For those that bother to look closely at wine labels, it may be a surprise to see sometimes written there in the small print 'may contain traces of fish, egg, milk and nut products'.
What's this all about?
Newly fermented wine contains a lot of solid material in suspension. This can settle out naturally but will generally take a long time. To make the process quicker a process called 'fining' is used.
Fining means the addition of either natural or synthetic materials to wine to clarify and stabilize it. Particles in suspension are carried down with the fining agent to form a deposit on the bottom of the holding vessel, in our case a tank. The clarified wine is then racked (decanted) off this layer.
Historically, a wide variety of agents have been used for fining eg. ox blood, egg whites, milk casein, fish bladders, horse gelatins, seaweed, clay and others. In fact, almost any protein will work to some extent by binding to other proteins and forming the solid deposits.
During the prebottling filtering stage most of the remnants of these fining agents are removed but minute quantities can remain. In this litigious age however, wine companies will put a warning on their labels to cover themselves.
Here we use bentonite, a special clay, to fine white wine. Not only does it aid in clarifying the wine but it also removes heat unstable proteins which can cause hazes in the finished bottled wine. We make up a water slurry of the material based on 0.8g/L wine and add it to the ferment.
I guess this clay is not an allergen so it's never mentioned on labels.
For fining red wine we use egg white which is carefully mixed into a solution with water and a little salt which helps dissolve the protein. One egg white/200L wine seems to do the trick. The yokes we use in omelettes.
Other wineries in the area use milk to clarify their white wines. Here it is the casein content that does the work.
Isinglass is made from the swim bladders of certain fish and is also used to fine white wines.
So what about nut products?
This was new to me also.
It is allowed to add tannin to wine, predominantly reds, that are lacking it. Tannins come from the grape skins (and seeds and stalks) as well as the oak and give that dry puckering feeling in the mouth after swallowing red wine. They are an important component as they help give the wine structure and texture. Most tannin products used in the industry are grape derived but some are indeed tree nut derived eg. chestnuts.
Hence the allergen warning for the latter, not the former.