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Raisin-drying secrets for success

Growers have been seeking advice from UC on fire effects on drying.

Lee Allen, Contributing Writer

August 12, 2021

3 Min Read
A vineyard grows raisin grapes at the University of California’s Kearney Agricultural Research and Extension Center in Parlier, Calif.Matthew Fidelibus/UCANR

A rose by any other name is still a rose.  Likewise, a raisin by any other name is still a dried grape.  If you’re curious about entomology, the word raisin dates to Old France where a dried grape — raisin sec — morphed into the modern, raisin.

There are a variety of ways to dry grapes, on-the-vine or on-the-ground, shade drying or solar drying, as well as hot-air drying, freezing, microwave, vacuum impulse, and more.

As the popularity of the healthy snack continues to increase, raisin drying was one of the topics for discussion during the San Joaquin Valley Virtual Grape Symposium.

“We’ve been getting a lot of questions from growers regarding fire effects on raisin drying and how to reduce future risks,” noted speaker Matthew Fidelibus of the UC Davis Department of Viticulture and Enology.

“We had five of the Top 20 most destructive California wildfires burning last year during the August-September critical drying period, affecting solar radiation and minimizing direct sunlight that normally increases berry temperatures,” he said.  “Direct sunlight drives drying and has a huge effect on how long it takes to dry the fruit and berry temperature is a real key for both on-the-ground and DOV (direct-on-vine) drying.

“If you can increase the exposure of your fruit to sunlight on DOV, you can increase temperatures and thereby decrease drying time.”  And that’s a good thing because as drying times are extended, the fruit stays moist longer and is therefore more susceptible to pests and disease.  “The longer you stretch out the drying period, the greater the chance of getting rained on,” he said.

Unfortunately, weather events of 2020 may have been precursors of what 2021 would bring.

“The National Weather Service is suggesting we’re going to have continuing drought conditions, resulting in yet another bad fire season,” Fidelibus said.  “We can’t do anything about the fires, they’ll either happen or not, but what growers can try to do is achieve timely drying by taking a holistic approach --- make sure the grapes are ripe as soon as possible.”

Early-ripening rootstock

He suggests pest-resistant early-ripening rootstock and paying particular attention to irrigation and fertilization programs.  “Too much nitrogen can really delay ripening as well as promote too much vegetative growth which exacerbates powdery mildew and other disease problems.”

Sanitation is always a priority.  “If your vineyard floor is clean and dry during drying, your temperatures will be higher.  Excess weeds create problems and reduce temperatures.”

Trellis architecture also plays a part.  “There might be a role here for leafing or hedging depending on the kind of trellis you have.  Row spacing is another consideration, beneficial in terms of increasing yield if spaced closely, but posing a negative effect on vineyard temperatures if too close because the sun doesn’t reach the vineyard floor and provide reflective heat.”

It’s not just one thing growers can do to hasten ripening, it’s a combination of things lumped under the common-sense banner.

“None of these items taken singularly will make a dramatic change, but all of them taken together will give growers a better shot at success,” he said.

“Growers were challenged last year by fluctuating markets and roller coaster prices, yields were down, the coronavirus caused labor problems.  But good grower practices still need to be maintained.  If farmers slack on these things, it will put them more at risk of having another season like the one they had last year.”

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