Firmness and grape berry maturation. Definition of different rheological parameters during the ripening

Mechanical properties of Shiraz and Gamay grape berries were studied in relation with their maturity state using the PenelaupTM rheometer. The analysis of the constrains registered during berry crushing with the flat tool of the device, up to the pellicular tearing, allowed the definition of different rheological parameters and the characterisation of mechanical behaviour of grape and its evolution with the degree of ripening. The analysis of the deformability curves shows, independently of the cultivar, that berry behaviour is not elastical except for some berries at the beginning and at the end of the ripening. This behaviour can be characterised by two indexes expressing the curvature sense of deformability curves, the curvature degree in a way reflecting the turgescence state of the grape. Berry firmness was also considered in two different ways: the initial firmness which represents the elasticity coefficient of the fruit at the beginning of the deformation, and the bursting firmness which can be considered as the pellicular elasticity coefficient. Others parameters, as the pellicular strength which can be expressed from the value of the displacement at berry bursting and the energy used for the deformation were also defined. The evolution of these different parameters during ripening confirms that berry softening at the véraison time depends on the cultivar and on environmental conditions as the vintage. The analysis of the evolutions also indicates that pellicular strength is maximum at this crucial period of berry development