- Fall 1996 "Update" Newsletter Article -
Vacuum drying tried
New technology could be used to produce Golden Seedless raisins without use of sulfur dioxide
From CATI Publication #961001
Copyright © 1996. All rights reserved.
A specialist in vacuum-drying technology at Fresno State is testing a new method of producing Golden Seedless raisins-without the use of sulfur dioxide.
Results showing potential of the new technology are contained in a research publication just produced by the California Agricultural Technology Institute (CATI). Author is Carter Clary, Ph.D., research scientist for CATI's Viticulture and Enology Research Center (VERC).
While Golden Seedless raisins account for only about four percent of California's total annual raisin production, they do comprise a significant amount of tonnage (more than 17,000) and could possibly grow in popularity if the same taste and texture of the raisin were achieved without the use of sulfur dioxide.
Current methods for producing Golden Seedless raisins call for application of a liquid rate of from four to eight pounds of sulfur dioxide per ton of raisins. A sulfur concentration of about 2,000 parts per million (ppm) is needed to preserve color for one year.
The new drying method seeks to achieve the same color, texture and flavor associated with a Golden Seedless raisin without the use of sulfur dioxide.
In his experiments Clary used three vacuum flasks connected in series to a vacuum pumping system. The grapes were placed in the first flask in a liquid media which was then heated to evaporate the moisture from the grapes. When evaporation was observed to be complete, the media was drained off into the other flasks and the grapes were returned to normal atmospheric pressure. Their appearance, including color, shape and texture, was noted, and final moisture content was determined by vacuum oven.
Variations of temperature and pressure were applied in different treatments and produced different results, Clary said. For example, low temperature and high pressure produced a final raisined product with a soft, pliable texture and final moisture content of 13.5 percent. High temperature and low pressure resulted in a puffed, crispy product with a final moisture content of one to two percent.
In all cases, the dried product exhibited a bright golden color, he said.
"The results of these experiments have proven the concept of producing Golden Seedless raisins using liquid media," Clary said. Future work is planned and will address more detailed character of the dried raisin product, analysis of media residue, product shelf-life, scale-up of laboratory experiments, and economics.
Copies of the research bulletin detailing these experiments are available from CATI. The title is "Use of Liquid Media for Dehydration of Seedless Grapes." Copies are available and may be viewed or requested from the Research Publications page. For additional information, Clary may be contacted through VERC at (559) 278-2089.{ page top }