Fascinating fungi and the noble rot

The fungi are a fascinating group of living organisms. More than 100,000 species have already been identified and twice that number may exist.

One fungus, Penicillium roqueforti, was first found in caves near the French village of Roquefort. As the story goes, a piece of cheese was left in a cave and a few weeks later was found to have acquired a tart, pungently fragrant character from the fungus infection. Today, only cheeses from around these caves can use the name Roquefort. Similarly, the fungus Penicillium camemberti give Camembert cheese its unique flavor.

Another useful by-product from the fungus world is the antibiotic penicillin. An antibiotic is a substance produced by a living organism that inhibits the growth of another organism. In 1928 it was discovered that a strain of the fungus Penicillium produced a substance that halted the growth of bacteria responsible for pneumonia, scarlet fever, gonorrhea, syphilis, diphtheria and rheumatic fever.

Of particular interest to the wine-maker is the group of over 50 different types of parasitic fungi that grow on grapes producing what is commonly called bunch rot. Despite the efforts of grape growers to avoid bunch rot, moldy grapes (fungal infections) are responsible for considerable crop loss each year.

Bunch rot organisms emit unpleasant odors as they grow. Sour rot, a bacterial/bunch rot complex produces acetic acid which imparts a vinegary smell. Aspergillis growth gives off a "smelly-sock" odor while Penicillium generates a rotten smell. Moldy grapes readily transmit their odors which are described as "earthy", "dirty", "stinky", or "funky" and such grapes cannot be used to make salable wines.

These three common bunch rots are called secondary pathogens because they can grow only where a crack or break already exists in a grape such as from insect or bird damage or where grapes split from growing too closely together in a cluster.

While bunch rot is the bane of the grape grower's existence, there is one fungus, Botrytis cinerea, which certain winemakers depend on to make some of the most delicious, sought-after and expensive desert wines in the world.

The fascinating aspect of Botrytis is that under normal circumstances it is considered a lowly bunch rot and is responsible for annual crop losses of 110% in California. Growers specially trellis their vines and apply anti-fungal spray frequently to guard against Botrytis bunch rot. However, under the proper set of climatic conditions this fungus is responsible for the phenomenon known as "noble rot" which transforms the plump, healthy grape into a shriveled, fuzzy mass--the raw material for the decadently sweet Sauternes of France, the exotic and rich Trockenbeerenauslese (TBA) and Beerenauslese (BA) of Germany and the honeyed and nectar-like dessert wines of California.

Each region grows its own traditional grape varieties: Semillon and Sauvignon blanc in Sauternes, Johannisberg Riesling in Germany and in California, which is less bound by tradition, all of the above including Gewurztraminer.

These varieties are more susceptible to bunch rot for two reasons: Either they have thin skins which are more easily pierced by the fungus or the grapes grow tightly packed together in the clusters causing poor air circulation between berries. Merlot and Cabernet Sauvignon, on the other hand, are thick-skinned, loose-clustered varieties and are therefore less susceptible to bunch rot.

Botrytis cinerea is a primary pathogen because it can attack undamaged fruit. The Botrytis spores are present in the vineyard throughout the entire year but remain dormant until the proper weather conditions prevail; a combination of cool temperatures and high humidity.

Humidity can come from rain or fog. A common scenario is for morning fog from a nearby body of water to settle on the grape surface initiating an infection. Since Botrytis is a cool weather fungus, the grower hopes that the temperatures remain in the narrow range which permits a clean Botrytis infection unaccompanied by other molds. The fungus perforates the skin of the berries leaving the pulp open to the air.

Extremely hot weather (more than 90 [degrees] F) will dry up the infection altogether and cause raisining. Continued wet weather or high humidity following the initial Botrytis infection results in a mixed bag of fungal infections without the simultaneous dehydration of the fruit.

Under ideal conditions, the afternoon sun dries up the moisture halting the growth of Botrytis and causing desiccation as water evaporates from the grape pulp, now exposed to the air.

After this cycle of morning sun and afternoon fog continues for 3-10 days, the grapes dry and shrivel into what resemble raisins. The single berries affected with the noble rot are harvested from the cluster individually, hence the terms special select or individually selected late harvest. If temperature and humidity continue to be favorable, further noble rot occurs and successive harvests are made.

Dirk Hampson, winemaker at Dolce, a winery founded by Far Niente in Oakville, Calif. produces a very fine late harvest Semillon/Sauvignon blanc blend. Hampson started out in 1985 making six barrels for fun only and now produces 65 barrels of Dolce which is sold throughout the United States. He explains, "On average, we pick a vineyard five to six times, picking clusters, parts of clusters or single berries. The selection process is perhaps the most important step because when the conditions are right for Botrytis they are also right for other undesirable bunch rots."

Not only must specific weather conditions prevail but they must do so at the right time of the season. Jim Klein, who makes late harvest Riesling at Navarro Vineyards located in California's Anderson Valley says, "The timing of the rain is critical. The rain must arrive when the grapes are at normal maturity of 22-25% sugar. Below 20% there is too much acidity in the grape. In 1992 the rain came when the grapes were at 19% sugar so the Botrytis just concentrated acid not sugar. Above 22%, you get a nice concentration of sugar."

Of the 20,000 cases of wine Navarro produces annually, a maximum of 4,000 cases will be late harvest dessert wine. "Our location in northwest Mendocino County just six miles from the Pacific Ocean provides the moisture and cool temperatures which Botrytis likes. When the conditions are right, the Botrytis just takes off and the wines almost make themselves."

When the right combination of relative humidity and temperature do not exist can you create them artificially? Klein explains, "It doesn't do any good to force the issue. We have tried overhead sprinklers and spraying Botrytis spores directly onto the clusters. We got black rot and other bunch rots but no Botrytis growth." It appears you can't fool mother nature.

Don Van Staaveren, the Winemaster at Chateau St. Jean in Kenwood, Calif. has been up to his elbows in Botrytis since 1974 when he began making dessert wines from Riesling with Richard Arrowood. Van Staaveren comments, "Alexander Valley is not a cool growing area and in theory Botrytis should not be found here. But I feel that the humidity level inside the canopy is more important. Three of our Botrytis vineyards, Hoot Owl Creek, Belle Terre and Frank Johnson all get fog from the Russian River providing plenty of humidity."

Van Staaveren agrees with Jim Klein about the importance of the onset of the Botrytis infection with regard to maturity levels adding, "It is critical that the infection occurs near maturity with respect to sugar and acidity. Wines made from early Botrytis are sour because the balance isn't correct inside the berry. At 20-23% sugar, the Botrytis has ripe fruit flavors to work on and concentrate. Also, the infection must be clean Botrytis because other bunch rots have a mustiness or moldiness like a stinky piece of cheese."

Hampson also agrees, "I am a proponent of getting ripeness before Botrytis. Botrytis flavors are best when they are in harmony with ripe fruit flavors."

Chateau St. Jean makes late harvest wines every year. "We start out the year intending to make some. We give Botrytis every opportunity to happen. We evaluate its progress during the summer and when we find a point of infection we try to promote it by increasing irrigation, eliminating fungicide spray and leaf removal and hope for the best."

Chateau St. Jean is committed to German-style late harvest wine production. Van Staaveren adds with excitement, "We are the only winery in California actually planting a site for Riesling. We have picked the clone of Riesling and the trellis system. The vineyard is adjacent to the Russian River and is surrounded by trees. The river gives us the humidity which stays in the vineyard because the trees restrict air movement."

Navarro has also planted an acre of Riesling solely dedicated to the production of late harvest Botrytis wines. "We intend to plant an additional ten acres over the next three years experimenting with three different clones", chimes in Klein.

But the number of acres planted to Riesling has fallen off, and therefore, so has the production of the late harvest botrytized versions. Today, Riesling costs about $650/ton and the wines sell for $7-$9 per bottle. Because of the greater return (it costs nearly the same to farm an acre of grapes regardless of the variety) growers have been steadily pulling out Riesling and replanting with Merlot, Chardonnay and Cabernet Sauvignon which fetch $1,500/ton and sell for $15-$20 per bottle.

Craig Williams, Joseph Phelps' veteran winemaker recalls the heady days of the 1970s when Riesling was king. "When I left U.C., Davis in 1975, everyone made a Riesling. It was the most expensive grape at $1,000/ton; Chardonnay was $600 to $700/ton."

Williams has resisted the temptation to pull out the Riesling. "I thoroughly enjoy making the late harvest wines. We made our first Botrytis wine from the Stanton Vineyard in Yountville in 1975. At that time Walter Schug at Joseph Phelps, Dick Arrowood at Chateau St. Jean and Jerry Luper and Brad Webb at Freemark Abbey were the leaders in drawing attention to what had heretofore been considered a rot and dumped back into the vineyard. It was their creativeness and vision to take these grapes and make what is now the industry standard for dessert wine."

Yountville's proximity to San Francisco Bay provides both the cool temperatures and high humidity necessary for Botrytis. What is unique to California is that it is dry in terms of rainfall but certain locations in the Napa, Sonoma and Mendocino counties get fog and cloud cover until 11:00 to noon due to the marine influence. This fog, thick enough to be mist, deposits on the grapes stimulating Botrytis growth.

Like Dirk Hampson at Dolce, David Ramey, winemaker at Chalk Hill Winery in Sonoma County, also produces a Sauternes or Barsac-style dessert wine from a blend of Botrytis-affected Semillon and Sauvignon blanc. Ramey, a lover of French winemaking traditions and practices comments, "Semillon-based wines have a distinct structure and balance. The classical constructs in France dictate harvest sugar of 30-34% yielding wines of 14% alcohol and acidity of 7-8 grams/liter. If harvest sugar exceeds these levels the yeast are unable to produce enough alcohol to give the power and structure expected of a Sauternes-style wine. If you're working with Riesling (alcohol levels of 7-9%), there is no upper limit to harvest sugar which suits the California attitude of 'if a little is good then more is better.'"

Preferences aside, there are fundamental differences between dessert wines made from Riesling and those produced from Semillon and Sauvignon blanc. Riesling is high in a class of highly aromatic compounds called terpenes which give Piesling its flowery and honeysuckle scents. Semillon is more neutral, says Ramey. "Just as Chardonnay is a blank pallet for oak and malo-lactic, Semillon by itself is kind of bland and is a blank pallet for Botrytis giving the Semillon-based dessert wines aromas of peaches and apricots as the fungus concentrates the grape's contents."

Hampson's motivation for Dolce, which is 60-75% Semillon and the remainder Sauvignon blanc was simple, "I chose the Sauternes style because I considered it the most challenging and therefore the most fun." He defends his position by saying, "The German-style wines are bottled 3-6 months after harvest reflecting the fruit of that year versus Sauternes which is more a reflection of wine making influences."

Dolce follows the time-honored wine making practices of Chateau d'Y Quem, the world's most famous and expensive dessert wine. "We age Dolce in 100% new oak barrels for three years and use the tightest grained oak available."

One would think that due to its viscosity, a late harvest wine would not be prone to leakage and would not require tight grained oak such as Alliers, Vosges or Nevers. "But quite the opposite is true", says Hampson. "It appears that the high sugar concentration counteracts the ability of the alcohol in the wine to swell and block the pores of the oak. Dolce is the hardest wine we have to keep from leaking during barrel aging."

The trademark thickness of these wines results from the high sugar concentration as well as the fungus' production of large quantities of glycerin. This viscosity not only adds to the exquisite beauty of late harvest wines but also causes some of the many difficulties associated with their handling and vinification. Besides attracting bees, making harvesting hazardous, the high sugar musts provide an adverse environment for the yeast to grow in. Fermentation may be difficult to start and may require several months to complete compared to a week for table wine fermentation. The finished wines are so thick that they are difficult to pump and clog conventional filters.

Another attribute of these late harvest wines is their dark color. Botrytis produces an enzyme that causes browning giving the wines their straw to yellow color which intensifies to a golden hue as they age.

Dessert wines made from botry-tized grapes have considerably greater aging potential than table wines. Like Madeira and Port, the high sugar concentration preserves the wine against microbial growth as it ages in the bottle or after it is opened.

The aromas by themselves are seductive and the taster may find himself deriving great pleasure from simply smelling the rich apricot and honeysuckle aromas of a late harvest Johannisberg Riesling. The intense sweetness followed by the scintillating acidity makes the wine drinker's mouth gush with pleasure. Why else have these wines been given the accolade, "nectar of the gods?"

(Ross, a graduate in fermentation science from U.C., Davis, represents Chateau St. Jean in the Eastern United States).

COPYRIGHT 1994 Hiaring Company
COPYRIGHT 2004 Gale Group