Delayed harvest can lead to more problems with pitting and pebbling of cherries.
Courtesy of Peter Toivonen, Agriculture and Agri-Food Canada
In an increasingly competitive fresh cherry market, buyers are looking, above all, for consistent, reliable quality, Dr. Peter Toivonen, a postharvest physiologist with Agriculture and Agri-Food Canada at Summerland, British Columbia, Canada, reminded growers during the 2012 B.C. Tree Fruit Horticultural Symposium.
That means growers must be able to deliver the same quality day after day so that buyers develop a trust and come back for more. Quality fruit has a low incidence of defects, such as pebbling, pitting, stem browning, shrivel, decay, and softening, and is not infested by insects. Above all, the cherries must be firm.
Contrary to what growers might expect, problems with soft cherries are more likely to occur early during the shipping season than after they’ve been stored for a week or two, said Toivonen, who has been conducting research on fruit firmness.
“People feel that if you have any problems with your cherries, you should try to ship them off quickly, but this looks like it’s not the thing to do,” he said. “Cherries do get firmer in storage—we’ve shown that time and time again. Particularly with the Summerland cultivars, if you leave them in storage for a week or two, they get much firmer. That’s why the firmness problems disappear. If you’re having problems at the beginning of storage, it’s not like a hot potato. You may want to think of holding on to them for a week or so instead of trying to get them to the customer right away—as long as you keep them in cold storage.”
Toivonen said the reason for the increase in firmness is not understood.
He’s found that there tend to be more problems with soft cherries in orchards where the fruit has lower Brix levels. Sugar levels can be increased by improving light penetration to the fruit on the trees.
Harvesting at the right maturity is critical for the best quality fruit, Toivonen stressed. More mature fruit is sweeter and has better flavor because of both higher sugar and acid levels. Growers can improve flavor by delaying harvest. However, delaying harvest too long can have negative effects, such as increasing pebbling and pitting of the fruit. If the cherries are left on the tree until they are dark mahogany color, there can be stem browning and the fruit can lose flavor as the acid level drops.
“It’s a bit of a balancing act getting them at the correct maturity—trying to get the quality and flavor into the fruit but not allow the fruit to get so old it’s starting to degrade on you,” Toivonen said, recommending that a good compromise is to harvest at stage 5 on the chart of the Ctfil (Centre Technique Interprofessional des Fruits et Légumes).
A significant amount of impact and compression injury can occur during harvest, he said, urging growers to pay attention to how their workers are harvesting the fruit. “There has to be a lot more effort and training in managing pickers and getting fruit to the packing house without damage.”
Corrugated plastic containers that are more flexible than buckets can help reduce impact injury that leads to bruising and pitting. Foam pads placed in the bottom of plastic kidney buckets or metal pails can help break the force as the cherries are dropped into them. Cherries can be damaged when they are dropped from a height of more than 8 inches, so if pickers are dropping the fruit from above the top of the picking container, that’s a problem. It’s easy to hear the distinctive sound of cherries being dropped from too great a height into the containers, he said, and he recommended that growers go through the orchard to spot the offenders and spend some time training them.
An inexperienced crew can cause compression injury by grabbing the cherries instead of picking them by the stems. Compression injuries cause bruising and softening of the tissue that ultimately leads to rotting. Cherries with high soluble solids levels are less susceptible to compression injury because they also contain more water, and that increases the turgor of the cherries, making it harder to compress them. For Sweetheart cherries, a level of 20° Brix should ensure minimal susceptibility to compression injury, Toivonen said.
Compression bruising tends to be worse when the fruit is warm and can be caused by jostling fruit when it is transported from the orchard. Cherries can also suffer compression damage at the warehouse if they are stacked above the water level when they’re emptied into the flumes. “Feed the packing line at the same rate as the fruit is moving out,” Toivonen advised.
Small changes in the orchard can make big differences in the ultimate quality of the cherries, Toivonen said. For example, growers should avoid leaving bins of cherries in the sun in the orchard, which can make the stems turn brown and shriveled.
The cherries should be cooled with potable water, and a shade cloth (with the silver side down and the white side up) should be placed over the bin to prevent the temperature of the cherries from rising and to maintain high humidity. This also prevents shrinkage of the cherries through shrivel.
The cherries should be carefully transported to the warehouse, avoiding injury, and packed as soon as possible.
Toivonen has done experiments to find out the impact of various postharvest temperatures on fruit quality. Titratable acidity, which is important for good flavor, is best maintained when the temperature is about 0.5°C (33°F), he has found. Low temperatures lead to less bruising and fewer symptoms of pebbling (because of less evaporation of water from the fruit). A low temperature can also help maintain the stem pull force, which might be important if the grower is worried about the stems falling off or becoming loose.
Firmness is also better at lower temperatures. Fruit in warm temperatures will become softer and softer.
Hydrocooling removes heat from the fruit, but Toivonen said producers cannot rely just on hydrocooling to bring the temperature down as low as it needs to be. Even with a well-designed system, it will be difficult to get the fruit cooler than 3° to 5°C (37° to 41°F).
Once the fruit is palletized, no more heat will be removed from the boxes that are in the middle of the pallet. In fact, the cherries continue to respire, generating heat, and the fruit warm up. Refrigerated shipping containers, trucks, and trailers are designed to remove heat from around the pallets but not to remove heat from the fruit, so the interior boxes will continue to warm up during shipping.
It’s best to bring the temperature of the fruit down to 0°C ( 32°F) before they are palletized, either through postpack cooling, or forced-air cooling in storage, Toivonen said. Cherries that are at 0°C when shipped should still be below 5°C (41°F) after 28 days, and he believes that’s probably the best that producers can do in terms of preserving quality. If the temperature of the fruit starts out at 5°C, it can rise to almost 15°C (59°F) after 28 days.
“You have to get the heat down as far as possible before it goes on the truck or container,” he stressed.