Vol. 12, No. 5 September - October, 1997

Bruce Zoecklein

Department of Food Science and Technology

VPI & SU

Blacksburg, VA 24061-0418

I. Managing Red Wine Phenols

Nature of Grape Phenols 2

Skin, Seed and Cell Maturity 2

Maceration Enzymes 2

Maceration Period 3

Role of Polysaccharides 3

Evolution of Phenols 5

Understanding the quantitative and qualitative influences of grape phenols on wine quality is of great importance to premium red winemakers. Phenols are grape metabolites composed of many types of compounds with different chemical structures and sensory properties. Table 1 list some of the sensory effects of wine phenols.

Dr. Delteil (head of the research and development department at the ICV, Montpellier, France) suggests that a red wine with a positive phenol profile has good body, smooth tannins and a low perception of astringency, bitterness and dry or dusty tannins. At a recent symposium on tannin management co-sponsored by Lallemand, Scott Labs and Vinquiry, Dr. Delteil also suggested that a positive phenol profile results from the interaction among tannins, anthocyanins and certain polysaccharides. This issue explores some thoughts and considerations regarding phenol management.

Table 1. Sensory Effects of Phenolics in Wines

Nature of Grape Phenols

Reactions involved in the evolution of grape phenolics are listed in Figure 1 (adapted from Ribereau-Gayon and Glories, 1986). Procyanidins are fundamental phenol compounds, their oxidation and condensation in association with other molecules produces tannins (T) which provide maximal astringency. Procyanidins may also undergo non-oxidative polymerization producing condensed tannins (TC) which have diminished astringency. Further polymerization leads to highly condensed polymerized tannins (TtC) which precipitate when they become sufficiently large. Condensation and polymerization reactions also involve other molecules such as polysaccharides and peptides, which appear to inactivate the astringency of tannins by forming a condensation product (TP) that may enhance suppleness. It appears that this association of tannin phenols and polysaccharides may be quite important in creating a supple wine (see below). Additionally, reactions occur between anthocyanin and colorless procyanidin molecules to produce a tannin-anthocyanin complex (T-A). Work conducted by Dr. Roger Boultons' group at UC Davis has demonstrated the importance of the extraction colorless phenols. This extraction increases anthocyanin stability by a process referred to as co-pigmentation.

Skin, Seed and Cell Maturity

Evaluations of red fruit maturity should include an assessment of skin, seed and cell wall maturities (Delteil, 1997). As grapes mature, skin astringency decreases along with the drop in acidity corresponding to increased suppleness. Tannins evolve from astringent-to-dusty-to soft or supple. As discussed often in this series, red wine producers should base their harvest dates, at least in part, on the tactile response to skin tannins, looking for mature, 'ripe' tannins which develop just before over-ripeness. Polymerized skin tannins have a large molecular weight and are smoother on the palate than smaller low molecule weight tannins - considered to be 'hard' and astringent. Seed maturity may also be an important red wine consideration. Vol. 9 No. 3 (1994) of this series showed data from my laboratory on the relationship between maturity, total seed phenols and seed tannins in Virginia grown Cabernet Sauvignon. Immature seeds (green seed) contain harsh phenols which may be extracted during fermentation. Cell wall maturity refers, in part, to berry softness and it's role in red wine production (discussed in Vol. 6, No. 4, 1991).

It is important to note that the total phenol concentration of the grape may double from one season to the next, a result in quantitive differences and berry size. Such changes have a major effect on red wine style, an important reason for measuring mean berry weights as an index of 'oenologically active' phenols Somers 1986.

Macerating Enzymes

The importance of cold-coaking to color stability has been discussed (Vol. 9 No. 4, 1994). Many use macerating enzymes to aid extraction. In most of the world, macerating enzymes are rapidly replacing traditional pectinases used in winemaking. These enzymes typically contain pectinases, cellulase, hemicellulase and other carbohydrate activities. They improve juice yields by degrading structural polysaccharides that interfere with juice extraction, clarification, and filtration. Cellulases and hemicellulases degrade cell wall polysaccharides, breaking down and causing solubilization of the middle lamella. The reported benefits of these enzymes include improved body, mouth-feel and structure as a result of the 10-30% increase of condensed tannins and tannins bound to polysaccharides. These are water soluble compounds not easily extracted during fermentation.

Maceration Period

The importance of gentle fruit handling to reduce the nonsoluble solids levels in the must has been discussed in several previous editions(Vol. 6, No. 4, 1991, Vol. 8, No. 4, 1993, Vol. 9, No. 4, 1994). Another important factor influencing color and style is the duration of the cuvaison. Maximum color extraction is reached about half-way through fermentation because anthocyanins are easily extracted (Ber and Akayoshi 1956). Other phenols such as tannins, on the other hand, are extracted more slowly. This is the principal reason why traditional dejuicing prior to dryness, particularly with mature fruit, produces wines with good initial color, little astringency, low total phenols and which are generally floral, light in body, complexity and depth. In many regions early dejuicing to allow the fermentation to complete in tanks has been replaced by dejuicing and barrel fermentation (the so-called Australian red method-see Vol. 9, No. 4, 1994).

Extended cuvaison effects the evaluation of tannins, and creates more body, complexity, depth of character and enhanced color stability. In contrast to anthocyanins, tannin phenols are extracted throughout the skin-contact period. This extraction has a significant effect on color and, particularly, color stability. Tannins derived from extended skin-contact appear to stabilize anthocyanins by combining to form larger polymeric complexes with pigments (Figure 1). This pigment-tannin complex formation and its effect on color stabilization is important for grapes with limited color such as Pinot Noir, but is relevant for most varieties.

Extended cuvaison affects the evaluation of tannins, creating more complex wines as a result of phenol condensation and polymerization reactions. As the duration of maceration increases, so does the extraction and polymerization of phenols to form tannins with higher molecular weights. This is reflected in the sensory analysis of the wines, that have less, yet stable color, less bitterness, and more, yet 'softer' tannins. The increased extraction and the polymerization of phenols may allow wines to be produced which are 'round' and 'firm' in the palate, often with considerable ageing potential. Maceration is frequently extended by filling the tank completely or by gassing the head-space of a partially full tank with CO₂. The length of maceration is determined by a host of factors including fruit maturity (perceived presence of 'ripe' tannins), age of the vine, source of fruit, history of wines produced from a particular vineyard block, stylistic goals, and of course, taste.

Unripe fruit with 'immature' tannins will be unlikely to benefit from extended skin-contact. There is an increasing awareness that some vineyards benefit from extended maceration, while others do not, probably because of qualitative and quantitative differences in grape phenols. Maceration does not have a pre-determined duration, but is generally determined by tasting a sample of the young wine from the racking valve. Red wines are dejuiced when the tannins start to soften and are said to be 'well behaved', not less structured, less 'green' and 'raw'. For Californian Cabernets maceration can last for 6 weeks.

Role of Polysaccharides

Wine polysaccharides can be divided into two main groups, those extracted from the fruit and those released by yeast and bacteria during processing. The first group includes pectins and glucans. As stated above, the use of macerating enzymes increase the extraction of tannins bound to polysaccharides which are the most supple (Figure 1). Release of polysaccharides during fermentation parallels yeast population growth, with significant differences among yeast strains reported. As yeast contact time increases the polysaccharide concentration increases. Lees contact, as occurs with Australian red procedure, extended maceration or storage sur lie may be an important feature influencing tannin suppleness.

Storage of wine in contact with yeast lees is a common method used to alter wine aroma and flavor of certain white wines and for the production of method Champenoise. The practice of storing red wines on the lees (usually light lees) is gaining interest. Sur lie results in the loss of intercellular organization and the release of cellular proteins, lipids, nucleic acids and polysaccharides into the wine. Liberated components such as amino acids may undergo secondary reactions influencing wine aroma. Dubourdieu et al (1988) reported that B-glucosidases are excreted in to media as a result of yeast autolysis. The true affect of this on wine aroma and flavor is unknown. However we have been evaluating the influence of sur lie on grape derived secondary metabolites, specifically glycosides.

Figure 1.

Table 2 shows the effect of sur lie on grape glycosides, a reduction signifies hydrolysis and the liberation of free aroma volatiles into the wine (Zoecklein et al 1997). Storage of red wines on the fine lees during barrel aging may result in improved body (as occurs with white wines) and the 'rounding' of tannins. Table 2. Effect of Four Strains of Saccharomyces cerevisiae the Glycoside Content (µmol) of White Riesling Wines immediately following fermentation and storage on heavy gross lees.

Table 2

Evolution of Phenols

Reactions involving wine phenols during ageing are governed by:

1. The nature and concentration of different phenols

2. Temperature

3. The pressure of oxygen

4. The presence of lees

5. The bisulphite concentration.

Controlled aeration of red wines is a production tool to help evolve and soften tannins, and to help lightly structured wines by providing body and aiding in flavor development. Controlled aeration increases the rate of reaction between coloring matter and tannins, resulting in condensation and polymerization reactions that allow wines to mature as quickly as possible in the barrel. It is believed that maximum color and color stability can be enhanced through such controlled oxidation. Judgements are based on cultivar, style, phenol content and pH, with air exposure accomplished by controlled aeration during tank racking, barrel-to-barrel racking and/or ageing of barrels with the bung in the upright position. Aeration is most effective in the months following the fermentation and usually after the malolactic 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. Aeration too late in the development of a wine can cause precipitation (Dournel 1985).

Sulphur dioxide is not frequently added to many reds until after the malolactic fermentation is complete. Sulphur dioxide can slow or inhibit the formation of anthocyanin-tannin complexes by binding with acetaldehydes. Free sulphur dioxide levels should be kept below 20 mg/L to help facilitate this development. Note-if you are measuring sulfur dioxide using the Ripper titration vs aeration oxidation method you cannot achieve the needed accuracy to help you obtain your stylistic goals (see Vol. 12, No. 2, 1997, Vol. 9, No. 6, 1994).

The goals of premium red wine producers should be good body, balance, smooth, supple tannins with no bitterness and a relatively low perception of astrin- gency and dryness. Quality wines have a fine balance which allows for flavor development. An imbalance of tannins, acid or alcohol can both conceal and distract from flavor (Long, 1997). Supple, balanced wines provide winemakers and more importantly consumers with complete flexibility-a wine for drinking or for keeping.

REFERENCES

Berg, H.W. and M. Akiyoshi, (1956). The effect of contact time of juice with pomace on the colour and tannin content of red wines. Am. J. Enol. Vitic. 7, 84-90.

Delteil, D. (1997). Managing tannin management. Vinquiry, No. 15.

Dournel, J.M. (1985). Recherches sur les combinaisons anthocyanes - flavanols. Influence de ces reactions sur la couleur d'un vin rouge. Bordeaux: Universite de Bourdeaux II. These Doctorat oenologie.

Long, Z. (1997). Developing wine flavor in the vineyard. Practical Winery and Vineyard. July/Aug.

Ribereau-Gayon, P. And Y. Glories. (1986). Phenolics in grapes and wines. In: Proc. of the Sixth Australian Wine Industry Technical Conference. T. Lee (Ed). Adelaide, South Australia. AIP, Adelaide, South Australia, AIP, Adelaide. pp. 247-56.

Somers, T.C. (1986) Assessment of phenolic components in viticulture and enology. In: Proc. of the Sixth Australian Wine Industry Technical Conference. T. Lee (Ed). Adelaide, South Australia, AIP, Adelaide. pp. 257-60.

Zoecklein, B.W., J.E. Marcy, and Y. Jasinski. (1997). Effect of fermentation, storage sur lie or post-fermentation thermal processing on White Riesling glycoconyugates. Am. J. Enol. Vitic. Vol. 48, No. 4. In press.