Vintner's Corner

Vol. 13, No. 5 September - October, 1998

Bruce W. Zoecklein

Department of Food Science and Technology

VPI & SU - 0418

Blacksburg, VA 24061, E-mail: bzoeckle@vt.edu

Table of Contents

I. Co-pigmentation 1

II. Controlled Aeration 1

III. Accurate Sulfur Dioxide Analysis 3

IV. Upcoming Event - Oak from Forest to Glass - Practical Management of Oak and Wine 4

I. Co-pigmentation

There is evidence that the extraction and retention of anthocyanin pigments is limited by their solubility. To increase solubility, anthocyanins must be co-joined to other non-pigmented compounds. This joining or co-pigmentation results in significantly enhanced spectral color. For example, Dr. Roger Boulten, UCD, has demonstrated that the prefermentation addition of caffeic acid increased red color adsorption at 520 nm by 60%. Such co-pigmentation can result in a positive deviation from Beers Law where the color intensity increases more than the increase in anthocyanin concentration. This is believed to be a principle reason why blending a small volume of white grapes (presumable with a high concentration of cofactors such as caffeic acid) can enhance spectral color of Sangiovese and some Rhone reds.

As discussed at the last Virginia Winemakers Roundtable, prefermentation maceration or cold soak (particularly in the presence of maceration enzymes) may aid in enhancing co-pigmentation. Maceration enzymes are a relatively new generation of products. These are different than standard pectinolytic enzymes in that they also contain cellulases and hemicellulases which may allow for more rapid extraction of both anthoganim and non-colored phenols.

II. Controlled Aeration

As stated, red wine color results from the integration of various phenolic compounds such as the monomeric anthocyanins and polymeric structures. The color of a young red wine is due primarily to the monomeric anthocyanin pigments. During fermentation and storage, however, condensation reactions occur, producing polymeric structures. Aeration is most effective in the months following fermentation when the tannins are only slightly polymerized. At this stage, the condensation of tannins with anthocyanins in the presence of air will help stabilize pigments. At the same time, tannins become more supple as a result of condensation with anthocyanins. Following controlled aeration, the total content of phenol compounds remains almost constant while the free anthocyanin level decreases.

The ratio of tannins to anthocyanins is an important factor in providing long-term color stability. Tannins stabilize anthocyanins by binding to form larger polymeric pigments. Production strategies such as stem return, addition of tannin, use of low profile tanks, extended cuvaison and barrel aging all increase the tannin to anthocyanin ratio and thus help provide color stability. In addition to management of phenols in the fermenter, winemakers should continue to look at controlled aeration of reds as a production tool to enhance color stability, evolve and soften tannins and help lightly structured wines by providing body. Controlled aeration is thought to increase the rate of reaction of coloring matter with tannins resulting in condensation, polymerization and enhanced suppleness. Judgements are based on style, phenol content and pH, with air exposure accomplished by aeration during tank and barrel to barrel racking. Too much air exposure or aeration too late in the development of a wine can cause precipitation of the tannin-anthocyanin complex.

Acetaldehyde is produced as a result of the oxidation of ethanol which can occur quickly as a result of splash racking or slowly due to barrel aging. This aldehyde formation provides the chemical bridge to bind anthocyanins and tannins together. The rate of oxidation is a function of temperature, pH and sulfur dioxide content. During the development of red wines, it is possible to modify the formation of the anthocyanin-tannin complex (and, therefore color stability and palate structure) by adjusting either the quantity of dissolved oxygen or sulfur dioxide or both. The higher the pH, the greater the rate and extent of oxidation. This may be an important consideration this year. High malic acid levels will result in fairly high pH's following MLF. Sulfur dioxide slows or inhibits the formation of the tannin-anthocyanin complex by binding acetaldehyde, tying up the bridge needed in the formation of the complex. Thus, free SO2 should be kept below 15 mg/L in young red wines if the goal is to stabilize color and evolve tannins. The lower the free sulfur dioxide the faster the rate of complex formation. This requires the use of an accurate method of SO2 determination (see below).

In practical terms, the questions are 1) how much oxygen is needed for optimal maturation without provoking phenolic instability or excessive oxidation? and 2) how late in wine maturation can controlled aeration occur? The answers to both questions relate to the level of total phenolics present.

For any variety, cultural and environmental factors influence the phenol composition of fruit at harvest. Vintage conditions and vineyard location or management primarily influence the amount of phenols. For example, at a given Brix, fruit of the same variety grown in a moderately warm region will have more (but the same mixture) anthocyanins than fruit from a cooler region. Flavonoid phenols (tannins) seem to vary more than nonflavonoids in their response to the environment.

Ripeness effects can involve qualitative differences, but are generally not very large. Overcropping can affect fruit ripening and can significantly reduce the anthocyanin content. There is a net synthesis of phenols in the grape berry over much of the period from veraison to commercial ripeness, but since it is less than the berry enlargement, the concentration tends to drop except for anthocyanins. Overripe fruit, especially if shriveling, can lose phenols including anthocyanins, perhaps by conversion to unextractable polymers and oxidation products.

Physical characteristics have a more important effect on the phenol concentration than chemical characteristics. For example, small differences in berry size may have large effects on the pulp to skin ratio. This has obvious influences on the phenol concentration in the must and subsequent wine which may be an important factor in the degree of postfermentation aeration. As indicated in a previous edition of Vintner's Corner, measuring the mean berry weight of field samples is a useful way of noting seasonal differences in phenol concentration, an important factor in stylistic winemaking decisions.

High tannin wines are often provided with air exposure 3-4 times in the first year. Depending upon the phenol load and desired style, wines are usually stored anaerobically during year 2 and beyond. Naturally, the use of controlled aeration must be appropriate to the structure and pH of the wine. In light red wines, which are low in phenolic compounds, exposure to air is generally limited. This is generally the case with Pinot Noir. In order to limit air exposure, low phenol varieties are sometimes barrelled and left sur lie to provide the wine with some autolysis products to help buffer the effects of oxygen and add palate weight.

III. Accurate Sulfur Dioxide Analysis

There is an industry-wide trend toward reducing sulfur dioxide use whenever possible. The reasons include health concerns, possible desire for a malolactic fermentation, and the enhanced suppleness of red wines that have had only limited SO2 additions. Reduction of sulfur dioxide levels is often consistent with such stylistic goals and should be used to help obtain these goals. Unfortunately, many winemakers continue to use an analytical procedure which is insufficiently accurate for stylistic wine making.

The Ripper method for sulfur dioxide, which is more than one hundred years old, uses a standard iodine to titrate the free or total SO2 in a sample. Although it is universally recognized that this method is inaccurate (particularly in red wines), the procedure is so simple that it is the most commonly employed. 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:

H2SO3 + I2 > H2SO4 + 2HI

The completion of this reaction is signaled by the presence of excess iodine in the titration flask which is complexed 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; and 3) difficulty of end point detection in red wines.

Commercial kits for conducting the analyses of sulfur dioxide by the Ripper techniques are available. These 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. With reds the problem is not simply the inability to see the end point, but also the production of a false high reading due to phenolic compounds which react with iodine. In highly pigmented wines, this inaccuracy can be quite extensive. The accuracy is compounded by the fact that different sizes and forms of the tannin-anthocyanin complex will react differently with the iodine.

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 > S3 + 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 in relatively inexpensive and the analysis is easy to perform. The AO procedure eliminates the interference from pigments and tannins. To enhance phenol polymerization in red wines, the free sulfur dioxide should be no more than 15 mg/L. This requires the use of a low sulfite producing yeast and analytical accuracy. Because of the importance of accurate analysis, I highly recommend the AO procedure for free SO2 determination. For further details regarding this procedure, see Production Wine Analysis, Zoecklein et al., 1995 or call me. If you are in the premium red wine business, you are using sulfur dioxide to help manage the evolution of phenols. Therefore, you must use the most accurate analysis possible.

IV. Oak From Forest to Glass

July 14 - 17, 1999

As a member of the 1999 Symposium Committee of the American Society for Enology and Viticulture - Eastern Section, I am pleased to announce the 1999 Symposium on Oak and annual meeting, July 14 - 17, 1999 in St. Louis, Missouri. This meeting will bring together suppliers, winemakers, and research personnel to discuss the practical applications of oak in winemaking. The event begins July 14 with a tour (for the first 200 registrants) sponsored by World Cooperage of their forest and production facilities. The program continues July 15 - 16 followed by the annual meeting of the American Society for Enology and Viticulture on the 17th.

The symposium topics discussed will include:

1. Chemistry and composition of oak wood. Characteristics of various oaks (American, French, etc.) and their sensory significance.

2. Wood transformation during natural seasoning of oak wood and its influence on wine composition and sensory qualities. Influences of leaching, chemical oxidation, hydrolysis, and chemical transformation. Microbial activity and wood maturation. Optimal conditions for wood maturation. Influence of flora on chemical composition of oak wood.

3. Effect of heating and toast levels on composition and organoleptic characteristics of wine.

4. Extraction of oak wood constituents during wine maturation and their contributions to wine aroma and flavor.

5. Barrel fermentation and maturation of red wine.

6. Barrel fermentation and maturation of white wine.

7. The physiology of sur lie.

8. Principles and practices of "sur lie" aging of white wine with emphasis on:

A.Influence of yeast fermentation on volatile oak extractives, and

B.Influence of lees contact time and stirring regime on wine quality.

9. Modern method of cooperage production

10. Microbial contamination of barrels (bacterial and Brettanomyces sp.). Rehabilitation of contaminated barrels.

11. Barrel alternatives in wine production (oak chips, oak inserts, others).

12. Barrel management in cellar

A.Buying, treating, and conditioning of new and used barrels.

B.Handling and storage of full as well as empty barrels.

C.Barrel defects, repair, and maintenance.

D.Barrel cleaning and sanitation; using ozone as a sanitizer.

E.Ullage, topping, and racking.

13. Conducting oak trials in the winery.

Further details to follow - MARK YOUR CALENDARS!