The mechanical harvesting of grapes is an important operation in order to contain the operating costs of the viticultural farms. One of the limits of the mechanical harvesting of grapes is represented by the production of juice that is mostly due to the energetic action of the shakers that knocks against the grape clusters to allow the detachment of the berries. As a consequence, under the same structural conditions of the plants, the production of grape juice depends on the physical-mechanical characteristics of the berry and particularly on its breaking strength and on its detachment from the pedicel. In this paper the strength of the berry to the compression force is evaluated through the measurement of the crushing force and the strength of the berry linked to its pedicel measuring its detaching force. The study was performed on two variety of grapes very widespread in Sicily: Catarratto comune and Nero d’Avola. The research showed that the evaluation of the breaking strength of the berry and its pedicel detachment resistance, correlated with the sugar content, allows to find out the optimal mechanical harvesting time in order to obtain a low production of grape juice and, then, wines of quality from mechanically harvested grapes.