The Effects of Harvest Date on Thompson Seedless Grapes and Raisins. II. Relationships of Fruit Quality Factors

Abstract

The fresh fruit and raisin composition and quality factors measured during a three-year Thompson Seedless raisin harvest study were compared by regression analysis to determine their relationships and suitability as predictors of raisin quality. Raisin quality was measured with the airstream sorter as percent B and better (%B&B) and percent Substandard (%Sstnd) raisin grades. The relationship of fresh fruit soluble solids (SS) to the drying ratio of raisins was also analyzed. The main fruit measurements analyzed as a predictor of raisin quality were fresh fruit SS and titratable acidity (TA) and raisin reducing sugars (RS) and TA. Soluble solids and drying ratio showed a highly significant linear relationship, with a coefficient of determination (R²) of 0.9445, but with a different calculated product of their values as compared to earlier reports. Fresh fruit SS and TA were curvilinear and linear, respectively, to %B&B raisin grade. The relationship of fresh fruit TA to %B&B raisin grade is explained by TA's curvilinear relationship to SS rather than a direct effect of TA on raisin quality. The regression comparison of the RS and TA content of raisins showed a high coefficient of determination (R² = 0.7474) and both showed a highly significant linear relationship to %B&B and %Sstnd raisin grades. Many of the parameters showed relationships similar to those found in earlier studies, including the fresh fruit SS and raisin TA correlation to raisin quality; some differences indicate need for further study under more current raisin grape growing conditions.