Vintner's Corner

Vol.15, No. 5 November - December, 2000


Bruce W. Zoecklein

Department of Food Science and Technology

VPI & SU - 0418

Blacksburg, VA 24061


Web site:

Table of Contents

I. Winery Planning and Design Workshop 1

II. Web Address 2

III. Effects of Fruit Rot on Wine Stability 2

I. Winery Planning and Design Workshop

I have organized a one-day workshop, Winery Planning and Design for table wine production scheduled for February 14, 2001, at the Omni Hotel in Charlottesville, VA. This program is for those seriously interested in entering the commercial wine industry. The workshop will include discussions and printed materials on winery planning, economics, winery design and space allocations, processing equipment, federal, state and county compliance issues, etc. This event is being sponsored by the Enology-Grape Chemistry Group, Virginia Tech and by the Virginia Wine Marketing Office.

The course will involve the following:

Design areas:

raw production handling


barrel fermentation/storage






Refrigeration , temperature control,


Sanitation, steam and hot water



Electrical requirements

Water and waste water requirements

Gravity flow considerations


Expansion considerations



County zoning regulations

The course fee is $75 payable to Bruce Zoecklein Foundation Account, Virginia Tech, mailed to address below. Pre-registration is required and enrollment capacity is limited. The registration deadline is January 31. Additional information is available by contacting Terry Rakestraw, Department of Food Science and Technology, Virginia Tech - 0418, Blacksburg VA 24061. Phone 540-231-6805. E-mail: Please help spread the word on this event.

II. Web Address

As indicated in previous communications, there have been some difficulties in accessing the Enology-Grape Chemistry Group web site. Please note that we have modified the address in an effort to alleviate problems. THE NEW WEB ADDRESS:

III. Effects of Fruit Rot on Wine Stability

Controlling the incidence of fungal degradation from Botrytis cinerea and sour rot was particularly difficult this year with our early whites. This season, once again, highlighted the need for fruit inspection and fruit culling at the winery.

Both Botrytis cinerea and sour rot have significant influence on wine chemistry (Table 1). The largest quantitative changes occurring in the fruit as a result of Botrytis growth are those of sugars and organic acids.

TABLE 1. Comparison Between Virginia White Riesling Musts

'Clean Grapes' Botrytis cinerea Sour Rot
°Brix 18.5 21 16.0
Titratable Acidity (g/L) 8.0 6.5 5.0
pH 3.3 3.5 >3.4
Gluconic acid (g/L) 0.5 1-5 0.5
Acetic acid (g/L) 0 1.1 1.5
Glycerol (g/L) trace 1-10 trace
Ethanol (%, v/v) 0 0-trace 0.2%
Laccase (g/mL) trace 0.1-8 trace to 0.5
Glucan (mg/L) 0 247 65

Botrytis and sour rot use ammonia nitrogen, reducing the levels available for yeast metabolism. Additionally, thiamine (vitamin B1) and pyridoxine (vitamin B6) are depleted. This is a primary reason why I have suggested that wines produced from Botrytis and/or sour rot infected grapes generally require supplementation with nitrogen and vitamins to help avoid stuck fermentations and possible H2S/mercaptan formation.

Like other fungi, Botrytis cinerea produces laccase, which catalyzes phenolic oxidation. This coupled with loss of varietal aroma is a major problem with fruit rots. The main nonflavonoid phenolic compounds of grapes are caffeic and p-coumaric acids, both free and esterified with tartaric acid. These are transformed to quinones by laccase, with resultant polymerization responsible for browning of the fruit. Excessive browning and the concentration of laccase can generally be limited by whole cluster pressing, the elimination of the first 10-15 gallons/ton and the use of PVPP. Laccase is resistant to sulfur dioxide, cannot easily be removed with bentonite, and is active in the presence of alcohol. Therefore, the reduction pre-fermentation is important.

Elevated levels of acetic and lactic acid are frequently seen in wines made from Botrytis-infected fruit. These spoilage acids arise from growth of yeast and bacteria associated with the mold. Aspergillus, Botrytis, and Penicillium sp. oxidize glucose to produce gluconic acid. Since gluconic acid is not utilized by yeast or bacteria it may be used as an indicator of fruit deterioration. Gluconic acid levels in "clean" fruit and in wines made from clean fruit are near 0.5 g/L, whereas in wines produced from fruit infected with B. cinerea levels range from 1 to 5 g/L. In the case of sour rot or vulgar rot, where bacterial growth occurs along with the mold growth, levels may also reach 5 g/L.

Polysaccharide Instability

One of the greatest impacts of Botrytis growth and sour rot is the formation of polysaccharides that create clarification problems. Pectins are hydrolyzed by mold-produced polygalacturonase, with the formation of beta-1,2- and 1,6-glucans. In wine, ethyl alcohol causes the glucan chains to aggregate, thus inhibiting clarification and filtration. Commercially, several glucanases are available to minimize these clarification problems.

Polysaccharides can form protective colloids in juices and wines inhibiting clarification, fining, and filtration. In grape juices and wines polysaccharides may be in the form of pectins and/or glucans, each forming gelatinous aggregates in an alcohol solution. Several Virginia producers have reported polysaccharide instabilities this season. The following, adapted from Zoecklein et al. (1995), are two simple lab procedures for determining pectin and glucan instability.

Pectin Instability

Pectins are structural components of plant cell walls. If pectins are present, the addition of pectolytic enzymes to a laboratory sample and subsequent pectin precipitation test is recommended.

Procedure: To a 25-mL aliquot of the wine containing unidentified haze, add 50 mL of a 95% ethanol: 1% HCl or alternatively, isoproanol: 1% HCl reagent.

Interpretation: Formation of gel after several minutes is indicative of pectin.

Glucan Instability

Dubourdieu et al. (1981) developed two precipitation tests for glucans. The first procedure given is for the presence of glucans in concentrations greater than 15 mg/L, the second for levels as low as 3 mg/L. Even at low concentrations, glucans can cause filtration problems. A positive test for the presence of glucans should be followed by a laboratory fining trial using glucanases and retesting.

Procedure for Glucans > 15 mg/L: Add 5 mL of 96% ethanol (vol/vol) acidulated with 1% HCl to a tube containing 10 mL of juice or wine.

Interpretation: The formation of a white filament is indicative of the presence of glucans at levels greater than 15 mg/L. Because much lower levels can cause problems, an additional test that will detect glucans at concentrations above 3 mg/L may be warranted.

Procedure for Glucans > 3 mg/L:

Interpretation: The formation of filaments is indicative of glucans.

IV. Virginia Vineyards Association Meeting.

The 2001 VVA and State Horticultural Society annual meeting will be held January 22-24, in Williamsburg, VA. See Tony Wolf's newsletter for details.