Controlled delayed cooling helps boost stone fruit flavor, shelf life

By Vicky Boyd


After years of urging packers to quickly chill stone fruit after harvest and packing, researchers have made a 180-degree turn and are now recommending holding fruit at 68 degrees for two days before cooling.

The result of the controlled delayed cooling or as many in the industry call it, preconditioning, is a softer, juicier, tastier peach, nectarine or plum with a potentially extended market life.

The industry used to market based on the firmness of the fruit, says Carlos Crisosto, a University of California post-harvest physiologist. This is a completely differently approach that is changing everything that they have been told before.

Key to the controlled delayed cooling process is avoiding the killing temperature zone--36- to 50-degree temperatures that promote internal breakdown or dry, mealy fruit, Crisosto says.

During the process, the fruit softens, the aroma is produced, the juice is released and the texture changes, says Crisosto, who's based at the Kearney Research and Extension Center near Parlier, Calif. There is a higher perception of sweetness because the sugar-to-acid ratio changes. We are getting the fruit closer to the optimum eating stage.

Educating growers, buyers

But changing the mindset of growers, packers and buyers has been a long educational process, says Doug Luther, quality assurance manager for Summeripe Worldwide LLC of Reedley, Calif.

For the past six years, the group of Central Valley tree-fruit shippers has followed a strict preconditioning protocol and has even set up a quality control laboratory to monitor the fruit.

With growers, this is the way their fathers have done it. Now we are talking about breaking a generation of tradition, Luther says. We have to do it with tact and understanding.

Receivers have been trained to receive firm fruit, almost rock-hard fruit. Now all of a sudden, we've changed that.

Since Summeripe began offering preconditioned fruit, volume has increased 20 percent to 30 percent annually. Currently, it accounts for about 80 percent of the stone fruit the group sells.

Preconditioned fruit has become the norm; conventional fruit has become the exception, says Mike Thurlow, Summeripe sales manager.

Delivering an entire program

Ripe N Ready has taken a slightly different approach since its inception in 1997, says President Steve Kenfield.

Preconditioning is a process and a very specific process, where Ripe 'N Ready is delivering against a taste strategy, says Kenfield, who's based in Parlier, Calif.

Although the marketer of California and Chilean stone fruit may use the preconditioning process, it may also use several other tools to help produce a tastier product to keep buyers and consumers returning.

You can precondition right and still fail the consumers, he says. We've shifted our focus away from marketing a process, even a well-executed process, to really making sure we are delivering against consumer expectations.

What is preconditioning?

Preconditioning evolved from research Crisosto and his UC colleagues conducted into internal breakdown of stone fruit. When stone fruit is subjected to the killing temperature zone, a series of undesirable enzyme reactions occur that damage the cell wall and break it into medium to large pieces

Water then attaches to the cell wall pieces, making it unavailable when a consumer bites into a piece of fruit. In the process, the normal softening that occurs during ripening is disrupted.

The result is a dry, mealy piece of fruit or one with internal breakdown.

But Crisosto found that when fruit is held at 68 degrees F for about 48 hours, ripening continues and internal breakdown is slowed considerably.

During normal ripening, enzymes break the cell wall into small pieces. Water within the cells is not attracted to the small cell wall pieces and is available when somebody bites into the fruit and ruptures some of the cells. The fruit also softens.

At the end of the two-day holding period, fruit pressures have dropped to about 8 pounds-force, when measured with a penetrometer. Pressures may vary among varieties.

Then he recommends packers quickly cool the fruit to as close to 32 degrees F as possible to put it to sleep.

We are not telling people that they don't have to cool down the fruit, Crisosto says. What we are telling them is give it 48 hours at 68 degrees F, then cool it down to 32 degrees F.

By the time the fruit reaches the grocery store and is put on display, pressures typically have dropped to 6 to 8 lbf, making it ready to eat.

Crisosto's work also found that cutting corners produces a less-desirable product.

In a trial with Flavorcrest peaches, only 28 percent were graded as juicy after 24 hours at 68 degrees F followed by 20 days of storage at 41 degrees F. When the variety was held for a full 48 hours at 68 degrees F and then stored for 20 days, 93 percent of the fruit was ranked juicy.

Because holding the fruit at 68 degrees F even for 48 hours promotes decay, Crisosto says packers should apply a Food and Drug Administration-approved wax and post-harvest fungicide to all fruit. They also should tray pack the fruit since softer pressures contribute to increased bruising.

If done correctly, preconditioning can potentially double the fruit's shelf life, Crisosto says.

More work but better returns

Although many packers receive a $1- to $4-per-box premium for preconditioned fruit, Luther says additional management, labor and quality control offset most of that.

Seven years ago, this [quality assurance] department didn't exist, Luther says. Now we have 10 people to test soluble solids on each and every load. We take pressures and do TAs (titratable acids) to figure out sugar-to-acid ratios. But it's necessary because we have to be able to deliver what we promise to our customers, and this is what it takes to do it.