The reason not all wines are vegan- or vegetarian-friendly is down to the way a wine is clarified (i.e. made clear and bright) via a process called 'fining'. Young wines naturally contain proteins, tartrates, tannins and phenolics. These are in no way harmful, and most wines will eventually self-clarify. However, to hasten the process, many winemakers use fining agents. The most commonly used are casein (milk protein), albumen (egg white), gelatin (animal protein) and isinglass (fish bladder protein). When it comes to assessing what's in there, it's worth noting the label is generally not going to be of much assistance.