HERSHEY, Pa. — Sweet corn, though it seems firm and sturdy, is among the most perishable of crops. It is vulnerable to bruising, pathogen contamination, and loss of sweetness — and consumers have high expectations for safe, attractive sweet corn.
Steven Sargent, a University of Florida Extension post-harvest specialist, discussed sweet corn quality and safety issues Jan. 30 at the Mid-Atlantic Fruit and Vegetable Convention.
Sargent said he started studying post-harvest quality issues because he saw how hard it was to grow crops, and it seemed a shame to lose 5 to 15 percent of the crop after it was picked.
Although nearly 60 percent of U.S.-grown sweet corn gets processed, the value on the fresh market is 2.5 times larger.
“That’s where the market is,” Sargent said.
With the rise of supersweet varieties that stay sweeter longer, “people started to get careless in handling the corn,” he said.
Water loss is an issue because the leaves continue to transpire water after the ear has been picked. That water flows from the kernels through the cob to the leaves, weakening the kernels’ taste, he said.
Sweet corn actually stays fresher when the husk is completely removed because the leaves cannot drain the water, he said.
Value-added plastic packaging of corn cobs can help reduce moisture loss. These packs often create a low-oxygen environment, which slows senescence, he said.
The handling process offers numerous opportunities for bruising or breaking the kernels. Kernels are packed with water at harvest, and “no water really escapes through that pericarp,” the impervious outer layer of the kernel, he said.
Breaking the pericarp dumps moisture and invites decay, he said.
Delayed corn loss can also occur through impact and compression. The kernels can get injured if too much weight is on top of them in a bulk bin, for instance, he said.
Compressed kernels remain sweet but become less juicy and “very much chewier,” he said.
Even if an ear is handled gently, “it’s very rapidly consuming its own reserves at room temperature,” he said.
At room temperature, produce can take nine hours to lose seven-eighths of its heat, the magic number for cooling.
The most efficient way to cool produce is to rapidly remove seven-eighths of the heat and let the storage system remove the remaining eighth of the excess heat, he said.
Commercial cooling can cut the heat in a fraction of that time and is imperative to maintaining the corn’s quality. Sweet corn is best kept near freezing during transport, he said.
Water is the most common cooling medium, and it must be kept a constant, cold temperature to be effective, he said.
Immersion hydrocooling is one of the less common methods for cooling produce. Sargent showed a picture of a grower-constructed case that held racks of flat containers. Water circulated through the whole box.
The grower is using the system to cool lychees, a type of tropical fruit, from 80 to 40 degrees in 45 minutes, Sargent said.
In tunnel cooling, workers or a conveyor belt push the flats through a housing that showers water from above. The water is then re-cooled and pumped through the system again, he said.
Water flows better through a stack of reusable plastic crates than through wire-bound wooden boxes. Wooden crates are often packed so tightly that the water does not penetrate to the lower crates, he said.
Vertically oriented cobs also help the water move better, he said.
Slush icing is acceptable but is no longer used much as a cooling method. Spreading ice over the top of corn is ineffective, Sargent said.
Maintaining a cold chain throughout the entire handling process preserves the corn’s quality and saves energy by not letting the corn warm up and have to be cooled again, he said.
While water is useful for cooling, it can also be a great way to introduce human pathogens into otherwise safe food.
Sargent offered several examples from his home state. Growers in certain parts of Florida irrigate by drilling a few inches into the bedrock, which sends water spraying everywhere.
“If that water isn’t clean, you’ve just contaminated your whole crop,” Sargent said.
Florida has many miles of canals, and farmers sometimes draw water from them. Sargent finds himself reminding farmers that pesticides do not sanitize tank water
Sargent related a phone call with a grower, who said, “ We’re washing our summer squash in the canal,’ and I’m going, OK, that wouldn’t work,’ ” he said.
Once microorganisms get inside fresh produce, the pathogens are essentially protected unless the food is cooked, he said.
Hydrocooling water has to be sanitary because the re-circulating water can quickly spread a disease from one ear to crates of ears, he said.
Chlorine at 100 to 200 parts per million is a favorite broad-spectrum disinfectant. “It’s cheap. It’s very effective. It has some issues with corrosion, but those can be controlled,” Sargent said.
Proper hand washing and sanitation practices in the field also help keep corn clean, he said.