Properly cooling fruits and vegetables post-harvest ensures they will likely last longer – and be less susceptible to disease – before reaching consumers.
Photo by Sally Colby

by Sally Colby

Dr. Penelope Perkins-Veazie is a post-harvest physiologist at North Carolina State University who knows the importance of post-harvest handling.

“Post-harvest is a big picture,” said Perkins-Veazie, who also worked with the USDA on post-harvest issues. “I refer to it as mature science because a lot has been worked out over the years, but it comes back to your own eyes and experience.”

Although the appearance of fresh produce is important, looks aren’t everything. Perkins-Veazie said growers want people to return and purchase again, so flavor and crispness are critical. “The big driver is nutrition,” she said. “The other is food safety. First we worried about not having dirt all over it, then it was pesticide residue, and now it’s human pathogens.”

Perkins-Veazie pointed out produce is not dead after harvest. “It’s still alive, sometimes for a year,” she said. “There’s a lot going on – it’s still respiring and maturing.” Growers spend time nurturing a great crop – and sometimes pickers go out and mix decayed produce with the good. “Make sure what you start with is the best quality you have,” she said. “Obvious problems like malformed or decayed fruits should not be mixed in with the good quality produce.”

Color changes such as yellowing or browning can easily occur with sloppy post-harvest care. “Cucumbers turn yellow, broccoli turns brown,” said Perkins-Veazie. “We want lots of firmness – the softer the fruit, the easier it is to injure or bruise, and the more obvious the bruise. And if there are cuts on the fruit, that’s a source of decay, and it’s unattractive.”

Relative storage life is an important factor in post-harvest handling. Perkins-Veazie said while apples, potatoes and onions will be good for a year, many crops quickly lose quality. Berries may become soft and moldy within two days. On a diversified farm, the timing of every crop harvest must be considered.

Ideally, growers have planned post-harvest handling prior to growing. “Think about what you need and what you’re going to do with it – what’s the market?” said Perkins-Veazie. “Are you sending it to another state, or doing direct-market only? Are you going to farmers markets or U-pick? How are you going to transport it?”

For U-pick operations, determine what’s attractive about the crop. Perkins-Veazie said growers can get away with softer heirloom tomatoes if fruits are going to a farmers market or distributed through a CSA. That’s where genetics of certain varieties play a role, along with sustained performance throughout the growing season.

Storage and cooling needs should be considered for each crop. “Everyone figures they aren’t going to have that big a crop, and they’ll be fine, then it’s a bumper year and everything is backed up,” said Perkins-Veazie. “Make sure you think about the whole picture. Invest in a walk-in cooler – they cost a lot less now, and loans are available.” Perkins-Veazie warned against using a home refrigerator for cooling or storage because each time the door opens, a wave a warm air enters.

Perkins-Veazie recommended harvesting as early in the day as possible. “If you can reduce field heat, you’re 10 times better off,” she said. “Later in the season, if you’re harvesting watermelons, apples or pumpkins, you might have to be careful about it being too cold.” Early morning can be an issue with some crops such as watermelons, which can crack after heavy dew.

“Be selective when harvesting – know your audience,” said Perkins-Veazie. “Do customers want large zucchini or small? It’s the same with tomatoes – what stage are you going to harvest, and how do you train labor to recognize what stage to pick each crop?”

Minimizing heat buildup in harvested crops is essential. “We used to say ‘put it under a tree,’ but food safety changed that,” said Perkins-Veazie. “You can try to use what you have available, like a canopy for shade. The alternative is pop-ups for sections of the field that are hard to get to, or arrange it so you can move items quickly from the field to the packing shed.”

Most guidelines for cleaning produce after harvest are based on the Food Safety Modernization Act. “Be sure you’re using potable water,” said Perkins-Veazie. “If you’re a larger operation with a packing line, bin dumping and a wash station, that’s another avenue.”

With cold storage, the ultimate goal is the keeping the cold chain intact. “The first thing is to remove field heat,” said Perkins-Veazie. “Fifty percent or more of the heat in a crop is from the field. Keep produce at the right temperature while in storage. Make sure it stays cold – you just spent time and money removing heat and putting it at the right temperature, so you don’t want it to get warm again or warm then cold – you’ll end up with condensation and mold issues.”

The most common method for removing field heat is room cooling because there isn’t as much management involved, but it’s slow. Forced air cooling is another popular method. Top icing works for small scale cooling. Ice, preferably fine for more surface contact, is added directly to the product. Be sure workers are aware of the slipping hazard present with top icing.

Hydrocoolers can be used for some crops, and involve a degree of management.

For room cooling, be sure there are spaces, especially with pallet loads of boxes, because cooling has to work its way around everything in the room. “Also make sure there are vents,” said Perkins-Veazie. “Not only does the cooling have to go around, it has to go through. You don’t want boxes so tightly packed that there’s no ventilation – it’ll be 98º in the center of the load.”

Once vegetables are pre-cooled, don’t put them back in warm temperatures. “Cold rooms are usually between 33 and 41º Fahrenheit,” said Perkins-Veazie. “Big growers can get cold rooms down to 31 or 32º – most things won’t freeze at that temperature. Thirty-three degrees is best – usually you can do that without freezing the coils.” Many growers prefer 41º, but some produce such as berries won’t last as long at 41º as they will at 33º.

Additional details for post-harvest handling are available from UC-Davis at postharvest.ucdavis.edu/library and from Purdue University at ag.purdue.edu/hla/fruitveg/pages/harvest.aspx.