University of California research may allow raisin growers to take advantage of the culinary mystique formally reserved for producers of fine wines.

What is the color intensity? Do the raisins impart a hint of spice, caramel, bitterness or astringency?

Panelists trained by a UC Davis scientist were able to detect these subtleties when comparing two varieties of raisins that were dried on the vine (DOV) with fruit from those varieties dried on trays spread on the vineyard floor.

In a separate study, 120 consumers who tasted the raisins reported to UC Davis researchers that they liked them all — the Fiesta and Selma Pete grapes that were dried both ways — but statistical analysis of their responses was able to tease out distinct preferences for certain fruit. Some liked DOV raisins better whether they were Fiesta or Selma Pete, and some liked tray-dried raisins better regardless of the variety. A sizeable group of consumers preferred Fiesta raisins dried on trays well above all others.

“We can conclude that there are clusters of consumers who do like certain raisins intensely,” said Hildegarde Heymann, a sensory scientist in the UC Davis Department of Viticulture and Enology who conducted the taste tests. “Raisin growers might be able to distinguish their product by targeting a market that wants their raisins dried one particular way or another.”

Costly labor

Tray drying has been the standard in California since commercial raisin production began in the San Joaquin Valley some 80 years ago. Wooden trays were replaced by paper trays in the 1960s, but otherwise the practice of cutting grapes, laying them out on trays and later collecting them demands farm labor expenditures for raisins that are among the highest as a proportion of total agricultural production costs. In a typical season, more than 50,000 field workers may be needed for raisin harvest.

These facts set scientists to work. The most popular labor saving device today is a mechanical harvesting machine that shakes off grapes and places them on a continuous paper tray. After drying, the machine picks up the tray and raisins. However, the mechanical harvester can be prohibitively expensive, especially for small-scale raisin producers.

Drying on the vine involves severing the vine, but leaving the grapes in the canopy where they hang to dry. A mechanical harvester is also needed for this method. The main barrier to adoption of DOV is the annual uncertainty about whether the crop will dry adequately. The temperature is significantly lower in the canopy compared to the ground, so drying takes longer — pushing it perilously closer to shorter days, cool nights, rainy weather and the need to deliver the fruit to a dehydrating facility for costly finishing.

In 2005, about 20 percent of the raisin crop was dried on the vine. As the proportion is increasing, so are questions about the raisins' quality. Some growers observed that their DOV raisins achieved higher USDA grades than tray-dried raisins. Proven better quality would be another factor for farmers to consider when contemplating a change in their raisin production methods.

UC viticulture specialist Matthew Fidelibus, working with viticulture farm advisors Stephen Vasquez of Fresno County and George Leavitt of Madera County, researched the raisin grading issue scientifically.

Graded higher

Controlling for other factors, the scientists compared Selma Pete and Fiesta grapes dried using the two systems and found that DOV grapes were graded higher.

“They seem to have a softer texture and finer wrinkling,” Fidelibus said. “The DOV raisins are more football- or teardrop-shaped, less flat like tray-dried raisins would be. The wrinkling and shape of the raisins are factors that could affect USDA quality grades.”

Fidelibus also noted that the DOV raisins had a “fruitier taste,” in his own experience. But he wondered what the grade difference and his own observations would mean for the ultimate raisin judge — the consumer. Fidelibus contacted Heymann to conduct the sensory analysis.

Heymann took on the two studies — one in which she trained panelists to objectively identify certain flavor and texture characteristics, such as caramel, sourness, astringency, grittiness, chewiness, stickiness and moistness. The second study, targeting raisin consumers, simply asked how much they liked each of the samples.

The panelists determined the dried-on-the-vine raisins were moister and the tray-dried raisins were more sticky, chewy and gritty. In terms of appearance, the biggest difference was the fact that tray-dried raisins have larger wrinkles than raisins dried on the vine.

For the consumer taste test, the scientists recruited 120 people who eat raisins at least once a week and asked them to rate the raisins on a nine-point scale, with 1 representing “dislike extremely” and 9 representing “like extremely.” Each of the raisin samples was rated overall between 5.2 and 5.7. Heymann placed the responses on a statistical “preference map” that revealed certain trends.

Sensory differences

“There were distinct sensory differences between Selma Pete and Fiesta and distinct sensory differences between DOV and tray drying,” Heymann said. “On average, the consumers do not care. But a market for those with the distinct preferences might be developed. For example, if most all raisins go to on-the-vine drying, a market niche for traditional tray-dried fruit could emerge.”

Fresno farmer Earl Rocca might agree. Rocca began working in vineyards when he was five years old and bought his first raisin farm in 1950. He said the DOV raisins might pass a USDA test with flying colors, but his own tasting tells him nothing beats the traditional tray-dried raisin.

“When you start cutting the canes and drying on the vines, the earlier you cut, to me, they have more tannin. The DOV raisins have the sour tannin flavor that you catch in some wines. I don't like the high tannin wines either,” Rocca said. “I think a dried raisin should have a certain amount of sweetness and caramel.”

But, he said, the UC research does offer much information to consider for someone interested in establishing a new vineyard with special DOV trellising and an early maturing grape variety.