Vol.16, No. 5September - October, 2001
Bruce W. Zoecklein
Department of Food Science and Technology
VPI & SU - 0418
Blacksburg, VA 24061
E-mail: bzoeckle@vt.edu
Web site: http://www.fst.vt.edu/Zoecklein
I. Juice and Wine Analysis Short Course
The Enology-Grape Chemistry Group will offer a two-day juice and wine analysis short course January 8 and 9, 2002. This program will be a hands-on, practically-oriented laboratory course. It will be conducted in the teaching laboratory of the Food Science and Technology Building at Virginia Tech.
This program will include the following:
Registrants will participate in hands-on analysis. Analyses will be supplemented
with discussions concerning the practical winemaking significance of each test.
Enrollment is Limited and Restricted. The short course will be limited to a
total of 14 participants. Registration is for both days. Preference will be
given to bonded winery representatives that register BEFORE DECEMBER 3, 2001.
After December 3, open enrollment will be offered if space is available.
Registration: Register by sending an email message to my secretary, Terry Rakestraw,
at rakestra@vt.edu. If you are associated
with a commercial winery, please indicate. Detailed information will be mailed
to registrants prior to the short course.
Cost $300 per person. This includes the short course fee plus two lunches and
an evening reception.
The complete registration fee is due NO LATER THAN December 10, 2001. Checks
are to be written payable to Bruce Zoecklein, Foundation Account, Virginia Tech
and mailed to Bruce Zoecklein, Department of Food Science and Technology, Virginia
Tech (0418), Blacksburg, VA 24061-0418. On the memo section of the check list
"Gift."
Supplemental Text: It is suggested (but not required) that registrants have a copy of Wine Analysis and Production, Zoecklein et al. (1995) for this short course. If you have a copy, bring it to Blacksburg. Books will be available at the time of the program at the discounted rate of $80 each. Books will be available through my office to registrants only.
Each bonded winery in the state should be able to conduct alcohol, pH, TA
and sulfur dioxide in-house. These are basic assays that should be part of each
and every winery's HACCP plan.
As of January 15, 2001, the Enology-Grape Chemistry laboratory stopped conducting analysis on alcohol, pH, TA and sulfur dioxide (unless a winery is having a problem). The laboratory service remains in effect to provide supplemental quality analysis, to help in trouble-shooting problems, and to provide sensory evaluations. For example, we will continue to conduct HPLC analysis for organic acids, run electrical conductivity tests for bitartrate stability, and nephelometric analysis for protein stability.
III. Results of Phase I of the Enology-Grape Chemistry Group Laboratory Certification Program
Phase I of the Laboratory Certification Program involved measurements of pH, titratable acidity (TA) and ethanol. Values for the specified measurements were determined experimentally in the Enology-Grape Chemistry Lab at Virginia Tech.
The pH values reported had a mean of 3.29 with a standard deviation of 0.04.
77% of the results were within 2% of the mean, which is considered acceptable.
The balance had values which deviated significantly and may have been due to
single point pH meter calibration or lack of recent calibration.
The correct value for TA in the sample was 4.1 g/L (as tartaric acid). Responding
wineries reported values for TA, ranging from 3.6 to 4.8 g/L. The mean and standard
deviation were 4.1 and 0.3 g/L. 54% of the results were within 0.1 g/L of the
actual value, and 77% were within 0.4 g/L. Ideally, all results should be within
0.1 g/L. Small variations for this analysis may be due to endpoint uncertainty
and incorrectly reading the buret. The probable sources of larger deviation
include difficulty in endpoint determination, failure to use a pH meter to determine
the endpoint, and failure to standardize the NaOH solution. Large deviations
require careful examination of the experimental method.
The final test was determination of ethanol concentration. The value determined
in the Enology-Grape Chemistry Lab was 11.3 +/- 0.04 %, by ebulliometry. The
values reported ranged from 11.2 to 13.25%. Ignoring three obvious outliers
(11.8, 12, and 13.25%), the average value was 11.38%, with a standard deviation
of 0.15. This is excellent reproducibility. Errors in measurement by ebulliometry
can be due to insufficient cooling of the condenser, allowing ethanol to boil
off, or failure to compensate for atmospheric pressure or pressure changes by
measuring the boiling point of distilled water. Ethanol can also be measured
by distillation and hydrometry. Hydrometry errors include dirty hydrometers,
incorrect reading, and failure to compensate for temperature.
Overall, most of the wineries were reasonably accurate on at least two of the
analyses, but 70% had one deviation, with the deviations evenly spread among
the analyses. Only 23% of the wineries were accurate on all three determinations.
This demonstrates that there is still some work to do on winery lab accuracy.
The results of the fermentable nitrogen certification have been reported to each winery and will be discussed in a subsequent email.
As of this writing, the 2001 season appears to be one of the best ever, possibly
eclipsing 1998. The most important parameters governing potential wine quality
are the so-called secondary plant metabolites, aroma, flavor and phenolic compounds.
For the most part, we are seeing fruit with rich, ripe aroma and flavor potential,
with mature tannin phenols which will provide both suppleness and structural
carpentry or depth.
The best temperature for secondary metabolite development during late-stage fruit ripening is below that for optimum vine growth. Aroma, flavor and phenol production is the result of enzymatic activity in the plant. This activity increases in relatively low and intermediate temperature ranges, but falls at high temperatures. An optimum mean temperature during grape ripening is about 20°C (68°F) or slightly higher. It is a little below the mean temperature of around 23-25°C (73-77°F), at which grape vines reach their greatest rate of photosynthesis. Below that temperature a surplus of sugars can accumulate, which is available to be used as luxury products such as aroma/flavor/phenol compounds. As the temperature increases above the optimum, the demand for sugar due to rapidly accelerating respiration results in a smaller amount of sugar for growth and secondary metabolites. This is why anthocyanins, for example, disappear at high growing temperatures. The maximum rate of secondary metabolite synthesis depends on a good supply of sugar, as the basic chemical substrate and energy source, together with favorable intermediate temperatures for maximum enzyme activity.
One of the reasons why this is a good vintage may be temperature. There is evidence that cooler growing/ripening temperatures result in more aromatic fruit. Cooler temperatures (to a point) may result in a wider range of grape volatile compounds retained. It appears that many grape-derived aroma/flavor compounds are in a constant cycle of formation, accumulation, and conjugation with sugars into odorless forms. The free forms contribute directly to odor, while those bound to sugars are odorless. The size of the free active odor pool may vary directly with their rate of formation, which is a function of temperature. There may indeed by a qualitative effect. For example, high boiling point compounds such as terpenes (character impact compounds responsible for the varietal notes of White Riesling, Muscat and Gewurztraminer) may have an optimum temperature of accumulation, which is different from that of other types of compounds. Indeed, the importance of temperature on the development of varietal aroma/flavor is why some choose to use a different fruit zone leaf removal program, based on the side of the grapevine canopy and the variety.
A desirable tannin profile can be achieved only with a complete understanding of tannin management from vineyard to bottle. There are a number of factors which influence a wine's tannin profile including: fruit maturity and uniformity of maturity, fruit handling, sulfur dioxide, oxygen, yeast species and strain, and must nitrogen status. Processing variations such as cold soak, the use of maceration enzymes, tannin addition, and alcohol content at dejuicing, can also influence a wine's tannin profile. Each of these has been discussed in previous editions of the Vintner's Corner, which are available on-line at www.fst.vt.edu/zoecklein/index.html.
Enology Notes are electronic email communications which cover various enological
and viticultural topics. They are different in content from this subscription-based
Vintner's Corner newsjoural. To be added to the Enology Notes list serve send
an email message to bzoeckle@vt.edu with the word ADD in the subject line.
All past Enology Notes and Vintners Corner newsjournals are posted on the Enology-Grape
Chemistry Group's web site at: www.fst.vt.edu/zoecklein/index.html.
VII. Winery Planning and Design Workshop Proceedings Available
Proceedings of the Winery Planning and Design Workshop conducted in July are available. The 104 page proceedings covers establishing a business plan, winery design considerations, including gravity flow, winery tank selection, sanitation, etc. Send $45 payable to Dr. Bruce Zoecklein, Foundation Account, Department of Food Science and Technology, Virginia Tech (0418), Blacksburg, VA 24061