Vintner's Corner

Vol. 12, No. 2 March - April, 1997

Bruce Zoecklein

Department of Food Science and Technology


Blacksburg, VA 24061-0418

Table of Contents

I. Adding Complexity to Red Wines 1

II. Sulfur Dioxide Analysis 2

III. Coming Events

Wine and Juice Production and Practical Monitoring Workshop 3

Annual Meeting of the American Society for Enology and Viticulture

and International Riesling Symposium 5

IV. Proceedings Available 5

V. Department Internship 5

I. Adding Complexity to Red Wines

Cold soaking is a production tool which may increase the complexity (aroma), color and color stability of our red wines. If you have not used this procedure in the past, consider it for the 1997 vintage. The process involves juice and skin contact for 1-4 days prior to fermentation. During "soaking" chemical transformations occur which may include native grape glycosidases hydrolysis of some bound aroma components and the interaction of wine phenols. In the absence of alcohol, chemical bonds are formed between anthocyanin pigments and other phenols which are believed to stabilize color in the resulting wine. Additionally, cold soaking is an aerobic process (frequently involving pump mixing the must) which allows increased phenol polymerization possibly increasing suppleness. The term cold soak refers to cold or cool temperatures during prefermentation maceration. Relatively cool temperatures are required to prevent spontaneous fermentations. Temperatures above 10oC, however, do not detrimentally affect the benefit of improved color and color stability. The best approach may be to crush and add pectinolytic enzymes during the cold soak period. This allows maximum extractions. In Burgundy, soaking may be performed in the presence of sulfur dioxide to aid in the control of fermentation. Because sulfur dioxide binds phenols at the site available for anthocyanin binding (needed for color stability) such additions may not be desirable if they exceed 15 mg/L free sulfur dioxide. This requires an accurate method of SO2 analysis (see below).

Cold soaking lowered hues (A420nm/A520nm) and the rate of phenol polymerization while increasing color intensity (A420nm + A520nm) and total anthocyanins. The practical result of cold soak is to produce wines with brighter color, less tawniness with added complexity.

II. Sulfur Dioxide Analysis

There is an industry-wide trend toward reducing sulfur dioxide use whenever possible. The reasons include public health concerns, better quality fruit, possible desire for a malolactic fermentation, and the perceived delicacy of wines (lower levels of phenols in white wines) that have had only limited SO2 additions. Reduction of sulfur dioxide levels particularly in the juice is often consistent with such stylistic goals. Additionally, wines produced with low levels of sulfur dioxide are thought to be softer on the palate.

A number of sensory characteristics in wines have been directly attributed to the presence of sulfur dioxide. High levels of SO2 impart a metallic (tinny) and harsh character to wines. Furthermore, excessive levels of free sulfur dioxide add a pungent aroma, a sharpness in the nose, and a "soapy" smell.

Because of the general concern for sulfites in foods as well as sensory considerations, accurate analysis of these compounds is essential.

The Ripper method for sulfur dioxide, which is more than one hundred years old, uses standard iodine to titrate the free or total SO2 in a sample. Although it is universally recognized that this method is somewhat inaccurate, the procedure is so simple that it is the most common method employed in the winery laboratory.

In this procedure standard iodine is used to titrate free sulfur dioxide. Free sulfur dioxide is determined directly. While total sulfur dioxide can be determined by first treating the sample with sodium hydroxide to release bound sulfur dioxide.

The analysis for free and total SO2 is dependent upon the redox reaction:

H2 SO3 + I2 H2 SO4 + 2HI

The completion of this reaction is signaled by the presence of excess iodine in the titration flask which is competed with added starch (blue black end point).

The Ripper procedure for free and total SO2 suffers from several notable deficiencies: 1) volatilization and loss of SO2 during titration; 2) reduction of the iodine titrant by compounds other than sulfite; 3) difficulty of end point detection in red wines.

Commercial kits for conducting the analyses of sulfur dioxide by the Ripper technique are available. These kits generally involve the same chemistry as is utilized in the Ripper titration. The accuracy, therefore, can not be any greater than the Ripper titration method and may be less under optimum conditions. The Ripper titration for total SO2 is accurate to 7 mg/L. The main problem with either the kit or titration form of the Ripper analysis occurs with red wines and white wines made from fruit which has been degraded by Botrytis cinerea. With reds the problem is not simply the inability to see the end point, but also due to phenolic compounds which react with iodine producing a false high reading. In highly pigmented wines, this inaccuracy can be quite extensive. White wines generally possess fewer phenols and other interferences. Wines which have been made from grapes with Botrytis may contain metabolities which cause problems with the analysis.

Improved accuracy, particularly in red wines can be attained by other analysis methods such as the Aeration Oxidation (AO) procedure.

In this procedure, sulfur dioxide in wine or juice is distilled (with nitrogen as a sweeping gas or with air aspiration) from an acidified sample solution into a hydrogen peroxide trap, where the volatilized SO2 is oxidized to H2SO4:

H2O2 + SO2 SO3-2 + H2O- H2SO4

The volume of 0.01 N NaOH required to titrate the acid formed to an end point is measured, and is used to calculate SO2 levels.

The glassware involved is relatively inexpensive and the analysis is easy to perform. The AO procedure eliminates the interference from pigments and fungal metabolites. Because of the importance of accurate analysis. I highly recommend the AO procedure for free SO2 determination. For further details regarding this procedure see Zoecklein, et al., 1990, 1995.

III. Coming Events

Wine and Juice Production and Practical Monitoring Workshop

Historically, winemakers succeeded with slow but gradual improvements brought about by combinations of folklore, observation, and luck. However, they also had monumental failures resulting in the necessity to dispose of wine or convert it into distilled spirits or vinegar. It was assumed that even the most marginally drinkable wines could be marketed. This is not the case for modern producers. The costs of grapes, the technology used in production, oak barrels, corks, bottling equipment, etc., have increased dramatically and continue to rise. Consumers are now accustomed to supplies of inexpensive and high-quality varietals and blends; they continue to demand better. Modern winemakers now rely on basic science and the systematic application of their art to produce products pleasing to the increasingly knowledgeable consumer base that enjoys wine as part of its civilized society. To help assure that our industry remains competitive, we are offering a short course program on juice and wine monitoring.

This course will be a regional meeting of the American Society for Enology & Viticulture -Eastern Section held on June 7 - 9, 1997 at the Holiday Inn Washington Dulles, 1000 Sully Road (Route 28), Dulles, Virginia 20166.

The program is presented and sponsored by the Enology-Grape Chemistry Group, Department of Food Science and Technology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia and Viticulture and Enology Research Center, California State University- Fresno, Fresno, California.

This workshop is for persons interested in receiving an integrated program of information of importance to the Eastern Grape and Wine Industry. You will receive practical information on technical management of wine and juice production plus on-site monitoring and analysis of grape, juice, and wine.

About the Presenters: Bruce W. Zoecklein is an Assistant Professor and Enology Specialist in the Department of Food Science and Technology at Virginia Polytechnic Institute and State University, Blacksburg; K. C. Fugelsang is Winemaster and Adjunct Professor of Enology at California State University, Fresno; Barry H. Gump is Professor of Enology and Analytical Chemistry at California State University, Fresno.

The presenters have coauthored Production Wine Analysis (1990), Wine Analysis and Production (1994) among numerous technical papers and books including Wine Microbiology (1996) by K.C. Fugelsang, and Beer and Wine Production (1993), edited by B.H. Gump.

This conference is recommended for: Winemakers, Juice Processors, Laboratory Personnel, Researchers, and Educators.


The registration deadline is Friday, May 9, 1997

Preconference Session Registration Fees:

Registration Fee: $40 per person

Late Registration Fee: $75 per person

Workshop Registration Fees:

Registration Fee: $200 per person

Late Registration Fee: $250 per person

Student Fee w/valid ID: $150 per person

Additional registrants from the same company: $150 per person

Complete the registration form and make check payable to: California State University, Fresno Foundation

Mail your check and completed form to:

Viticulture and Enology Research Center (VERC)

Attention: Cynthia Wood

California State University, Fresno

2360 E. Barstow Avenue M/S 89

Fresno, CA 93740-8003

Phone: (209) 278-2089 FAX: (209) 278-4795

Your workshop registration fee includes admission to the workshop, refreshments, wine reception and lunches.

If Your Plans Change: Individuals may send a replacement at any time for no additional fee by calling VERC prior to the event.

Overnight Accommodations in Virginia

Holiday Inn Washington Dulles (703) 471-7411

$83 + 10% tax - Single or double room

$93 + 10% tax - Triple or quad room

For rate mention: Wine Production & Analysis Workshop

Preconference Session, Saturday, June 7, 1997

Registration/Coffee: 7:30 am - 8:30 am

Presentations: 8:30 am -12:00 pm

Review of the following analytical methods:


Titratable Acidity/pH

Sulfur Dioxide

Volatile Acidity

Malolactic Fermentation (paper chromatography)

Bitartrate Stability

Protein Stability

Reducing Sugar

Workshop, Saturday, June 7, 1997

Registration: 12:30 pm - 1:30 pm

Presentations: 1:30 pm - 5:30 pm

Wine Reception: 5:30 pm - 7:30 pm

Sensory techniques in winemaking:

Establishing a sensory evaluation program in the winery

Vineyard management and grape chemistry:

Defining grape quality

Use of secondary metabolites as maturity gauges

Chemical and sensory analysis of aroma/flavor

Grape and wine phenolic components:

Management of wine phenols

Analysis of grape and wine phenols

Prefermentation nutritional issues:

FAN deficiency and related problems and correction

Analytical methodology for prediction of nitrogen status

Native yeasts and bacteria - other microbiological issues

Sunday, June 8, 1997

Coffee: 7:00 am - 8:00 am

Presentations: 8:00 am - 5:00 pm

Lunch: 12:00 pm - 1:30 pm

Fermentation issues:

Selection, utilization of commercial yeast cultures

Native yeast fermentations

Factors affecting hydrogen sulfide and other

odor- active sulfur compounds

Post-bottling hydrogen sulfide

Sensory examination of hydrogen sulfide/ mercaptans

Post-fermentation issues:

Commercial and native lactic acid bacteria

Monitoring the progress and completion of MLF

Post-MLF handling of wine

SO2 and ascorbic acid

Monday, June 9, 1997

Coffee: 7:00 am - 8:00 am

Presentations: 8:00 am - 4:00 pm

Lunch: 12:00 pm - 1:30 pm


Utilization of selected fining agents

Metal instabilities, diagnostic and analytical measurements and their correction

Carbon dioxide:

Utilization, measurement and sensory considerations

Techniques for addition/removal

Potential for using other gases

Tartrate and protein stabilization:


Bottling concerns:

Sterile bottling and alternatives




Quality issues (physical, chemical, and microbiological)

Sensory examination of selected cork taint


Proceeds of this conference are to benefit the Enology-Grape Chemistry Program at Virginia Polytechnic Institute, Blacksburg, Virginia and the Enology Research Program at California State University, Fresno, Fresno, California.

The Wine Reception is sponsored by the Virginia Wineries Association.

Annual Meeting of American Society for Enology and Viticulture and International Riesling Symposium

The 22nd Annual Meeting will be held at the Radisson Hotel in Corning, New York on July 9 - 11, 1997.

The International Riesling Symposium will involve:

The Technical Program will involve:

The technical committee of the Virginia Wineries Association is working on a possible bus trip to the meeting. If this develops, information will be mailed to both growers and vintners.

IV. Proceedings Available

Proceedings are now available from the 4th International Symposium on Cool Climate Viticulture & Enology on CD ROM or Print. Contact ASEV/Easter Section, Ellen Harkness: Phone: 317-494-6704; FAX: 317-494-7953 EMAIL:

V. Department of Food Science and Technology Internship Program.

In 1995, our department, in cooperation with the Williamsburg Winery, began an annual student internship. This program allows qualified students to gain practical skills while also participating in an applied research activity. This mutually beneficial experience serves the student, our department and certainly the industry. I want to publicly thank the Williamsburg Winery for their generous support. Any Virginia vintner interested in an Enology student intern should contact my office.