Vol.15, No. 4 July - August, 2000
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
Table of Contents
I. The 2000 Harvest Season 1
II. Overview of Factors Influencing Fermentation 1
III. Canada's VQA 3
IV. Enology-Grape Chemistry Group's Extension Lab Service 3
V. Wine Crystals Formed by Calcium Mucate 4
The 2000 harvest is rapidly approaching. July temperatures averaged 74.7F at National Airport, 5.3F below normal resulting in one of the coolest Julys on record. Additionally, we are about five inches of rain above average. What will these factors mean relative to wine quality? The rain may dilute the increased varietal character and impact the incidence of fruit rot. I recommend fruit sorting tables at the winery to allow for the removal of rot degraded berries and clusters. If rains continue, steps to enhance varietal character may be important and include bleeding and the use of pectic enzymes vs bentonite for juice clarification. Consider the use of pectic enzymes post-fermentation for increased aroma development. As indicated in Vol. 15, No. 1, each winery should establish a HACCP (hazard analysis and critical control points) plan. An HACCP review helps determine the production steps that should be carefully monitored, namely those that influence quality. A review of HACCP planning and the other topics discussed in the Vintner's Corner since 1996 are posted on our web site at www.fst.vt.edu/Zoecklein.
II. Overview of Factors Influencing Fermentation
In years past, successful fermentations were the norm, and winemakers usually had to be careful only about the most high-risk musts such as drought-stressed Chardonnay. However, problems with stuck fermentations have become more and more common for a variety of reasons. Winemakers must be proactive, approaching each processing step with as complete an understanding as possible.
There is no one universal cause of fermentation problems. Difficulties can arise from a combination of factors and a variety of sources. Frequently it is the impact of two or more conditions that result in greater difficulty than would be predicted by a single parameter acting alone. Figure 1 shows some of the factors which can act to limit or inhibit fermentation.
Figure 1. Environmental and Processing Factors Influencing Viability and Fermentative Performance of Wine Yeasts. (Zoecklein et al., 2000)
Naturally, stuck or sluggish fermentations can impact sensory quality. Even in the absence of sluggish fermentations, low yeast nutrient concentrations can impact wine quality. For example, a low level of fermentable N can increase the production of stress metabolites such as hydrogen sulfide. Additionally, low levels of fermentable N increase the production of long-chained alcohols and reduce the production of esters, resulting in less fruit intensity. This is a primary reason why we have suggested that wineries perform the Formol titration for fermentable nitrogen. If you are unable to conduct this assay during the busy harvest season, freeze a representative sample of must for later analysis. It is important to gain an understanding of the nitrogen status of your fruit.
In grapes there can be large seasonal and varietal differences in the concentration of fermentable N. Additionally, yeasts vary considerably in their requirements for nitrogen. As outlined in previous editions, a balanced nutrient/vitamin supplement is preferred to adding only DAP. Yeast nutrient cocktails such as Fermaid K contain a variety of compounds such as amino acids, sterols, yeast hulls, and vitamins required by yeasts . This product also contains a limited concentration of fermentable N (25g/HL = 25 mg/L fermentable N verses DAP at 25g/HL = 50mg/L N). Healthy fermentations require a bare minimum of 140 mg/L N with the optimum concentration frequently greater than 250 mg/L. We recommend a two stage addition - first a general nutrient such as Fermaid K, followed by DAP. We have simplified the Formol titration method-see the posting on the Enology and Grape Chemistry web site under Extension, On-line Publications & Current Topics
III. Canada's VQA
Not long ago I was invited to British Columbia's wine producing regions including the famous Okanagan Valley. This region has undergone rapid growth. In discussing the increased volume and quality of wines I was struck by the importance both winemakers and university personnel placed on their Vintner's Quality Alliance (VQA) program.
The British Columbia and Ontario VQA systems have been in place for some time. These alliances were originally established in response to the competitive nature of the international wine market (NAFTA for example) and the desire to replace poor quality grapes with high quality ones. The programs are paid, in part, by the industry through a check-off. The main goal in both Ontario and BC is to promote the region's wine industry. This goal has been attained by establishment of formal standards. Wines which are defect-free and true to varietal character are allowed to carry the VQA stamp and are promoted as supplier products.
In BC, the alliance maintains about twenty-five certified judges. Each winery may submit people for certification which requires passing sensory tests for defect recognition. An 80% or better score is required for initial judge certification. Certification testing continues on a regular basis. All judges are required to attend monthly workshops on sensory evaluation to maintain their certification. Judges retain their position on the VQA panel for only a certain period.
For VQA wine evaluations, judges meet once per month. Wines are reviewed by twelve judges not by flights but simply one wine at a time. It takes three or more judges to reject a wine. If that happens an additional bottle is opened and the wine reevaluated. This program is mainly sensorial, although some chemical analyses are also conducted.
Wineries can submit pre-bottling tank samples for preliminary approval. This allows the producer to affix the VQA stamp during the bottling operation. Immediately post-bottling, bottled samples are submitted to the VQA and compared to the previously submitted tank sample. In the case of BC, only 100% BC wines are eligible for VQA certification.
Of what value is this program? It has been used to create a positive image for BC wines, particularly in the Okanagan Valley. Because it has been properly promoted, it is recognized. Consumers understand that wines with the VQA stamp have undergone a careful sensory review. To me, however, the greatest surprise was to learn that the VQA system has increased, not decreased, the sense of community among the producers. I am told that it has brought the industry closer together in the shared interest of improving both the image and quality of BC wines. This is a very desirable consequence in view of the ever more competitive global market place!!!!
I learned more about the Canadian VQA system and about the newly formed New York Pinot noir Alliance at our recently concluded meeting of the American Society for Enology and Viticulture. New York state Pinot producers have banded together to work on issues of common interest such as quality improvements and marketing. Currently, there are 14 members, each of which has pledged $750 as the first years dues!
IV. Enology-Grape Chemistry Group's Extension Lab Service
The Enology-Grape Chemistry Group offers extension laboratory analyses, the details of which are outlined in the group's web site. This service, supported in part by the Virginia Winegrowers Advisory Board, is designed to provide supplemental, quality control and production analyses. The analysis program has been operational since 1986. Since then, the number of Virginia wineries has increased and so have the demands on this service. Figure 2 shows the number of assays conducted since 1996.
A significant percentage of these assays are HPLC analyses of organic acids, frequently to determine MLF status. To help maximize efficiency, effective October 1, 2000, we will be performing HPLC analyses on a fixed schedule which will allow timely reporting of the results. Wines for HPLC analysis must be received by our lab by the first or third Monday of the month. Samples received between these dates will be held for the next scheduled analysis. With this schedule, HPLC results will be available by the next day. Additionally, it is helpful to email the Enology-Grape Chemistry Lab (contact our lab at bzoeckle@vt.edu) in advance of sending samples stating the desired assays. This will help to maximize efficiency and allow us to better serve the industry needs.
V. Wine Crystals Formed by Calcium Mucate
Mucic acid is formed in must degraded by Botrytis cincerea, and possibly sour rot, and can result in an unusual crystalline precipitate. We have seen this precipitate in some 1999 Virginia wines. The acid is believed to form from enzymatic oxidation of galacturonic acid, which is the structural backbone of grape pectin. With heavy rot degradation (10 to 25% of the berries), the pectin can be broken down to produce as much as 1-2 g/L of mucic acid. The acid can form a salt with calcium (calcium mucic) which has a solubility product at 20C of 1.3 x 10-7 mol/L which means that the solubility is very low. The low solubility results in a precipitate, which is characterized by small white clumps. Wines should not contain more that 0.1 g/L of mucic acid to prevent this occurrence. The quantity of the acid in the reactive form is pH dependent. It doubles from pH 3.0 to pH 3.25 and again from pH 3.25 to pH 3.5 (Boulton et al., 1996).
The calcium content of a table wine ranges from 6 to 165 mg/L. The saturation levels for the salt are pH dependent and are about 90 mg/L at pH 3.50.
The addition of calcium containing sprays in and of itself does not cause the problem. However, late season calcium sprays would not be advisable in view of the general nature of the problem. Calcium
can also form another insoluble salt, calcium tartrate, which can also result in post-bottling precipitation.
It is strongly suggested that individual clusters be sorted prior to crushing or pressing. The elimination of rot degraded fruit will not only influence the likelihood of this problem but also be an overall quality enhancer.
Boulton R., V.L. Singleton, L.F. Bisson and R.E. Kunkee. Principles and Practices of Winemaking. Chapman and Hall, New York. 1996.
Zoecklein, B.W., B.H. Gump and K.C. Fugelsang. Nutritional Status of Grape Juice. Food Micobiology Protocols. J.F.T Spencer and A.L. Ragout de Spencer (Eds.). In press. 2000.