Submitted to: American Journal of Enology and Viticulture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/27/2008
Publication Date: 6/15/2009
Citation: Ramming, D.W. 2009. Water Loss from Fresh Berries of Raisin Cultivars under Controlled Drying Conditions. American Journal of Enology and Viticulture. 60:2:208-214. Interpretive Summary: Raisins are an important commodity produced in California and were valued at $527 million in 2006. The traditional method of harvesting by hand and drying the grapes on paper trays in the field is labor intensive and the crop is at risk from fall rains. Raisins dried-on-the-vine (DOV) are mechanically harvested and have less risk of rain damage. Thompson Seedless is late ripening and in many years does not dry fully under the DOV raisin production system, and as a result need supplemental drying. USDA has released a number of new raisin cultivars that ripen early, making them more suitable for DOV raisin production. Differences in drying rates have been observed in the field. Berries were dried under controlled conditions to determine if these differences are due to berry traits. Summer Muscat, Diamond Muscat, and Primus dried the fastest and Thompson Seedless always dried the slowest. This shows that there is genetic variation between cultivars and that breeders can develop raisin cultivars that dry very quickly, as well as ripening early. When the waxy surface on the skin was removed, Summer Muscat continued to dry quicker than Thompson Seedless. This shows that there are other skin characteristics in addition to the surface wax that affect the drying rates of raisin cultivars.
Technical Abstract: Traditional production of raisins by hand harvesting and drying grapes on paper trays is labor intensive and at risk to inclement weather. Dried-on-vine (DOV) raisin production uses mechanical harvesters and the risk of rain damage is reduced. Earlier ripening raisin cultivars have been developed and released that are suited for this new raisin production system. It would be advantageous for breeders to know if differences in drying rates existed between cultivars in addition to their early ripening characteristic. If raisin cultivars had both characteristics, they would be suited very well for DOV raisin production. Berries from Thompson Seedless and 5 raisin cultivars were dried under controlled conditions to determine their drying rates. There were significant differences among the cultivars for drying rate for all three years. Summer Muscat, Diamond Muscat, and Primus dried the fastest and Thompson Seedless always dried the slowest. Water loss was affected more by berry size than by berry Brix. Removing the epicuticular wax by a chloroform dip increase drying rates more than by rubbing. Summer Muscat dries faster than Thompson Seedless even when wax was removed by chloroform or rubbing. This indicates that skin characteristics in addition to the epicuticular wax might play a role in the differences in raisin cultivar drying rates. There were large differences between cultivar for amount of cuticle compared to epicuticular wax. These findings show that breeding advances can be made in the development of raisin grapes with high drying rates.