No. 4 July - August, 2003
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
Web site: http://www.vtwines.info/
Table of Contents
|ICV Mouthfeel Sensory Evaluation Protocol||4|
I. Closure Review
At the recently concluded Closure Symposium, associated with the annual American Society for Enology and Viticulture, eastern section meeting, I presented commercial wines bottled with natural cork versus synthetic closures.
Identical wines were bottled using two different closure types. Prior to the sensory evaluation, we quantified 27 head-space volatile compounds in each wine, using microextraction solid-phase gas chromatography-mass spectrometry (as described by Whiton and Zoecklein, 2000). The wines evaluated were from two Virginia producers, Horton Cellars and Jefferson Vineyards.
The aroma components of grapes and wines consist of hundreds of individual constituents, representing a number of chemical functional groups. However, the most important fermentation-derived volatiles can be grouped into three main chemical classes. Ethyl esters of medium-chain fatty acids (ethyl butyrate, hexanoate, octanoate, decanoate, and dodecanoate) represent compounds which are fruity, wine-like, and contribute to wine odor. Acetate esters, such as isoamyl and hexyl acetate, are responsible for the tropical fruit and banana-like notes in some wines. The third group of chemical compounds we evaluated were the higher alcohols, which include compounds such as isobutanol, isoamyl alcohol, and hexanol. These compounds are unpleasant when isolated and characterized alone, but may provide complexity to wines if their collective concentration is below 300 mg/L.
The analysis of head-space volatiles is of interest in helping to categorize differences among wines. However, there are few compounds whose concentrations can be directly correlated to sensory responses.
The sensory analysis was an informal paired comparison test, in which three sets of wines were presented. Wines were reviewed blindly. Each set contained one wine bottled with a natural cork closure, and its twin with a synthetic closure, presented in a random order. Sixty participants were asked to indicate which wine was preferred in each set. The results were as follows:
Natural cork vs. SupremeCorq Preserva (sulfur dioxide impregnated synthetic). 45% preferred the natural cork. 55% preferred the synthetic.
The wines did not differ much in the concentration of headspace volatiles measured. There were higher concentrations of hexylocetate, and ethyl decanoate phenethylacetate in the natural cork wines.
Cabernet Franc, 1994.
Natural cork vs. SupremeCorq. 65% liked the natural cork vs. 35% preferred the synthetic.
These wines represented a unique opportunity to evaluate wines which had been in the bottle for some time (bottled in 1996). Generally, there was a higher concentration of alcohols present in the synthetic closure wines, and a lower concentration of esters. Specifically, the synthetics had a lower concentration of ethyl hexanoate, ethyl octanoate, and ethyl decanoate.
Red Blend, 2001 Estate Reserve.
This wine is 38% Merlot, 25% Petit Verdot, 25% Tannat, and 12% Malbec, bottled in natural cork and Neocork. 60% of the evaluators preferred the natural cork, 40% the Neocork. The natural cork had a higher concentration of ethyl hexanoate, octanoate and decanoate, as well as two higher alcohols, isobutanol and isoamyl alcohol.
Red Blend, 2000.
Thirty-two percent Mourvedre, 27% Carignan, 25% Syrah, 5% Tinta Cao, 5% Touriga Nacional, 3% Tannat, 1% Cabernet Franc, 1% Malbec and 1% Nebbiolo, natural cork vs. SupremeCorq. This wine was not reviewed sensorially, due to time constraints. However, the analysis of the head-space volatiles indicated a trend similar to that noted for the Jefferson wines, namely a reduction in the ester concentration in the synthetic closure wines.
II. Atypical Aging
Grape nitrogen appears to be related to a sensory phenomenon known as untypical (UTA) or atypical (ATA) aging. Wines with this taint lose their varietal character very early, and take on atypical aromas/flavors, which have been described as naphthalene (moth balls), dirty dish rag, and wet towel, and are also characterized by a metallic bitterness. Since it was first reported, it has been identified in wine regions of Europe, the Pacific Northwest, California, and New York. We have confirmed its presence in Virginia. This sensory problem has been linked to nitrogen metabolism in the vine. Aminoacetophenone and two other compounds (indole and methyl indole) related to the metabolism of the amino acid tryptophan, are believed to contribute to this taint. Under certain growing conditions, the grape may accumulate excessive concentrations of these compounds in the bound, glycosidic form. These bound components may later be hydrolyzed, or broken, releasing the free odor-active volatiles, resulting in the taint. The problem appears to be related to insufficient assimilable nitrogen in the plant, but cannot simply be solved by the addition of nitrogen fertilizer in the vineyard, or addition of nitrogen to the fermenter.
Wine with atypical aging (ATA) defect loses its varietal flavor very quickly – sometimes before the wine is one year old, or in its first and second year of aging. With the disappearance of the varietal flavor, atypical flavors appear. ATA primarily occurs in white wines, such as Chardonnay, Riesling, Cayuga White, Pinot Gris, and Seyval Blanc. It can potentially cause serious economic loss to the grape and wine industry.
Very little is known about the cause of ATA. In general, ATA is induced by vine stresses. The critical time appears to be before and during veraison. ATA occurs more frequently in dry vineyards and in dry years. Nitrogen deficiency may play an important role in ATA, as identification of ATA in Europe coincided with a dramatic reduction of nitrogen fertilizer use in vineyards. Sponholz (2001) reported that cultivation of the vineyard floor, and N application, reduced ATA development. Generally, under ATA-inducing conditions, fruit does not reach full maturity, and wines therefore lack intense varietal flavor and/or a sufficient pool of flavor precursors (for additional information see Enology Notes #14).
At a recent meeting of research scientists, called by Dr. Thomas Henick-Kling of Cornell University, the current understanding of ATA was reviewed. According to Dr. Henick-Kling, as much as 20% of the white wines produced in New York may show signs of ATA.
We have not seen anywhere near that incidence level in Virginia. However, it is becoming increasingly apparent that ATA is a world-wide problem, particularly for white wines.
The following is a simple screening test for white wines, prescribed by Dr. Henick-Kling.
1. Collect a sample of white wine as the control. Take another sample of the same wine, adjust to 40 mg/L free sulfur dioxide, and add 150 mg/L ascorbic acid (Vitamin C).
2. Pour the two samples into vials so that they are completely full, and seal. Label the control, and label the SO2 and ascorbic acid wine as treatment. Place both vials in an incubator set at 40º C for 12 hours.
3. Remove the samples, cool, and sensorially evaluate, comparing the control to the treated sample.
This simple artificial aging test can be used as an ATA screen. If the test is positive, the best course of action is to prevent oxidation, and to add both sulfur dioxide and ascorbic acid at bottling.
ATA is believed to result from oxidative degradation. This is a main reason why it is seen much more often in whites vs. reds (reds have a high concentration of oxygen buffers).
Any Virginia producers needing assistance in identification of ATA can call my office.
III. Short Vatting.
Perhaps the greatest challenge to the production of consistent wine quality in Virginia lies in the ability to adjust winemaking protocol to fit seasonal variations. A problem and concern this season has been the extensive rainfall resulting in poor fruit set, low crop, a great deal of vegetative growth, and fungal disease pressures.
The environmental conditions during bloom resulted in uneven set, which may provide a high degree of uneven ripening. If the wet weather continues, the extensive vegetative growth will continue, resulting in a delay in the rate of fruit maturity. This, coupled with asynchronous fruit development, can cause a lack of phenolic maturity, and a high concentration of undesirable aromas.
Winemaking adjustments to suit the season is a requirement for premium wine production. One such adjustment to consider is short vatting. This term is applied to wines made by dejuicing prior to dryness, sometimes considerably prior.
The goal of short vatting is selective extraction. The aims of traditional short vatting include the rapid diffusion of desirable pigments, tannins and polysaccharides from the skins and the pulp, and the stabilization of phenols and aromatic compounds (Delteil, 2000). Questions to ask include how to determine if short vatting is a desirable production method, and at what stage in the fermentation should wines be dejuiced.
Both determinations should be based upon an understanding of the fruit and objectives. The specific objectives include color, structure (the balance of phenolic and acidic elements), and aroma. These objectives have to be established, based on knowledge of fruit ripening parameters and the degree of asynchronous ripening.
Thinning at the time of veraison is a standard method used in Virginia to help minimize the degree of uneven development at the time of harvest. One concern this year may be the already depressed crop load and the problems associated with reducing the crop in the future, and possibly increasing vegetative growth.
At or near the time of harvest, each block should be monitored for the degree of uneven ripening. This can be done by berry sampling. For accuracy of +/- 0.5 Brix, 5 x 100 berry samples are required. Processing each 100 berry lot separately allows for the determination of the coefficient of variation, or the relative difference among samples. If the variation among samples from a individual block is not large, then the degree of uneven ripening is not large. The greater the degree of uneven ripening, the shorter the vatting period.
Processing involves the following considerations. Avoid excessive berry breakage. Enzyme addition helps in extraction. This will also lower the concentration of reductive tones, by allowing greater ability to separate heavy from light lees. Heavy lees are those which precipitate 24 hours immediately post-fermentation. Wines should be removed from heavy lees. Light lees are those which precipitate more than 24 hours post-fermentation, and those lees should be retained, depending on the varietal, fruit conditions, and wine style (See Enology Notes # 25, 41, 61 and Vintner's Corner Vol. 14, No. 3; Vol. 17, No. 5).
Yeast selection is also quite important. Those who were at our Mouthfeel Seminar with Dominique Delteil were able to evaluate a set of red wines, which demonstrated the impact of yeast on mouthfeel components. A high mannoprotein- or polysaccharide-producing yeast would be desirable in the case of uneven ripening.
Delestage, with or without seed deportation, allows for fairly rapid diffusion without over-extraction. Excessive extraction increases the phenol load, resulting in a high concentration of astringency, tannin intensity, dryness and bitterness. Delestage has the advantage of being gentle, avoiding over-extraction as well as being oxidative. The oxidative nature of this processing technique allows for increased polyphenol polymerization if this is done daily, resulting in increased color stability and mouthfeel softening (See Enology Notes # 23, Vintner's Corner Vol. 15, No. 3). Relatively cool fermentation temperatures of both the cap and juice have a large impact, by helping to avoid over-extraction.
After dejuicing, short-vatted wines should be racked off the heavy lees 24 hours post-fermentation. Microoxygenation or splash racking may help evolve the tannins, depending upon the variety and the degree of polymerization which has already occurred (See Enology Notes #23, 33, 36).
IV. ICV Mouthfeel Sensory Evaluation Protocol
At our recently concluded Mouthfeel Seminar, Dominique Delteil discussed his mouthfeel protocol. He has generously provided me with a written version, which I have posted on my Enology-Grape Chemistry Group website at www.vtwines.info. From the homepage click on Extension, then On Line Publications & Current Topics.
V. Enology Notes
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