A discussion of Sweet wines from Thomas Henick-Kling

Introduction

For the production of sweet wines it is essential that vineyard site, cultivar selection, vineyard management, grape maturity and harvest, juice extraction, and alcoholic fermentation are considered as one intimately related sequence of events. These special sweet wines presented here are not just sweet, they can not be produced simply by the addition of sugar to any grape must (or wine) or regular maturity. These special wines, the stickies, have unique, highly concentrated flavors which are derived from the particular concentration process the grapes underwent and from the fermentation of the musts of very high sugar content.

Sweet wines have a very long history. The wines of the ancients in the Near East, Egypt, Greece, were almost certainly made sweet.

Greek low trained vines and the hot Mediterranean climate were able to yield grapes high in sugar content.

The Romans initially grew vines up trees which allowed only low maturity and low sugar content. They concentrated grape juice by boiling. They also encourage oxidation of their wines to enhance their stability.

MAKING IT SWEET

NEW TECHNOLOGY

• stop fermentation before it completes to dryness (SO2, centrifugation, filtration)
  - add juice back to fermented wine, prevent refermentation by sterile filtration or pasteurization, and SO2
• fortification (adding ethanol to stop fermentation and to stabilize the sweet wine)
• cryoconcentration, reverse osmosis

• must of sugar content higher than 30% (30 Brix) so that the yeast could not ferment all the sugar (13-14% alcohol)
• drying of grapes
• infection by Botrytis cinerea
• chaptalization (leads to unbalanced wines because acidity is not increased)
• boiling juice to concentrate it
• Eiswein

BOTRYTIZED WINES

Wines made from grapes infected by the mold Botrytis cinerea. This fungus can cause the destructive bunch rot and under special conditions 'noble rot', 'pourriture noble', 'Edelfäule'.

The best made of these wines can live (improve) for 10-100 years

BOTRYTIS INFECTION

• Inoculum for infection: spores produced on fungal tissue that overwintered in the vineyard and spores from sclerotia (resting structures).
• Infections in aborted and senescing flower parts. Especially after rainy or long, cool bloom periods. Infected flower parts within developing clusters may initiate fruit infection later in the season.

bunch rot versus noble rot
susceptibility: skin toughness, open or tight clusters, prolonged wet periods

high humidity and berry splitting encourage bunch rot

noble rot: fluctuating humidity late in the season (humid nights, dry days) these conditions permit fungal infection but limit fungal growth and formation of new spores (conidia)

Hydrolytic enzymes produced by Botrytis cinerea, pectinases attacking the berry cell wall causing death and collapse of the infected tissue. Under dry conditions these berries dry out.

bunch rot (sour rot): if this drying out does not happen, other molds, yeast, and vinegar bacteria invade and bunch rot develops. Other fungi such as Penicillium, Aspergillus, Mucor, cause off-flavors (bitter, straw, mushrooms, plastic, moldy).

Changes in juice composition due to Botrytis infection

• Juice concentration due to evaporation.
• Acetic acid bacteria (Acetobacter, Gluconobacter) produce acetic acid, gluconic acid (no sensory significance), acetic acid & ethyl acetate (strong off-odor).
• Due to mold infection: increase in sugar content (even though 35-45% of the sugar is metabolized by the mold), increase in osmotic potential stops metabolism by the mold
• dehydration concentrates acidity along with sugar, keeping a balance of taste
• decrease of ammonia & thiamine content (fermentation difficulties)
• production of glycerol and other polyols: enhanced viscosity, sweetness
• loss in varietal aroma (hydrolysis of terpenes) esterases degrade esters that give wines fruity aromas
• Botrytis synthesizes sotolon (sweet aroma contributes to the distinctive botrytis aroma, honey-like), mushroom aroma, 1-octen-3-ol, derivatives of grape terpenes

Botrytis infection affects fermentation by:

• providing an environment for growth more yeasts and different population composition
• high sugar content (low osmotic potential) and fatty acids produced by molds inhibit growth and fermentation activity by yeasts
• sulfur dioxide is added to avoid development of lactic acid bacteria. Malolactic fermentation undesirable because it would lower the acidity (balance), increase acetic acid content in high sugar content.
• laccases produced by Botrytis will oxidize a wide range of important grape phenols (anthocyanins, tannins, phenols). These laccases are active in wine at low pH and in presence of ethanol affecting golden color in white wines and loss of color in red wines. Heat and large amounts of SO2 (>50 mg/L) will inactivate these enzymes
• polysaccharides produced by Botrytis increases viscosity and makes such wines difficult to filter

HUNGARY

• Tokaji Aszú : made from regular juice (non-botrytis infected) mixed with 'paste' of pulverized botrytized grapes, different amounts of Aszœ paste added, alcohol content up to 14%<
• Tokaji Aszú Eszencia
• Tokaji Eszencia: made from juice that spontaneously seeps from botrytized grapes (free-run), up to 50% sugar, 5-7% alcohol, up to 20 years of barrel maturation
• grape variety: Furmint and Hárslevelü

GERMANY

• Spätlese light Botrytis infection
• Auslese, Beerenauslese, Trockenbeerenauslese heavily Botrytis infected (sometimes just dried, dehydrated, berries without B-infection) typically 6-8% alcohol, 10 g/L acidity, structure determined by sugar and acid
• grape variety: Riesling, Sylvaner, Müller-Thurgau, Scheurebe

ALSACE

• Vendange Tardive, SŽlection des grains nobles
• grape variety: Riesling, Gewürztraminer, Muscat, Sylvaner, Pinot Gris

LOIRE

• grape variety: Chenin Blanc, Sauvignon Blanc, Pinot Gris, Chardonnay, others

BORDEAUX

• Sauternes generally more than 11% alcohol (14%, 10 g/L RS, 6 g/L TA)
• grape variety: Semillion, Sauvignon Blanc

New York State, Ontario (Canada)

• Excellent botrytized wines from Riesling and Vidal, mainly non-botrytized 'Trockenbeeren' styles from Vignoles (Ravat Blanc)

INDUCED BOTRYTIZATION

Harvested grapes are infected with Botrytis spores in climatized rooms (has been used in California and Australia)

NON-BOTRYTIZED STICKIES

• drying
  drying on the vine: Constantia (South Africa)
  tradition in Southern Europe: Moscato (Sicily), Vin Santo
  Recioto (Veneto, Italy) Recioto Valpolicella, sweet red wines, some Botrytis infection (amarone: dry; amabile: sweet)
• heating
  concentration of juice by heating or boiling: Madeira
• freezing, cryoextraction, reverse osmosis
  Eiswein (ice wine): over-ripe grapes frozen in the vineyard
  ice wine: might be produced from normal maturity grapes by freezing and reverse osmosis

FORTIFIED WINES: PORT, SHERRY, MADEIRA
fortified wine (18-20% alcohol): addition of grape spirit (wine distillate) stops fermentation, preserves sweetness

LIQUOR MUSCAT
fortified Muscat juice left to concentrate in barrels at high ambient temperature

SHERRY
Spanish Sherry: grapes: Palomino and Pedro Ximenez
Yet any grape variety can be (and is) used. The Sherry making process obscures grape varietal characteristics. The flavors produced by the oxidative handling and the Sherry yeast are dominant.

Flor Sherry
Grapes harvested at 18-20 ¡Brix, pH 3.0-3.4. Initially fortified to approx. 15% alcohol. Then filled into barrels which are left with large air space (ullage). Flor yeast is added. This yeast (a type of Saccharomyces cerevisiae) form a thick mat on the surface, producing the characteristic fermentation and oxidized flavors (acetaldehyde 150-300 mg/L). Later the wine is transferred to other oak casks (filled completely) and left for further maturation.

Other Methods:
submerged, aerated culture; column Flor growth

PORT
A large variety of grapes are used (including some white grapes). The high sugar content (26-40 Brix) of the grapes typically used for Port production requires a osmotolerant and alcohol tolerant yeast to start fermentation. Grape spirit added at 14-18 ¡Brix, fermentation slowly stops, residual sugar in Australian Port wines 7-12 Brix, Portuguese Ports: 6-8 Brix, 17-19% alcohol.

Vintage and Tawny Port styles.
Vintage Port is fruitier, darker color, more tannic, less oak aged, all one vintage.
Tawny Port is lighter in color, longer oak aged, blend of different years to maintain a consistent style.