Enology Notes

Enology Notes #26 August 29, 2001

To: Virginia Vintners

From: Bruce Zoecklein

Subject: Maturity Evaluations, Fermentation Issues, Sulfur Dioxide in the Fermenter

Maturity Evaluation. The evaluation of fruit maturity should be conducted with the same care used in determining what and when to bottle. Maturity evaluation, including proper vineyard sampling, is critical to both quality and style. A review of sampling and maturity evaluation including aroma/flavor, tannin maturity, sugar per berry, etc. was discussed in Volume 16, No. 1 of the Vintner's Corner newsjournal. This, along with all past newsjournals, is posted on my web site at http://www.fst.vt.edu/zoecklein/index.html. From the home page click on Extension, then Vintners Corner Newsjournals. A discussion of berry weight, shriveling and aroma/flavor was included in Enology Notes # 23, also available online. From the home page, click on Extension then, Enology Notes.

Fermentation Issues. The role of nitrogen, nutritional deficiency, inhibitory substances, physiological conditions of fermentation, and technological practices were discussed in Volume 14, No. 2 of the Vintner's Corner newsjoural, also posted online.

Each premium wine producer should conduct an evaluation of the nitrogen status of the juice prior to conducting the fermentation. This is easily done by the Formol titration method. We have modified this procedure to make it easy enough for all to run. The modified procedure is posted on my web site under Formol titration. From the home page click on Extension, then Publications On Line.

Remember, too low of a concentration of fermentable N can cause a sluggish, protracted fermentation, and may contribute to the production of volatile sulfur compounds. Remember, the principal factor influencing the production of desirable fermentation volatiles is the nitrogen source for the yeast. This is much more important than yeast strain!

Too much nitrogen can cause the fermentation to occur too quickly resulting in the loose of desirable volatile components and increased production of both volatile sulfur compounds and ethyl carbamate.

The nitrogen status of the fruit is influenced by a number of factors including: cultivar, maturity, vintage and site. We have seen greater than a 50% difference among cultivars and from one season to the next. Do not take a formula approach to winemaking and simply dump nutrients into the fermenter without knowing the nitrogen status before hand!

A healthy fermentation requires a minimum of 140 mg/L fermentable N. One pound per 1000 gallons (12 gram/hL) of DAP increases the fermentable N by about 25 mg/L.

One pound per 1000 gallons (12 gram/hL) of Fermaid K or Superfood increases the fermentable N by about 10 mg/L.

Sulfur Dioxide in the Fermenter. How much SO2 to add to the fermenter is a frequently asked question. While the answer depends on the intent, it is safe to say that we can run into problems by either using none or too much.

Sulfur dioxide binds oxygen. Oxygen plays important roles in the physiological status of yeast. Molecular oxygen is required for the synthesis of lipids and steroids needed for functional yeast cell membranes and, stabilize the membrane which helps to protect the membrane against alcohol. This increase correlates well with improved yeast viability during the fermentative phase.

As fermentation begins, oxygen present in must is rapidly consumed, usually within several hours. After utilization of initial oxygen present, fermentations become anaerobic. Because yeasts are not able to synthesize membrane components in the absence of oxygen, existing steroids must be redistributed within the growing population. Under such conditions, yeast multiplication is usually restricted to 4 or 5 generations, due largely to diminished levels of steroids, lipids and unsaturated fatty acids. This can result in stuck fermentation.

Methodology of starter propagation is important with respect to subsequent requirements for oxygen. Aerobic propagation has been demonstrated to significantly enhance subsequent fermentative activity. Yeast populations reach higher final cell numbers and fermentations proceed at a faster rate and are more likely to complete.

The group of enzymes responsible for catalyzing oxidative reactions in juice are the polyphenoloxidases, also referred to as phenolases or tyrosinases. Polyphenoloxidases catalyze the oxidation of phenols to their corresponding quinones (brown products). The reaction uses oxygen. It can result in a significant reduction in must oxygen concentration.

Traditionally, winemakers added sulfur dioxide to help inhibit this enzyme. Currently, many add sulfur dioxide only after the fermentation is completed.

Not adding any sulfur dioxide prior to fermentation may contribute to sluggish and/or stuck fermentation as a result of oxygen depletion.

Therefore, winemakers should have some air present during the first 30-72 hours of the fermentation. If no SO2 is present, the grape's polyphenyloxidase will greatly reduce the oxygen available for the yeast. Sulfur dioxide will inactivate the enzyme and thus allow a greater concentration of oxygen to be available for yeast lipid production. .

I recommend adding a small amount of SO2 (15-20 mg/L total) before fermentation to help avoid oxygen depletion. Oxygen should be considered an essential 'nutrient' required for proper yeast cell growth. Its management is essential for conduction a healthy fermentation.

Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head, Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg VA 24061-0418
Phone: (540) 231-5325
Fax: (540) 231-9293
E-mail: bzoeckle@vt.edu