To: Virginia Vintners
From: Bruce Zoecklein
Subject: Maturity Evaluation, Grape Skin Maturity Evaluation, Juice Aroma Evaluation
Maturity Evaluation. Some harvest fruit from the sides of the grapevine canopy at different time periods. Why ?
The effects of light and temperature on grape-derived aroma/flavor and phenols (the most important components of grape quality) can be significant.
During the summer, the sun is in the southern part of the sky. Vineyard row orientation and canopy management can influence both heat and light into the canopy. For example, fruit on the southern side of east-west rows receive more light and heat.
Heat and light interception can have an important influence on fruit chemistry as we and others have seen. Based solely on the side of the canopy, we have noted a 31% difference in skin tannin per berry. We have also noted differences in skin pigment components: a 13% difference in total skin color per berry and a 43% difference in the concentration of small polymeric pigments (SPP) in the skins (small molecular weight tannins associated with anthocyanins). These changes have been noted in the absence of differences in degrees Brix.
Chemically, grape and wine tannins are composed of subgroups (flavon-3-ols). In simplistic terms, those tannins which contain fewer than five subgroups are predominately bitter and contribute to tannin hardness. Those tannins which contain more than five are predominately astringent and contribute to suppleness.
As grape skins mature, the average number of subunits in the tannins increases, increasing tannin suppleness. This is an important sensory and maturity feature.
Mature tannins in the fruit are a requirement for premium red wines and, therefore, are an important harvest consideration. If the skins contain non-polymerized, harsh tannins, so will the wine.
Grape Skin Maturity Evaluation: A simple strategy for tannin evaluation of grape skins includes the following:
A random and representative sample of fruit is collected, and the pulp is completely removed from the skins. It is imperative that the pulp be removed due to the confounding influences of sugar and acidity on the perception of astringency and bitterness.
The skins are sensorially evaluated for the presence or absence of hard, astringent and/or bitter tannins. Tannins evoke a tactile response and the taste of bitterness. The textural descriptors used for astringency include hard, coarse, green, firm, chewy, dusty, supple, soft, fine, silky, etc.
Naturally, these terms are the same as those applied to wines. One can conceptualize skin tannin evolution as undergoing changes analogous to the textural differences between wool, linen and silk. For a more supple style, more silk-like grape tannins are required.
As indicated in the Norton Roundtable meeting, Australian researchers (Gawel et al., 2000) have published a mouth-feel wheel showing a hierarchical representation of terms that can be used to describe the mouth-feel characteristics of red wines (and grape skins). A copy is available through my office.
I suggest a portion of these skins be frozen and used as a reference for the evaluation of grapes at the next sample period.
If you are not evaluating skin tannin maturity, premium red wine quality and control of wine style is a matter of luck.
As discussed in editions of my hard copy news journal, The Vintners Corner, and previous editions of these Enology Notes, an evaluation of seed maturity is also essential. The Vintners Corner and previous editions of these Enology Notes can be accessed on-line; see Enology-Grape Chemistry Group web site at www.fst.vt.edu/zoecklein/index.html.
Juice Aroma Evaluation. Juice aroma can also be influenced by row orientation and canopy management. The evaluation of aroma is also an important maturity gauge, if done properly. The procedure suggested by Jordan and Croser (1983) is as follows:
Use a hand press or stomacher bag to press the chilled grape sample.
Add pectinolytic enzyme at the suppliers recommended level. Because many aroma compounds are present as glycosides (therefore not volatile) it is helpful to add pectinolytic enzymes. Pectinolytic enzymes cause some glycoside hydrolysis, and increase the free volatiles in the juice sample. This increase, in part, emulates what may occur during fermentation and aging.
If you desire to store the sample for later maturity comparisons, add 50mg/L ascorbic acid and up to 30mg/L total sulfur dioxide. Ascorbic acid is vitamin C an antioxidant, which along with sulfur dioxide, will help minimize degradation of aroma components.
If possible, lightly sparge the juice sample with nitrogen and sieve into sample bottles which have been CO2 filled, seal and cold settle. Decant the clear juice into CO2 filled bottles and conduct the sensory evaluation.
Note that the sample preparation method (including degree of pressing) and cold settling influences titratable acidity, potassium and pH (see Zoecklein et al., 1995).
Under optimum conditions, juices prepared using this methodology and stored at 0° C will remain viable for aroma assessment for several days or longer.
Processed samples can be retained and used to compare the changes in aroma which occur with additional fruit maturity.
For additional information on this subject, see either Enology Notes or Vintners
Corner on the Enology-Grape Chemistry Group web site at www.fst.vt.edu/zoecklein/index.html.
Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head, Enology-Grape Chemistry Group
Department of Food Science and Technology
Virginia Tech, Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293