Sulfur dioxide in must and wine

Monier-Williams, G, W. 1927. Determination of sulfur dioxide in foods. Repts. Public Health and Med. Subjects No. 43,1-56, Great Britain Ministry of Health, London. Moreau, L., and Vinet, E. 1937a. Determination of the real antiseptic power of sulfurous acid in musts and wines by the method of the iodine index. Compt. rend. 

In the case of musts and wines, however, additional compounds besides sugars, particularly acetaldehyde, help to bind an appreciable part of the SO2. For a long time, ever since SO2 came to be used generally in wine technology, the problem of bound SO2 occupied the minds of research workers. Rocques in 1897 maintained that SO2 in musts and wine was bound by glucose, but Ripper (1892) and Schmidt (see Kerp, 1904) thought that sulfur dioxide in wines existed in the greater part as a combination with aldehydes. 

Among secondary products, ketonic function compounds (pyruvic acid, a-ketoglutaric acid) and acetaldehyde predominantly combine with sulfur dioxide in wines made from healthy grapes. Their excretion is significant during the yeast proliferation phase and decreases towards the end of fermentation. Additional acetaldehyde is liberated in the presence of excessive quantities of sulfur dioxide in must. An elevated pH and fermentation temperature, anaerobic conditions, and a deficiency in thiamine and pantothenic acid increase production of ketonic acids. Thiamine supplementing of must limits the accumulation of ketonic compounds in wine (Figure 2.10). 

White musts and wines made without maceration contain very low amounts of flavonoids. However, when making white wine from white grapes, skin contact at low temperature is sometimes performed before pressing and fermentation to increase extraction of volatile compounds and aroma precursors. After 4h of skin contact, the concentration of flavanol monomers and dimers in must was increased threefold. Delays between harvest and pressing, especially if sulfur dioxide is added to prevent oxidation, as well as thorough pressing, similarly result in increased concentrations of flavonoids in white musts and wines. " " ... 

Certain enzymes present in grapes are responsible for wine problems such as clouding, darkening or an oxidized taste. To prevent this, wineries routinely treat must and wines with sulfur dioxide. In addition to its antimicrobial activity, SO2 has an antioxidative property which prevents browning and taste defects. Polyphenoloxidase has detrimental effects on wine quality. However enzymes are also responsable for the formation of certain desir able esters which are essential to the aroma or bouquet of the wine. 

Sulfur dioxide is also used in wine making for the most important operation known as the defecation of must. It is customary when making white wines to allow the must to clear by settling before fermentation, and then to draw off the supernatant clear must into the fermentation vats. The use of sulfur dioxide in this process, which may last from two to three days, is beneficial in several ways ... 

Mannitol. Whereas mannitic fermentations are seldom a problem where sulfur dioxide, pure yeasts, and temperature control are employed, Martucci (1941) has reported them in Argentina. He recommended control of the must acidity, since a high pH also favored such spoilage. A complicated polaiimetric procedure for mannitol (a sugar alcohol) in wines was presented by Salani (1937). Formation of mannite during dialysis of musts at low temperatures (8° to 10° C. (46.4° to 50° F.)) in the presence of chloroform was reported by Barbera (1933b) (possibly owing to enzyme action). 

Sorbic acid therefore exerts a selective effect on wine microorganisms and opposes yeast development without blocking bacterial growth. It has the opposite effect to that of sulfur dioxide (which favors yeasts at the expense of bacteria). Consequently, sorbic acid must never be used alone but always associated with an antibacterial product (sulfur dioxide). In wines exposed to air, the amount of volatile acidity formed by acetic acid bacteria can be greater in the presence of sorbic acid, due to the absence of an antagonism with yeasts. 

Cellar practices were shown to influence the extent of M-L fermentation in Eastern wines (60). The highest incidence occurred where must was fermented immediately after pressing. The lowest incidence was found where only a portion of the must was fermented immediately and the remainder was pasteurized, cooled, and held in cold storage until fermenter space became available. The effect of sulfur dioxide concentration on incidence of M-L fermentation was not evaluated in that survey. 

Copper. In the presence of sulfur dioxide, copper-protein cloudiness may develop in white wines. Only small amounts of copper (about 0.3 to 0.5 mg/liter) cause cloudiness. Widespread use of stainless steel in modern wineries has reduced copper pickup, but many wineries routinely test their wines for copper. Atomic absorption spectrophotometry is the method of choice (51) although reducing sugars and ethanol interfere, and correction tables must be used (107). To reduce this interference, chelating and extracting with ketone is recommended (108). Lacking this equipment colorimetric procedures can be used, especially with di-ethyldithiocarbamate (3, 4, 6, 9,10, 22,109). Neutron activation analysis has been used for determining copper in musts (110). 

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