Technical Bulletin - Calculating Headspace

Wine bottle drawings from glass manufacturers show the suggested fill point for wine at 68ºF. 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 64mm from the top. It is always best, however, to consult the drawing as a +/-3mm variance is possible.

The throat diameter of a standard, American 750ml bottle will vary slightly in the ullage area. On average, however, it is fair to say that the ullage with a 49mm cork and a 64mm fill height will average 4.8 ml. in volume. For a 45mm cork the correct volume would be 6.5 ml.

If the winery bottles at legal fill heights and with adequate vacuum to assure that there will be no more than 2 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 20ºC [68ºF] and 40ºC (104ºF] is .08%. This doesn't sound like much. It does, however, convert to .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 68ºF and internal bottle pressure will have risen significantly.

There are three ways to achieve proper ullage levels and bottle pressure.

• Bottle wine at 68ºF 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 0.55mm for every degree Fahrenheit above or below 68ºF.
• 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 2psi (relative) at 68ºF.

Fill Chart by Temperature*

Bottle Pressure by Temperature*

• 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. tests bottles that are out of spec for fill or vacuum at a specific temperature, the associated product should be quarantined, [preferably] flipped upright and checked out. Only when the problem is resolved, should cases be returned to regular inventory.
• Q.C. should always check out the readings on cork probe gauges against one another in the morning and again at noon.
• These gauges should also be used to check the functioning of the corker gauge [not vise versa].
• If there is a problem with the vacuum on one or more of the corker heads, the line should be stopped until it is cleared. This should not be done "on the fly".