The final stage of sparkling wine production is known as disgorgement. After the second fermentation occurs inside the bottle, sediment forms, consisting of dead yeast cells and tartrate crystals.
Initially, the sediment is settled in the neck of the bottle through a process called riddling. This involves gradually turning the bottles until the sediment collects at the neck, where it is easier to remove. Next, the neck of the bottle is immersed in a freezing glycol solution, freezing the sediment plug. Once frozen, the crown cap is removed, and the pressure inside the bottle forces out the frozen plug, leaving behind a clarified wine.
Following this, a sugar addition known as dosage is mixed with wine and used to top off the bottle before sealing it with a cork and securing it with a metal cap and wire twist (or muselet). This process is carried out manually at Colonjes winery in The Netherlands, although automation is available for larger production runs.
For further details on the disgorgement process and other aspects of sparkling wine production, you can watch the video demonstration here.