To: Regional Wine Producers
From: Bruce Zoecklein, Head, Enology-Grape Chemistry Group, Virginia Tech
Subject: Winemakers Technical Roundtable on Lactic Acid Bacteria; Wine and Sulfur-Containing Compounds
Technical Roundtable on Lactic Acid Bacteria.
There are several important issues with regard to lactic acid bacteria (LAB) management in wine. We are becoming increasingly aware of the impact of bacterial fermentation on wine quality. The following is a brief prelude to the Technical Roundtable meeting on LAB issues and management, scheduled for March 27 at Horton Vineyards, beginning at 1:00 pm.
Wine yeast. Wine yeast have a significant impact on the growth of lactic acid bacteria. That impact is in several forms. Yeasts differ in their ability to reduce sulfate to sulfur dioxide, which can impact LAB growth. Yeast also differ in their polysaccharide production. These compounds are stimulatory to LAB (Delteil and Jarry, 1992).
Maceration period. Long maceration creates a desirable environment for LAB, due to the extraction of nitrogen-based growth promoters, and the increase in pH associated with skin contact.
LAB and lees management. Work conducted by Delteil (2000) suggests the importance of having the MLF conducted on wines which are on light lees only. The benefits of light lees storage was discussed at the pre-harvest workshop this past summer. Light lees can be defined as those lees remaining after the wine has been racked 24 hours-post yeast fermentation.
Completion time. The shorter the time of MLF completion, the greater the sensory benefits, and the lower the risk of growth of anaerobic spoilage organisms, including Brettanomyces, Lactobacillus, and Pediococcus. Naturally, temperature is an important factor. Temperatures lower than 15°C will delay the MLF.
Strain differences. There are notable differences in LAB strains, with regard to growth rate and sensory impact. LAB can use other substrates in wines, particularly following the MLF. These include citric acid and glucose, and can result in the production of high concentrations of acetic acid and unpleasant aroma/flavors.
and Sulfur-Containing Compounds
According to Lavigne-Cruege and Dubourdieu (2001), great white wines can only be produced with a prolonged period on the lees.
This statement relates to the power of macromolecules, as discussed in Enology Notes #6 and outlined during our Pre-harvest Workshop last season. At that time, I defined heavy vs. light lees, and discussed the benefits of light lees storage for reds and whites.
The main risk of lees storage is the production of reductive sensory defects, like hydrogen sulfide and mercaptans.
These can result from a long list of factors outlined in Enology Notes # 70. Measuring fermentable N, careful control of the level of non-soluble solids going into the fermenter, limited concentrations of sulfur dioxide (less than 8 g/hL), careful inspection of barrels before filling, and delaying sulfur dioxide additions post-fermentation (at least 10 days post-fermentation completion) aid in minimizing the production of sulfur compounds (Lavigne-Cruege and Dubourdieu, 2001).
When the yeast cells are compacted and with low oxygen, there is the likelihood of increased production of off sulfur-containing compounds. For this reason, most do not conduct sur lie storage in large stainless steel tanks.
Lavigne-Cruege and Dubourdieu (2001) conducted an interesting study. A batch of wine, with a reductive defect following fermentation, was divided into lots and underwent several separate treatments including: double racking with aeration, racking with aeration and total lees removal, and racking with aeration and temporary lees removal, then lees return.
The treatment which had the greatest impact was the lot where lees were separated in a barrel, stirred frequently, and the lees added back to the wine after 48 hours.
These results may seem odd, but demonstrate that yeast walls have the ability to absorb certain volatile sulfur compounds. It would appear that disulfide bridges could be formed between some of the wine's volatile sulfur compounds and the cysteine in the yeast wall manoproteins.
This treatment represents a far better option than simply separating the wine from the light lees. Simply treating a wine with a reductive defect, by aerating and removal from the lees, may not eliminate the problem. Without the benefit of the lees, a wine is subject to increased risk of oxidative degradation. Additionally, if the wine is in new wood, the oak tannins, in the absence of the yeast polysaccharides, may not integrate well.
As I have suggested repeatedly, lees are a far too valuable tool for a winery to discard.
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Dr. Bruce Zoecklein
Associate Professor and Enology Specialist
Head Enology-Grape Chemistry Group
Department of Food Science and Technology
Blacksburg VA 24061
Enology-Grape Chemistry Group Web address: www.vtwines.info or www.fst.vt.edu/zoecklein/index.html
Phone: (540) 231-5325
Fax: (540) 231-9293