Free sulfur dioxide

The free sulfur dioxide in the sulfite pulping liquor promotes hydrolysis of the more easily hydrolyzable constituents of wood. A wood such as spruce, which contains 70.1% carbohydrate substances, of which 85.3% are potentially fermentable, gives a 50% yield of a pulp containing 97.7% carbohydrate, of which about 80% is fermentable. The... 

Table XVI shows two experiments which compared the same wine stored under different conditions (46). In both cases, the wine richer in anthocyanins is also that which is less colored. The mediation of tannins is not enough to explain the differences in color these can only be explained by a different structuring of the anthocyanin molecules. More specifically, the anthocyanin molecules will be reduced to colorless flavenes (14) during fermentation, which is a reductive process. The reoxidation occurs more rapidly in wooden casks which allow better oxygen penetration than metal storage tanks or large capacity casks. However, the flavenes themselves are relatively instable and can be irreversibly hydrolyzed into dihydrochalcones (16). This explains the lack of relationship between anthocyanin concentration and color, independent, of course, of the eventual appearance of free sulfur dioxide.

Rankine et al. (70) observed in Australian wines that the critical factor governing growth of lactics in wine was sulfur dioxide content. These studies of the M-L fermentation in native American and hybrid wines seem to confirm this observation. With the range of pH and temperature noted for these wines, the M-L fermentation occurred quite regularly as long as the free sulfur dioxide content was low (20 ppm or less). However, no growth of lactics was found if levels of free sulfur dioxide approached or exceeded 50 ppm. 

The problem of accurately determining the non-bound (free) sulfur dioxide has not been satisfactorily solved (6, 7, 40). The best approach is to distill the sample in the absence of air and to recognize that the usual procedure overestimates the non-bound sulfur dioxide (41, 42, 43). 

The direct Ripper iodometric titration is still used, but it is subject to error. In its place, direct iodate-iodide titration is used (44). This is followed by fixing the sulfur dioxide with glyoxal in a second sample and retitrating. The difference represents the free sulfur dioxide. The second titration roughly represents the amount of reduction and the amount of ascorbic acid present. Formulas for calculating the amount of sulfur dioxide to add in order to produce a predetermined level of free sulfur dioxide have been given by Stanescu (45). 

Whitmore (26) and Deibner and Benard (27) claim that sulfonic acids are capable of distilling intact. However, it would seem more likely that at the temperature of the distilling process, 1-hydroxyethane sulfonic acid in the wine would dissociate into the very volatile free acetaldehyde and therefore rapidly distill along with free sulfur dioxide and recombine at the top level of the fractionating column. 

Bartroli, M Escalada, CJ Jorquera, J Alonso. Determination of total and free sulfur dioxide in wine by flow injectionp-aminoazobenzene as the colorimetric reagent. Anal Chem 63 2532-2535, 1991. 

All of these forms of sulfur are known as free sulfur dioxide. The bisulfite ion (HS03 ) can react with aldehydes, dextrins, pectic substances, proteins, ketones, and certain sugars to form addition compounds. 

The (Fe +)/(Fe +) ratio in wine depends on storage conditions, especially the free sulfur dioxide concentration. For this reason, wine is more susceptible to ferric casse after aeration, as this increases the proportion of the Fe + form responsible for this phenomenon. 

Ion-selective electrodes using a sulfur dioxide gassensing membrane probe have been developed into commercial models for the determination of sul-fur(IV) oxo-species. A typical procedure for the determination of free sulfur dioxide requires the probe to be immersed in a stirred solution at pH 1, whereas total sulfur dioxide requires the sample to be treated with strong alkali prior to acidification and analysis at pH 1. Molecular sulfur dioxide passes through the membrane into a solution of hydrogen sulfite ion and the pH of the SO2 H20/HS03 buffer can be determined with a glass electrode. 

The analyses that are routinely carried out in wine are alcohol content (vol.%), sugars (gl ), glucose and fructose (gl ), glycerol (gl , mgl ), pH, total acidity (meql ), lactic, malic, and tartaric acids, volatile acidity (meql ), malvidol digluco-side, and polyphenols. The total and free sulfur dioxide content is also routinely analyzed in wine however, since it is not a native constituent of wine its analysis will not be described in this article. 

The content of sulfur dioxide (SO2) in the atmosphere varies widely and may reach values as high as 1-5 mg m in heavily polluted air. Direct monitoring with an SO2 gas probe is often insufficiently sensitive and thus SO2 must be preconcentrated by absorption in a suitable solution. The most common procedure is based on absorption of SO2 in a tetra-chloromercurate(II) solution (TCM), in which Hg(S03)2 is formed. The pH of the absorption solution is maintained at a value of 6.9 that ensures virtually complete absorption of S02- Prior to analysis, amidosulfonic acid is added to remove nitrite and the pH is decreased to about unity, so that most of the sulfite present is converted into free sulfur dioxide, which is determined by an SO2 probe. With very low SO2 contents, a filter is soaked with a TCM solution and air is passed through the filter at a high flow rate and for a rather long time. These methods provide discontinuous values of the SO2 concentration in regular intervals. 

Experience has shown that the presence of sulfur dioxide retards the development of brown color in orange juice at high temperature. Ingram believes that if there is a relation between browning and loss of free sulfur dioxide, more should remain unbound if it is added before rather than after storage. His data are indicative only and certainly are not an ade-... 

Yang, H. Y., and Wiegand, E. H. 1950. A method for maintaining free sulfur dioxide content in wine. Food Technol. 6, 103. 

The combination of bisulfites with sugars is much slower than with aldehydes and ketones and the products formed are relatively more unstable. Only the glucose and arabinose bisulfite addition compounds have been prepared in pure or relatively pure form. The combination with other sugars is largely inferred from measurements of decrease in free sulfur dioxide content. The early work of Kerp, Schmidt, and others in the laboratories of the German Public Health Department during 1904-... 

Even when errors in true free sulfur dioxide content due to dilution or oxidation are avoided, the direct iodometric determination of sulfurous acid and sulfites is subject to error. Eolthoff and Menzel (1929) point out that an appreciable error occurs when sulfite solutions are titrated with iodine owing to oxidation by air and point out that correct results can be obtained only when the sulfurous acid or sulfite solution is allowed to flow into a solution of iodine. The titration of sulfurous acid or bisulfites is... 

The above substitutes for the iodometric determination with one or two exceptions have not been tested for specificity for free SO2. Mathers (1949) on the basis of the fact that free sulfur dioxide is removed early in the distillation of wine proposed that 10 ml. of the distillate from the alcohol determination be mixed with 0.5 ml. of 5% neutral lead acetate and the turbidity of the suspension formed be used to correct volatile acidity for sulfur dioxide. La Rosa (1950) assumed that the color formed on mixing the fuchsin-sulfuric acid-formaldehyde reagent with white wine was a measure of free SO2 but did not give any data to confirm this. The fuchsin procedure and the lead sulfite procedures are very sensitive, the former more so, and can be applied only to very small aliquots or to dilute solutions. 

In controlling fermentation, the usual practice is to add all the SO2 required at one time. In special fermentations, however, such as that of sweet table wines like Sauternes, several small additions of SO2 are used to check the fermentation while fermentable sugar still remains. Quinn (1940) has recommended frequent small additions of sulfur dioxide to wine rather than an occasional large addition to take advantage of the germicidal power of free sulfur dioxide. 

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