Enology Notes

ENOLOGY NOTES # 12 January 12, 2001

To: Virginia Vintners and Prospective Vintners

From: Bruce Zoecklein

Subject: Change in Laboratory Services, Protein Stability Continued, Staunton ABC Office has New Address

Laboratory Services. As I have reported previously, demands on the Enology-Grape Chemistry Group's no charge extension laboratory service have increased dramatically. Since 1996, the number of analyses performed has increased 146% with no increase in laboratory operational funds.

In 1986, I established the laboratory service to provide supplemental quality control assays. It was understood that many wineries had neither the instrumentation nor the technical training to conduct some needed quality control tests. Since then, the Enology Grape Chemistry Group has conducted numerous short courses, workshops and symposia on quality control and quality control analysis. We have written extension publications and news journals on various analysis subjects.

The laboratory service will remain in effect to provide supplemental quality control, to help in trouble shooting and provide sensory evaluations. As of January 15, 2001, the laboratory will not conduct the following assays: alcohol, pH, TA and sulfur dioxide. Each bonded winery in the state should be able to conduct these tests in-house. Any bonded winery that would like assistance in establishing a lab and/or in-house QC program can contact my office.

Wine Proteins Continued. As discussed in the previous edition of Enology Notes, the nature of wine protein instability has been difficult to elucidate due to the many factors involved. Proteins differ as a result of cultivar, maturity, climate, molecular size, and electrical charge as well as the interaction and precipitation with other components. To add to the confusion, what we call protein instability is usually complexes of protein, polysaccharide and polyphenol compounds. This helps to explain why heat tests, protein precipitation tests and analysis of total proteins are not always effective predictors of potential instability.

Maximizing aroma and flavor is an important wine production concern. As reported in previous editions of the Vintner's Corner, protein stabilization can have a significant reduction on wine aroma and flavor components, particularly if large concentrations of bentonite (greater than 2 lb/1000 gallons or 24 g/hL) are required post-fermentation. Bentonite, the most commonly used fining agent for protein stabilization, is not discriminatory. It can adsorb large concentrations of ethyl and acetate esters, thus lowering aroma and aroma intensity. Therefore, it is imperative that premium wine producers understand white wine protein stability to help minimize aroma/flavor stripping.

Grape maturity and processing can have a qualitative and quantitative influence on wine proteins. The concentration of proteins increases with fruit maturity and pre-fermentation skin contact, particularly in the presence of sulfur dioxide. Pressing de-stemmed grapes yields a higher protein content than pressing with stems. Tannins, including stem tannins, have an ability to bind with proteins, thus lowering the concentration. This is the principle reason why mechanical harvesting (which results in grapes with relatively few stems) produces wines which have higher protein concentrations. This frequently results in the need for a greater bentonite concentration to obtain stability.

Any processing activity that influences phenol extraction and the possible binding of phenols with proteins can influence wine proteins. Therefore, fruit maturity, the addition of tannin, change in presses or press pressure, and a change in the proportion of free run to press run juice can influence total protein and the potential for protein instability.

Tannins may precipitate proteins. However, some soluble protein-tannin complexes are heat sensitive and less easily adsorbed by bentonite. This helps to explain why white wines in new oak barrels not stored sur lie can demonstrate a greater degree of heat instability with time. When wines are stored sur lie the opposite is true. As discussed in Enology Note # 6 (Oct. 13), yeast lees have some remarkable characteristics including the ability to bind with proteins and render them heat stable. This conversion is not believed to be due to adsorption or digestion but is the result of an association with manoproteins released by the yeast.

As stated, what we call protein instability is usually complexes of protein, polysaccharide and polyphenol compounds. As a result, some protein stability tests overstate the risk of potential haze formation which can lead to higher levels of fining agents required for stabilization. The two common types of protein stability assays are precipitation tests and heat tests.

A common precipitation test is the TCA or trichloracetic acid test. This assay involves heating a wine sample for two to five minutes in the presence of the acid, cooling and evaluating haze formation. The Bentotest uses phosophomolybdic acid and is a similar precipitation test but is performed at room temperature. The difference in clarity before and after the test is determined either visually or more appropriately with the aid of a nephelometer and is measured in nephols units or NTU's . Both tests chemically denature wine proteins in a acidic media, making no differentiation between stable and potentially unstable proteins. Therefore, both somewhat overestimate the requirement for bentonite.

Heat tests range from high temperature short time (85 degrees C for one hour) to the more traditional 45-50 degrees C for 24 to 48 hours. Heat tests are a measure of heat labile proteins as opposed to precipitation tests which do not discriminate. A concern is that heat tests involving prolonged heating can cause oxidative changes resulting in phenolic polymerization, even in the absence of air and presence of sulfur dioxide or ascorbic acid. This may lead to combined precipitation of proteins with polyphenolic compounds, and thus overestimate the potential for heat labile protein precipitation. The co-precipitation of proteins and phenols is the principle behind hyper-oxidation of juice to attain protein stability.

Heat tests are perhaps the most consistently reliable predictors of the potential for protein instability, particularly if a wine has been stored sur lie. Maximum turbidity forms after cooling the heated sample. Wines are considered stabile if, following treatment and cooling, no haze is visible or there is only a limited nephols change ( less than 2 NTU).

Understanding the nature of wine proteins and when and how to test for protein stability should be part of every premium wineries HACCP plan. For future details on wine proteins and stability evaluations see Zoecklein et al (1995).


The Staunton Regional ABC Office's new address is: 460 Commerce Square, Staunton, VA 24401.