Physical and Physiological Heterogeneity within the Grape Bunch: Impact on Mechanical Properties during Maturation

Mechanical properties are emerging as complementary criteria to evaluate grape berry quality for wine-making. As texture is evaluated on a berry per berry basis, it is important to assess the impact of the within grape heterogeneity on mechanical properties. Evolution of textural maturity of Cabernet franc was assayed throughout ripening using double compression 20% tests. Berries were sampled from whole grape bunches rather than randomly picked in the vineyard. Results showed coherent and significant evolution of double compression parameters: F1, Grad0, Grad1, W1, and gumminess significantly decreased whereas cohesiveness significantly increased during maturation. Variations in mechanical parameters were related with major physiological and physical traits. Mechanical parameters differed significantly according to berry position. Among these parameters, Grad1 was highly correlated with berry density and Brix (r = -0.91 for both), and W1 was highly correlated with water content (r = 0.87). Within-bunch heterogeneity was illustrated by different distributions of physical, physiological, and mechanical parameters. A negative gradient was shown from the top to the bottom of bunches for dry weight, density, Brix, cohesiveness, and gumminess. Berries at the distal part of the bunch were more advanced in maturity and were softer than berries at the top. Results are discussed with regard to the sampling strategy in the assessment of grape berry quality based on mechanical properties.