Firmness of the grapes. Mechanical tests and definition of indices. Study of the evolution of berry skin resistance during alcoholic fermentation

Abstract

Context and purpose of the study: The mechanical strength or firmness of a fruit is considered an important parameter to characterize its state of maturity or conservation, as other parameters such as sugar level or color. The mechanical resistance of grapes influences the integrity and sanitary quality of the harvest. In this study, the mechanical characteristics of grapes berries are studied at harvesting time in order to determine their properties of firmness and the resistance of the berry skin during the alcoholic fermentation. Special indices are defined measuring the energy needed to crush 50% of the initial diameter of the berry. We applied these indices to different varieties and get different results either for the entire berry firmness or for the skin resistance.

Material and methods : To evaluate the firmness of grapes, INRA has developed a tool specifically adapted to measure the skin resistance of the grapes (Penelaup Robot, patented). We used here two grape varieties: Grenache Noir and Carignan Noir.Firmness of the entire berries were measured at harvesting. Right after, the fermentations were conducted at 21°C, in low volume tanks (<1kg) using “French Press” coffee plunger with similar and standard conditions. 1 kg of berries were crushed and poured in the tank. Lalvin ICV OKAY yeast (20 g/hL) and SO2 (250 µL of a 8% solution) were added simultaneously. Cap management was carried out every day during alcoholic fermentation (AF) by submerging pomace with the plunger. The decrease of sugar concentration was monitored by measuring the Brix degree and the density. Fermentations were considered done when the density remained stable (7 to 8 days) with density less than 995. At the end of AF the classical wine chemical parameters were determined. Skin resistance measurements were carried out at the beginning and at the end of AF plus several points in between.

Results: We defined mechanical indices dedicated to the firmness of grapes. Using these indices, the result of this study shows differences in firmness related to the grape varieties: Grenache Noir and Carignan Noir have different mechanical properties. Similarly, during the alcoholic fermentation, the resistance of the skins highlights different properties of the berries immersed in the fermenting must. This had never measured until now. These results give new information on the mechanical properties of the grapes. It would help the winemaker to better choose the type of fermentation and maceration adapted to his grapes depending on the type of wine he wants to produce.