Jamal Munshi, Sonoma State University, all rights reserved
Transpiration loss during barrel-aging of wines creates an air space. The oxygen in these air pockets is injurious to wine quality. Periodically, each barrel must be unbunged and topped off with wine to reduce oxygen contact. This process is the weak link in the production of high quality wines in a large scale because it is labor intensive and because uncontrolled air contact occurs in between and during topping operations. An additional problem in this process is that the carbon dioxide produced by fermentation is allowed to flow into the barrel room where it presents a health hazard. In terms of winery economics, the health hazard translates to an additional cost of health and casualty insurance.
To address these winemaking problems, a method is proposed for topping wine barrels continuously and automatically using gravity feed. The method is appropriate for small or large scale barrel arrays. The primary benefit of the process is savings in labor and workers compensation insurance costs involved with periodic manual topping operations. The use of the automatic method also reduces contact with air during the barrel aging process that is imposed by the topping operation and not within the winemaker's control. The process therefore offers increased quality control and the possibility of producing higher quality wines. An additional benefit is that barrel rooms that use this system are expected to be safer because they are free of fermentation CO2. All fermentation CO2 is piped and vented outside the barrel room.
On the downside, the use of such a system would require additional capital investment and would severely restrict the movement of barrels. Barrel room management systems that require movement of barrel pallettes to staging areas must be substantiallly re-designed. The proposed topping and venting system may not be compatible with wine making styles that require frequent racking and cleaning operations.
The method requires a stainless steel tank, a header pipe assembly, a pressurized CO2 source, and pressure regulators as shown in the figure below. Sufficient topping fluid is stored in a stainless steel tank that is designed to hold a positive pressure and sized to provide airspace at full liquid capacity. A line from the bottom of the tank is connected to a header that feeds into the barrel array. The airspace is vented to the atmosphere through a back pressure regulator valve V1 that is set to maintain a pressure P1 with a margin of D1. If the tank pressure rises above P1+D1 this valve will open and if it falls below P1-D1 it will be shut. Upstream of this valve a pressurized CO2 source is connected to the tank with a forward pressure regulator valve V2 set at pressure P2 with a margin of D2. If the tank pressure falls below P2-D2 this valve will open and if it rises above P2+D2 this valve will be shut.
For the system to work it is necessary that P1-D1 is greater than P2+D2 by an amount that is sufficient to prevent the CO2 bottle from bleeding directly to the atmosphere; and that P2-D1 exceeds atmospheric pressure by more than the pressure drop in the header (ajusted for height differences) so that topping flow can occur even when all fermentation has ceased and the barrels are no longer generating CO2. The actual values of P1 and P2 will depend on the anticipated pressure drops, elevation differences, and safety margins. The use of this apparatus will leave no airspace in the barrels themselves. In fact the barrels will be subjected to a positive pressure of P1 plus pressure drops through the header assembly. This means that the bung must be affixed to the barrel using a mechanism that will sustain this overpressure. Various bung designs including a screw design could be used.
The header assembly is designed to carry fluids in both directions and to operate under two-phase flow conditions. During fermentation activity a wine-CO2 mixture will flow from the barrels to the topping tank to be vented. These peak flows are the sizing criteria for the header assembly. Later in the aging cycle, as fermentation slows or stops, transpiration losses will flow from the topping tank back to the barrels.
Wine in barrels connected to a single header will become intermingled. Therefore, barrel lots must be isolated with each lot assigned to its own venting and topping system.
The diagram below is a schematic of the essential process.