Abstract
The pomace cap formation during the red grape fermentation is widely described in literature. However, the measurement of the cap buoyancy is a less debated issue despite it affects several oenological practices. Particularly, it could affect the pomace cap management strategy and the related operations in order to achieve the extraction of compounds from the skins of the berries. In fact, buoyancy affects the volume of the cap submerged by the juice, and consequently contact between the skins and the must. Thus, understanding the changes in cap buoyancy could help to improve the pomace cap management.
In our test we set up a measurement device able to record, with a load cell and a data-logger, the buoyancy of the pomace cap at different times. Afterward, we test the measurement device at laboratory scale.
The measurements show the relationship between the buoyancy and the juice density, the berries density (i.e. both real, and apparent density), and the carbon dioxide emission during the alcoholic fermentation.
During the alcoholic fermentation, the cap changes the force against the load cell as results of the change in cap buoyancy. At the beginning of the fermentation the berries skins are on the bottom of the fermentation vessel. Then, after roughly one day, the pomace cap raise up and start to float and to push against the load cell. The buoyancy force reaches its maximum in our conditions roughly between the fourth and the fifth days, and finally it decreases until the end of the fermentation.
Furthermore, the carbon dioxide escaping from the fermentation vessel produces a vibration on the pomace cap. This vibration is detected by the load cell, and could be related to the fermentation speed. Thus, the measurement of the vibration magnitude allows the monitoring of the flux of the carbon dioxide and, consequently of the fermentation speed. In fact, as expected the maximum has been recorded during the day 3 of the fermentation, namely during the tumultuous phase and when the decrease of the density is maximum. Coherently, the vibration magnitude increases until the third day and decrease up to roughly zero at the end of the fermentation. The relationship between the latter parameter and the buoyancy allows us to monitor the fermentation kinetic.
The control of the fermentation kinetic still remain an open issue in oenology, and the development of simple systems for its measure could lead to kept this objective. The presented results could encourage implementing the measurement system in the future vintages. However, further studies are required before the adoption of this system at industrial scale.
