In search of a decent sherry, we invested recently in an Australian product of the Emu brand, which is pleasant flavored and moderately dry. Curious to know more about this very reasonably priced wine, I was astounded on examining the label to read as follows: "contains milk, egg and fish."
How can that possibly be?
For very good reason, according to this short article by Richard Gowel: it is to "fine" the wine; fining being the process of removing harsh tasting phenolics, or unwanted color from wine made with the final squeeze of the grape. There you are then!
Or to be more specific, the milk protein casein is a fining agent that precipitates phenolics, as does albumen, a protein from egg whites. But of all, the best fining agent, apparently, is isinglass, a protein obtained from the swim bladder of certain fishes.
From this we can infer that Emu sherry is made from a juice so rough that it requires the combined fining power of all three agents, derivatives of milk, egg and fish, to make it drinkable. But drinkable it certainly is, with a distinctive and likable flavor.