Leaking wine bottles are often erroneously blamed on poorly performing closures, but unless the closure has serious physical defects the reason for leakage is usually due to improper bottling practices and excess bottle pressure after bottling.
Wine bottle drawings from glass manufacturers show the suggested fill point for wine at 68°F. The fill point is measured as the distance from the top of the bottle to the correct wine level in the bottle. These figures do not absolve the winery from their requirement to have a legal fill. They do, however, provide a good idea as to where the correct fill point should be. Generally, the fill point on the 750 mL bottle at 68°F will be approximately 64 mm from the top. It is always best, however, to consult the drawing as a +/-3 mm variance is possible.
The throat diameter of a standard, American 750 mL bottle will vary slightly in the ullage area. On average, however, it is fair to say that the ullage with a 49 mm cork and a 64 mm fill height will average 4.8 mL in volume. For a 45 mm cork the correct volume would be 6.5mL.
The key point here is that good bottling is the key to good results with corks. If the winery bottles at legal fill heights and with adequate vacuum to assure that there will be no more than two pounds relative pressure in the bottle at 68°F, it is very unlikely that the customer will ever complain about leaking corks. The classic reason wines develop excess pressure in the bottles is that the bottles were overfilled in the first place. When this is combined with pressures of warming and expanding wine, leaking can become inevitable.
Based upon figures from "Principles and Practice of Winemaking" by Boulton et al, the thermal expansion of wine between 20°C (68°F) and 40°C (104°F) is 0.08%. This doesn't sound like much. It does, however, convert to 0.166 ml in volume per degree Fahrenheit. Thus, if a winery bottles at 58°F with 4.5 ml in ullage, that ullage will be reduced to under 3 ml at 68°F and internal bottle pressure will have risen significantly.
There are three ways to achieve proper ullage levels and bottle pressure:
- Bottle wine at 68°F and fill to the level designated by the bottle manufacturer and confirmed by the winery.
- Adjust the fill level to compensate for temperature differences. A good rule of thumb is to adjust the fill level by 0.55 mm for every degree Fahrenheit above or below 68°F.
- Adjust vacuum levels to compensate for temperature differences. This method seems less reliable than adjusting fill levels because it places so much responsibility on the performance of bottling equipment. Internal bottle pressure needs to be equivalent to less than 2 psi (relative) at 68°F.
Fill Level Chart by Temperature
Fill Level from Top
Ullage w/ 45mm Cork
Ullage w/ 49mm Cork
One way of dealing with ullage calculation is for bottling managers to chart out target fill heights and internal bottle pressures by bottle type in advance of bottling. Though this will not eliminate their responsibility for a “legal fill”, it will provide an excellent guideline for good bottling. It is also critical that wineries keep good ongoing records during the bottling day.
At a minimum, the following protocols should be observed:
- Freshly corked wines from each corker head should be checked at a minimum every hour for internal pressure (suggested interval is every 30 min).
- Quality control should not rely on the temperature gauge at the filler. A thermometer should be dropped into one bottle ex-filler every half hour.
- If bottling line Q.C. test bottles that are out of spec for fill or vacuum at a specific temp, the associated product should be quarantined, (perferably) flipped upright and inspected.
Maintaining Legal Fill Volume
Legal fill levels are an important requirement. We recommend the following process:
- Consult the bottle drawing.
- Calculate the approximate fill height based upon the acutal temperature of the wine.
- Weigh one case of bottles empty. Record the empty weight of each together with its mold number. Run them through the filler. Weigh each individual bottle. Calculate the net difference (full versus empty). In order to convert this figure to mLs at 68°F, divide by the net by 0.9982 g/mL (the specific gravity of water at 68°F).
- If testing with wine you should re-calculate the specific gravity based on the wine used. Wine typically has a lower specific gravity than water.
- Adjust fill heights as required.
- If legal requirements force the ullage to be smaller than indicated by the internal pressure table, increasing the bottling vacuum can be used to compensate.