The airlock in winemaking should start to bubble when fermentation has begun, which typically occurs within 24 to 72 hours after the yeast has been added to the must (juice and other ingredients). Bubbling in the airlock indicates that carbon dioxide (CO2) is being produced by the yeast as they consume the sugars in the must. The CO2 is expelled through the airlock, allowing the pressure to be regulated and preventing oxygen from entering the fermenting wine, which can cause spoilage.
The airlock is an important tool in winemaking as it allows the winemaker to monitor the progress of fermentation. The frequency and rate of bubbling can indicate if the yeast are active and healthy, and if the temperature and other conditions are suitable for fermentation. The bubbling should gradually slow down as the yeast consume the majority of the sugars, indicating that fermentation is coming to an end. The airlock can also be used to check if the wine has finished fermenting, as the absence of bubbles means that the yeast have completed their work and the wine is ready for aging or bottling.