cooled to 0.5ºC within a few minutes, stored in cooled labs and put in lined containers to help retain moisture, will last eight weeks.
Such perfect conditions cannot be replicated in the field under commercial conditions.
“If you’re doing a good job your hydro-cooled cherries are at 3ºC by the time you’ve finished packing,” Toivonen says.
Cherries packed into crates at that magical half degree will rise to 5ºC after 25 days because, even in a cooled room, cherries generate their own heat.
Heat causes the cherries to dehydrate, which is easily observed in the stem because stems have a much greater surface area to volume ratio.
“The stem is an extraordinarily important indicator of the state of the cherry,” he says, and if more than five per cent of cherries are stemless that’s a warning sign.
The surface area-to-volume ratio is also why larger cherries travel better and longer. Dehydration can also cause a dimpling of the cherry skin, known as pebbling.
Another key measurement is sugar content. The optimum sugar content for Sweethearts is around 20 per cent, but Toivonen estimates cherries leaving Chile are only at 17 or 18 per cent sugar.
“One of the biggest problems is the softness and we’ve learned with more mature fruit the sugar content is very important to firmness,” he says.
While sugar content relates directly to fruit firmness it is only half of the story for taste. Good flavour comes from a balance of sugar content and the fruit’s acidity. As time passes sugar content drops slowly, but the acidity level goes down more quickly, resulting in declining flavour and unhappy buyers.
The Solutions
Toivonen wants to work on different parts of the processing chain to improve the quality of shipped fruit, starting with picking and handling, which can result in invisible damage.
“A lot of times decay problems in the store can be traced back to problems in the field.”
Toivonen says the use of a pulsed phase thermal imaging (PPTI) device can detect invisible damage by delivering a once-second blast of heat to the cherry. If damaged, the fruit can then be identified as unsuitable for long distance. However, it’s still good enough to receive top dollar so long as it sells quickly.
Toivonen believes packing lines would be the best place to install the device, offering up a good return on investment. As most fruit is tagged, this would also aid farmers in identifying which pickers have poor technique.
Toivonen says there are other issues as well that need to be examined, including testing for firmness without squashing the fruit, time-sensitive ways of testing for acidity levels, plus improvements to storage by moving to “pyramid stacking” using forced air cooling.
“It’s like buying an insurance policy,” he says, because the system allows air flow between crates, which keeps the internal crate temperature lower for a longer period of time.
Right now all the risk for a fruit shipment is borne by the grower, and Toivonen says growers would like to see that risk shared with buyers.
If fruit could be tested and verified as being in optimum shape at the moment it leaves the packinghouse, damage for subsequent poor handling or storage might spread some losses to other players within the system.
If these small changes allow the best tasting cherries to be shipped further it might also provide better returns to B.C. cherry growers. ■