fill level, an aspect of individual bottles of wine which can be closely related to the condition of the wine. The lower the fill level when a wine is bottled, the more the space between the top of the wine and the bottom of the cork (the so-called head space or ullage) in which oxygen may be trapped in the bottle and may hasten the ageing process. Most bottlers try to ensure that there is minimal ullage space in the bottle immediately after bottling, a depth of 5–10 mm/0.2–0.4 in to allow for expansion if ambient temperature ever exceeds the usual bottling temperature of 20 °C. On modern bottling lines, the bottle may be flushed with inert gas before and after filling in order to limit as far as possible the oxygen in the head space (see total package oxygen). Subsequent reductions in temperature cause a reduction in the wine’s volume, thereby apparently lowering the fill level.
