University of California research may allow raisin growers to take advantage of the culinary mystique formerly 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 with fruit from those varieties dried on trays spread on the vineyard floor.

In a separate study, UC Davis researchers lined up 120 taste-testers who reported that they liked them all: the Fiesta and Selma Pete grapes that were dried both ways. But statistical analysis of the responses teased out distinct preferences for certain fruit. Some liked dried-on-the-vine raisins better whether they were Fiesta or Selma Pete, and some liked tray-dried raisins better regardless of the variety. A sizeable group preferred tray-dried Fiesta raisins well above all others.

Marketing advantage?

"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."

Tray drying has been the standard in California since commercial raisin production began in the San Joaquin Valley some 80 years ago. 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 dried-on-the-vine raisins achieved higher U.S. Department of Agriculture grades than tray-dried raisins.

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

Controlling for other factors, the scientists dried Selma Pete and Fiesta grapes using the two systems and found that the dried-on-the-vine 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 dried-on-the-vine 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:

• In one, 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.

Larger wrinkles

The panelists determined that 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 9-point scale, with 1 representing "dislike extremely" and 9 representing "like extremely." Each of the raisin samples was rated overall from 5.2 to 5.7.

Heymann placed the responses on a statistical "preference map" that revealed certain trends.

"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."

This is a Spotlight story from the UC Agriculture and Natural Resources Web site.