A bladder system and apparatus for use with a beverage barrel or container, to reduce undesirable head-space in a wooden container of a beverage, especially a fermented beverage, such as wine. The system continuously protects the beverage contained in a wooden barrel from the harmful effects of oxidation at the air-to-fluid interface in the head-space and compensates for ullage, attributable to evaporation, leakage or sampling losses. The system includes an inflation stem connected to a barrel bladder inflatable by filling with an inflation gas. The inflation stem includes a liquid level sensor and inserts into a bung opening. The fluid level is sensed and a gas valve opens to fill the bladder, reducing head-space within the barrel. The system can combine the control of several barrels and optionally with other sensors, to create a barrel monitoring and control network.