As discussed in Importance of Nitrogen in Winemaking, proteins represent a significant portion of wine's total nitrogen content. In juice/must, proteins usually represent less than 10% of total nitrogen content. In wine, levels are far higher and reach up to 40%. There are several viticultural and enological factors effecting wine protein content.
Proteins synthesized during berry development account for approximately half of total wine protein. After veraison, protein synthesis in grapes occurs at a similar rate as sugar level increase. Higher protein levels are associated with:
Winemakers tend to give more thought to wine protein levels during maturation of wines than earlier in the process. It is always important to consider down-the-line effects of any winemaking activity. Indeed, pre-fermentation processes have a larger impact on protein levels in the resultant wine than many winemakers realize.
A small amount of protein is produced by yeast during fermentation, but this tends to not effect overall wine protein significantly. Post-fermentation processes have negative and positive effects regarding wine protein levels
The solubility of wine proteins is highly dependent on the ionic strength of the particularly protein and the wine's alcohol concentration, temperature, and the pH.
An interesting relationship exists between the isoelectric point of proteins and wine pH. The isoelectric point is where positive and negative charges are equal: proteins have a negative charge when pH is above the isoelectric point, and vice versa. Typical wine pH is very close to its proteins' isoelectric point, when proteins are least soluble. This relationship makes removing unstable proteins tricky, requiring the correct type and rates of fining agents.
Winemakers are primarily concerned with proteins in regards to wine stability, which is still largely undetermined. A long list of factors, from grape variety and climate to protein molecular size and interactions with other wine components, effect the exact type and concentration of proteins in wine.
The phenomenon known as protein haze occurs when soluble proteins precipitate in bottled wines. Protein haze makes the wine appear cloudy or highly turbid, considered a defect by most producers and consumers. It is likely composed of several compounds: soluble proteins, polysaccharides, insoluble protein-polyphenol complexes, and metal-protein complexes (proteins act as nuclei for soluble iron, copper, etc.).
Wines with high phenol concentrations will rarely have issues with protein haze since phenols will react and remove sufficient amounts of protein to make the wine stable. This is the reason that protein levels are far more of a concern in white grape varieties, since most red varieties have sufficient phenol levels to stabilize proteins. Protein levels and color instability is highly correlated in red varieties such as Pinot Noir. Due to phenol content, oak maturation also increases protein stability in wines compared to those held in stainless steel vessels.
As the largest source of wine protein, grapes are also the largest source of protein instability. It is believed that proteins originating from yeast do not pose issues with stability.
Evaluation of protein stability should only be conducted after all other winemaking procedures have been completed. In other words, just prior to bottling. Any change to the balance of temperature, pH, and alcohol content from processes such as acidification, malolactic fermentation, fortification, and cold stabilization can lead to precipitation of wine protein complexes.
Protein stability evaluation is not an exact science, and thus involves predictive techniques. These can include heat testing, heat-and-cold testing, and bentonite testing. Most winemakers err on the side of caution, resulting in wines that will be over-fined to ensure stability in their finished product.
The following is a method common to many wineries that I have used with good success.
If a haze does appear, the wine should be fined to remove excess proteins. There are several different types of bentonite available today, and most wineries have their favorite. A bentonite fining trial should be conducted by preparing samples at varying addition rates (addition rates will vary depending on varietal, location, type of bentonite, etc.).
Then, complete the heat stability test again. I often find that bentonite fining in a controlled environment like this results in over-fined wine in the cellar, so I advise choosing a slightly lower rate than determined in the trial. Of course, heat stability should be re-tested once bentonite fining is completed.