No. 1 January - February, 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
|Technical Roundtable Meeting||2|
|Control of Hydrogen Sulfide and Mercaptans in Wine||3|
I. France Study Tour Notes Posted
Twenty Virginia wine industry members went on a study tour to southwestern France, the Rhône Valley and Beaujolais.
It was a unique chance to see grape varieties some of which we are highly interested in for Virginia, including: Fer Servadou, Petit Mensing, Tannat, Auxerrois or Malbec, Mourvedre, Syrah, Viognier, Marsanne, and Rousanne.
After leaving VINITECH, the largest grape and wine equipment show in the world, and some famous Bordeaux producers: Château Pichon Comtesse de Lalande in Pauilac, Château Cheval Blanc in Saint Émilion, Château Suduiraud in Sauternes, and Château de Laubade in Armagnac, we traveled to Pau in the foothills of the Pyrénées mountains. We visited the region of Madiran including Château Montus and Château d’Aydie where we discussed and tasted Tannat wines. We toured vineyards and domains in Jurançon and Cahors, traveled along Gascogne to the southern foothills of the central Massif, visited Gaillac and Fronton, and then went to the fortified city of Carcassone. From there we traveled across the world’s largest vineyard area of Languedoc to the Papal city of Avignon, and finally traveled along the Rhône River, visiting Châteauneuf de Pape, Hermitage, St. Peray, St. Joseph, Condrieu and Côtes Roties, into Beaujolais, and ended the tour at the city of Lyon, the capital of gastronomy.
Each day I assigned two of our participants to take detailed notes of each vineyard and wine producer. These have been compiled and posted, along with some nice photos taken by Jennifer McCloud of Chrysalis Vineyards, on the Enology-Grape Chemistry Group Website at www.vtwines.info.
Some of the take home messages for the Virginia industry include the following:
Some interesting opinions also included:
II. Technical Roundtable, March 27
A Technical Winemaker Roundtable meeting is scheduled for March 27, 2003 at Horton Vineyards 1:00 pm – 4:30 pm.
Subject: Managing and monitoring malo-lactic bacteria
Two guests will help lead the discussion. Sigrid Gertsen-Breand and Dr. Sibylle Krieger are Technical Support Staff and Director of Research and Development for Malo-Bacteria, respectfully.
If time permits and attendance is adequate, I will pour a set of Syrah wines produced with different alcohol concentrations, and several other wines which were poured at the recently concluded VVA Technical meeting.
III. Sulfur-Containing Compounds
The formation of sulfur-containing compounds has been a winemaking problem for as long as wines have been produced. The problem remains, although our knowledge of the nature of the compounds, and the mechanisms influencing their control, are increasing. The following is a general review of reductive tone formation.
The number of factors which can influence the production of sulfur containing compounds like H2S seems unlimited, and includes:
Factors Increasing H2S Formation
Source: Bisson (2000)
Volatile sulfur-containing compounds are known to impart distinctive aromas to wines such as rubbery, skunky, or like onion, garlic, cabbage, kerosene, etc. The objectionable odor of hydrogen sulfide, generally described as rotten-egg-like, also has been observed. If no correction is made, hydrogen sulfide may undergo reactions with other wine components to yield mercaptans, which can have detrimental effects on wine palatability and may be difficult to remove.
Hydrogen sulfide contains sulfur in its most reduced, negatively charged form (S-2). Other sulfur containing compounds of interest to winemakers include oxidized forms such as S+4O2 or copper sulfate (CuS+4O4).
H2S sulfur moiety can come from
Mercaptans are the other principal group of sulfur-containing compounds. They all contain the sulfhydryl (-SH) group. Ethyl mercaptan possesses a burnt rubber, skunk or garlic-like character. Methyl mercaptan has a sensory characteristic of cooked cabbage. The sensory threshold of both mercaptans is approximately 1 ppb (part per billion).
Mercaptans can oxidize to disulfides when exposed to air. This oxidation not only influences the sensory attributes but influences the ability to bind with copper sulfate (see below). The sensory threshold of disulfide is around 30 ppm.
Sulfur, an essential element for yeast growth, is utilized in the formation of cell components such as protein and vitamins. Available as sulfate (SO4-2) in grape juice, it can be reduced to hydrogen sulfide (H2S). As H2S is an integral part of yeast metabolism, it is not possible to completely prevent its formation. However, vineyard management, including selection and timing of spray applications, and wine processing techniques may effectively minimize its detrimental effects. The following is a review of some of the causes and solutions to the production of sulfur-containing compounds.
Elemental Sulfur: Elemental sulfur is used as a fungicide in vineyards throughout the world. Because of increasing awareness of the problems associated with sulfur in winemaking, most viticulturists are using micronized sulfur, which consists of very small particles, ranging from 6 to 8 µm in size, which are readily miscible in water. An advantage of micronized sulfur is that the application rate is less than one-third the normal dusting sulfur rate for the same measure of fungal control. Only 5 mg/L of elemental sulfur in the must is enough to produce H2S concentrations, which cannot be removed. Therefore, sulfur sprays should not occur less than 35 days prior to harvest.
An additional source of elemental sulfur in juice is sulfur candles, used by some vintners to disinfect barrels. These candles may not burn completely, so that unburned sulfur enters the wine or juice. The use of dripless sulfur sticks and/or sulfur cups may effectively overcome this problem.
Redox State and Temperature: Hydrogen sulfide formation also is a function of the oxidation-reduction (redox) state of the must during fermentation. Higher levels of H2S are produced from fermentations carried out in tall (height to diameter) tanks. The design of such fermentors is conducive to a rapid drop in redox potential. The fermentation temperature also affects the overall formation of H2S; generally, less H2S is produced at lower temperatures. However, at lower temperatures, less H2S is lost through entrainment with carbon dioxide.
Yeast and Yeast Physiology: Yeasts differ significantly in their ability to form hydrogen sulfide. However, due to the complexity of factors influencing their production, no strain can be said to be problem free. Some yeasts are known to have deficiencies in their sulfur metabolism that promote increased production of H2S. Such yeasts appear to have an absolute requirement for the vitamins pantothenate and/or pyridoxine (vitamin B6). Although grape juices normally are not deficient in these two vitamins, must treatment, seasonal variations, rot, etc. may result in the depletion of one or both.
Free amino nitrogen (FAN) components of must, therefore, play a role in subsequent H2S formation. Specifically, assimilable free amino nitrogen content is inversely related to H2S levels. Deficiencies in total yeast assimilable nitrogen are not always correlated with the formation of H2S.
Yeast Autolysis: Upon yeast cell death, degradation and rupture of cell membranes release cytoplasmic components including free amino acids, peptides, and polypeptides. Other degradation products include fatty acids, as well as components of the yeast nucleic acids, and vitamins. Yeast autolysate may play a role in the character and complexity of wine. However, the process of sur lie with heavy lees (particularly in the absence of stirring or oxygen) can occasionally result in the production of 'off' flavors and aromas, including H2S and mercaptans. However, if reductive tones were not present at the completion of fermentation, they rarely occur later.
Proper utilization of lees is an important quality and stylistic tool (see Enology Notes #6, The Power of Macro-Molecules).
Sulfur Compounds and Metals: Copper, manganese, and zinc are components of many vineyard fungicides. Late-season application of metal-containing fungicides to the grapes is known to increase the production of H2S and possibly other sulfur-containing compounds. Because of the nature of our growing seasons, some are inclined to apply Bordeaux mix fairly late in an attempt to help minimize downy mildew. There is a definite relationship between the use of copper-containing fungicides, like Bordeaux mix, and increased incidences of H2S formation in wines.
Questions regarding how late Bordeaux mix can be applied and how much copper stimulates H2S formation are not resolved. Copper ions are constituents of certain enzyme systems, as well as known inhibitors of respiration. Yeast grown in the presence of copper adopt a protective mechanism of H2S formation, and consequently copper sulfide formation. The advantage of late season copper sprays for mildew control must be balanced with concerns for the production of sulfur-containing compounds. If late season fungicides are applied, juice settling of whites and the addition of a yeast nutrient is advisable.
The pre-fermentation addition of sulfur dioxide can impact H2S production. High initial levels of added sulfur dioxide bind acetaldehyde, which is normally reduced to form ethanol. If not enough acetaldehyde is present, juice sulfates may instead be reduced, forming H2S. Additionally, SO2 can convert H2S to elemental sulfur, which may later be reduced back to H2S.
For this reason it is essential that post-fermentation sulfur dioxide additions should not occur before 10 days after the completion of fermentation.
IV. Control of Hydrogen Sulfide and Mercaptans in Wine
Optimum Assimilable N. A key to minimizing H2S formation is the maintenance of optimum assimilable nitrogen in the fermentor, and avoiding yeast stress. It is recommended that the N status of the juice be tested prior to fermentation (see Formol Analysis on my Web site at www.vtwines.info). If supplementation is required, it is best to do it in two stages - first an addition of a nutrient such as Fermaid K, Superfood, etc., followed by the addition of DAP after the fermentation has begun.
It should be noted that excessive assimilable N increases the production of H2S and mercaptans. It appears that individual amino acids in the juice, rather than the total N concentration, play the major role in impacting H2S formation.
Some winemakers deal with excessive H2S by aeration at first racking, thus volatilizing the H2S. Increased H2S production will occur, however, if aeration is carried out during, or too soon after the completion of, alcoholic fermentation. In these cases, elemental sulfur is believed to act as a hydrogen acceptor, forming H2S.
Coincidental with H2S formation are increases in yeast populations arising as a result of transient exposure to oxygen.
Additional techniques for controlling H2S include sparging problem wines with nitrogen gas, shortly after the completion of alcoholic fermentation. This practice may be relatively effective in eliminating minor quantities of H2S, but desirable volatile wine components also may be swept away during excessive sparging. In cases where methyl mercaptan appears to be the problem, carefully controlled aeration may bring about oxidation of methyl mercaptan to the less-objectionable compound dimethyl disulfide.
Copper addition. Winemakers can remove objectionable H2S and mercaptans from a 'still' (non-fermenting) wine by direct contact with copper. The addition of 4 g copper (II) sulfate (CuSO4 * 5H20) per 1000 gallons raises the copper content by 0.2 mg/L. Although governmental regulations permit additions of up to 0.5 mg/L (as copper), residual levels in the wine cannot exceed 0.2 mg/L (as copper).
It should be noted that although mercaptans react with copper, dimethyl disulfide does not. Thus, if the wine in question has undergone any oxidation, it may be necessary to reduce dimethyl disulfide back to the reactive species, methyl mercaptan. This can be accomplished by addition of ascorbic acid (see Zoecklein et al., 1995). Generally, addition levels of 50 mg/L or more of ascorbic acid are used, and such additions usually are made several days prior to the addition of copper. (The sulfur dioxide analysis by Ripper titration cannot be performed accurately in wines containing ascorbic acid, as the latter also reacts with the iodine titrant.) Copper should not be added until the fermentation is complete, and the yeast titer reduced by racking, filtration, and so on. Yeast cells will bind copper ions to cell surfaces and may reduce reactivity with H2S.
Sulfur dioxide additions, the addition of SO2 to still wines, may reduce H2S levels. The addition results in an SO2-induced oxidation of H2S to yield elemental sulfur which, after precipitation, may be removed by centrifugation or filtration.
SO2 + 2H2S <---> 3So + 2H2O
Prior to the 2003 vintage make sure you are prepared to conduct the Formal Titration for fermentable N. See Enology-Grape Chemistry Group Web site at www.vtwines.info.
Bisson, L. 2000. Proceeding of Taint Necessary 50: Detecting and Indentifying Off-Flavors in Wine.
Zoecklein, B.W., Fugelsang, K.C., Gump, B.H. and Nury, F.S. 1995. Wine Analysis and Production. Chapman & Hall, NY.
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