Courtesy of The Drifting Winemaker
Proteins represent a significant portion of wine’s total nitrogen content. Proteins synthesized during berry development account for approximately half of total wine protein, a small portion is derived from yeast protein synthesis during fermentation, and the remainder from yeast autolysis. Protein levels in grapes and resultant wine depend on several variables, including viticultural and winemaking factors.
Higher protein levels are typical in more mature grapes, grapes sourced from warmer regions, grapes grown at low crop levels, and grapes harvest mechanically. Skin contact with white varietals prior to pressing will typically increase protein concentration (in turn, fining and solids-separation pre-fermentation reduces protein concentration). Fortification typically results in significant lees precipitation, including a large quantity of proteinaceous lees depending on the wine’s polyphenolic concentration. Extended lees contact (to be discussed in a later post) also attributes to increased protein levels.
Winemakers are primarily concerned with proteins in regards to wine stability. Protein precipitation in bottled wines (whites and reds with low amounts of polyphenols) causes ‘protein haze’ or crystalline deposits; these are likely a combination of soluble proteins, polysaccharides, insoluble protein-polyphenol complexes, and metal-protein complexes (protein act as nuclei for soluble iron, copper, etc.).
The solubility of wine proteins is highly dependent on temperature, alcohol concentration, and pH. Typical wine pH levels are very near most wine proteins’ isoelectric point (where positive and negative charges are equal). Proteins have a negative charge when pH is above the isoelectric point, and vice versa. This plays a major role in protein stability and determining what fining agents to use on particular wines.
Phenolics play a major role in protein stability due to their interactions with proteins. Wines with high polyphenol concentrations will often remove a sufficient amount of proteins to make the wine stable; thus, white wines and lower phenol red varietals have more issues with protein instability (color loss and instability in red varietals such as Pinot Noir are highly correlated to protein concentration). Due to the tannins in wood, wines fermented or stored in barrels also have far less problems with protein stability when compared to those held in stainless steel.
Evaluation of protein stability should only be conducted after all other winemaking procedures have been completed, just prior to bottling. Processes such as acidification, malolactic fermentation, fortification, and cold stabilization (will be discussed in a later post) can lead to precipitation of wine protein complexes (largely due to shifting pH levels). Since proteins react with phenolic compounds in red wine during fermentation, protein stability is usually only an issue in white and rose wines.
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. This is a method common to many wineries that I have had good success using:
Filter sample through a sterile filter (0.45 µm). If the sample is still cloudy (i.e. from tank sitting on lees), you may need to centrifuge it prior to filtering.
Fill one test tube with filtered sample as a control.
Fill a second test tube and heat to 80° C (180° F) for two hours (many wineries like to do so for six hours instead, while others heat at lower temperature for up to 24 hours; I believe two hours at this temperature sufficiently precipitate proteins).
After sample is heated, allow it to return to room temperature. Giving the sample several hours or overnight is advisable to allow precipitation; if this is to be done, refrigerate both the control and heated sample but make sure to allow both to return to room temperature prior to reading.
Compare the two samples. Ideally, this should be completed with a turbidity meter (nephelometer); protein stable samples are deemed those with an NTU <1.0. Visual comparison with a bright light can be completed in lieu of a turbidity meter but my not adequately assess stability.
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.