La Lectura

In water solutions, sulfur dioxide can be written to show the equilibrium:

SO₂ + H₂ [H₂SO₃]

[H₂SO₃] HSO₃^–+ H⁺

HSO₃^– SO₃^2–+ H⁺

The bracketed form indicates the sulfur dioxide associated with water.

The pK_a values for sulfur dioxide are 1.76 and 7.20, indicating a rather weak dibasic acid (Segal, 1968). A plot of the distribution of the three ionic forms can be calculated and is shown in Figure 5.1. Specific gravity for various water solutions of sulfur dioxide is shown in Table 5.1.

It is useful to have the sulfur dioxide in a salt form. The dry salts are easier to store and are less of a problem to handle than gaseous or liquid sulfur dioxide. Table 5.2 lists the main forms of sulfur dioxide, their theoretical yields, and solubilities of each in water. The metabisulfite is the anhydride of the acid sulfite:

2 HSO₃^– S₂O₅^2–+ H₂O

When these salts are exposed to air, they show increasing stability in the order sulfite > bisulfite

> metabisulfite (Mason, 1928; Phillips, 1928).

FIGURE 5.1 Distribution of the ionized forms of sulfurous acid at various pHs.

←→

←→

←→

←→

% of Total H2SO3

100

80

60

40

20

00 1 2 3 4 5 6 7 8

SO₂ HSO₃⁻

pH

SO⁻⁻₃

Sulfur Dioxide and Sulfites 145

REACTIVITY

Wedzicha (1984) briefly reviewed the chemical interactions of sulfur dioxide. The oxidizability of the sulfurous acid salts is indicated by the following equations:

2 SO₃^2–+ O₂ → 2 SO42–

SO₃^2–+ H₂O₂ → SO42–+ H₂O

Jacobs (1976) showed that the amount of sulfur dioxide that reacted (oxidized) after 60 days of storage in bottled red wine was proportional to the original dissolved oxygen content of the wine. After the oxygen disappeared, much slower changes occurred in the sulfur dioxide content.

If ascorbic acid is being used in combination with sulfur dioxide, the second equation listed previously is a key reaction (Heinmann et al., 1970). The sulfur dioxide scavenges the hydrogen peroxide formed and keeps further oxidation of the dehydroascorbic acid and other products to a minimum. This reaction is extremely rapid. Holt and Kumar (1986) found an observed k = 41.7 ± 3.4 s^-1 for H₂O₂ with SO₃^2- at pH 3.40 and 15°C. The sulfur dioxide stabilizes the dehydroascorbic acid by reacting with the ketone bonds (Wisser et al., 1970).

TABLE 5.1

Specific Gravity of Various Sulfur Dioxide Water Solutions at Two Temperatures

SO₂ (g per 100 g)

Specific Gravity^a

15.56°C (60°F) 20°C (68°F)

1 1.0040 1.003

2 1.0091 1.008

3 1.0191 1.018

6 1.0292 1.028

8 1.0393 1.037

10 1.0493 —

a Corrections are approximately 0.0001°F^–1

TABLE 5.2

Sulfur Dioxide-Bearing Chemicals

Compound Formula

Theoretical Yield

(%) H₂O Solubility (g/L)

Sulfur dioxide SO₂ 100 110 (20°C)

Potassium sulfite K₂SO₃ 33 250 (20°C)

Sodium sulfite Na₂SO₃ 50.8 280 (40°C)

Sodium sulfite heptahydrate Na₂SO₃· 7H₂O 25.4 240 (25°C)

Potassium bisulfite KHSO₃ 53.5 1000 (20°C)

Sodium bisulfite NaHSO₃ 61.6 3000 (20°C)

Potassium metabisulfite K₂S₂O₅ 57.6 250 (0°C)

Sodium metabisulfite Na₂S₂O₅ 67.4 540 (20°C)

Bisulfite addition products, the hydroxysulfonic acids (Suter, 1944), form rapidly with alde-hydes:

HSO₃^–+ R-COH R-CHOH-SO₃^–

All aldehydes form the hydroxysulfonates, but not all ketones react. Diethyl ketone reacts slowly and to a limited extent. Otherwise, only ketones with a methyl group adjacent to the carbonyl or carbonyls that are part of a four- to seven-member carbon ring system will react. Reactions with the sugars are limited to those with a free aldehyde and are much slower and the products are less stable (Gehman and Osman, 1954; Joslyn and Braverman, 1954). Ingram and Vas (1950) found that galactose, mannose, and arabinose reacted rapidly with bisulfite; maltose, lactose, and glucose reacted less rapidly; raffinose reacted very slowly; and fructose and sucrose did not react at all.

Those reacting the most rapidly formed complexes that dissociated the least. Relative percentages of aldehydes reacting with sulfur dioxide and their equilibrium constants are shown in Table 5.3 (Burroughs and Whiting, 1960; Aerny, 1986 a,b; Navara, 1985).

In moldy apples, fermented ciders, and wines, 2,5-diketogluconic acid, 2-oxogluconic acid, 5-oxofructose, L-xylosone, d-threo-2,5-hexodiulose, acetaldehyde, pyruvate, α-ketoglutarate, and galacturonic acid were significant bisulfite binding forces (Burroughs and Sparks, 1964a, 1973;

Lea et al., 2000). In botrytized grapes made into wine, as much as 80% of the total sulfur dioxide may be bound by these types of carbonyls (Blouin, 1963). Rhem (1964) noted that the rate of formation and the amount of sulfonate formed depended on the concentration of the reactive substances, the pH, and the temperature. He also noted that phosphoglyceraldehyde would likely react.

Glucose is by far the most abundant of the reactive aldehydes and ketones in most fruit juices.

Compared with model solutions, natural fruit juices always bind more sulfur dioxide than would be calculated from the glucose present in the juice (Joslyn and Braverman, 1954).

TABLE 5.3

Relative Percentage of Aldehydes that React with Sulfur Dioxide and Their Equilibrium Constants (K)

Compound 1 2 3 K^a

Acetaldehyde 100 99.5 50 1.5 × 10^–6

Pyruvic acid 66 72 32 3 × 10^–4

α-Ketoglutaric acid 47 44 18 5 × 10^–4

Glyoxylic acid — 98 — 8 × 10^-6

L-Xylosone 27 — — —

Oxaloacetic acid — 66 — 2 × 10^–4

Glucuronic Acid — 1 — 5 × 10^–2

Monogalacturonic Acid 2.5 2 — 1.7 × 10²

Rhamnose — — 26 —

Trigalacturonic Acid 2.1 — — —

Arabinose — — 45 —

Xylose 1.1 — 32 —

Fructose — 0.1 0.6 15.0

Glucose 0.12 0.1 7.7 0.9

Malvidin-3-glucoside — 87 90 6 × 10^–5

Note: 1 = Burroughs and Whiting (1960); 2 = Aerny (1986a,b); 3 = Navara (1985).

a Aerny (1986 a,b).

←→

Sulfur Dioxide and Sulfites 147 The amounts of aldehydes, ketones, and other SO₂ binding substances limit the effective use of the added sulfite. Lafon-Lafourcade (1985) reviewed the role of yeast and bacteria in the amounts of these components found in wines. Farris et al. (1983) tested 30 strains of Saccharomyces cerevisiae for production of SO₂ binding materials. They measured acetaldehyde, pyruvate, and α-ketoglutarate. The respective ranges and means for these components were 72 to 287 and 115.5 mg/L, 16 to 42 and 27.5 mg/L, and 15 to 57 and 31.6 mg/L. Farris et al. (1982), from winery trials, selected four yeasts that produced low levels of SO₂ binding compounds.

Farkasˇ et al. (1985) found a reduction in the content of the SO₂ binding materials by the addition of 0.5 mg/L of thiamine. This treatment allows more effective use of SO₂ in wine. Piracci and Spera (1983) reported similar results. Uzuka et al. (1984) also showed that sulfite additions increased acetaldehyde, but they found no increase in pyruvate or α-ketoglutarate.

Bisulfites may also react with nucleotides such as nicotinamide adenine dinucleotide (Meyerhof et al., 1938). Shapiro and Weisgras (1970) also demonstrated that cytosine was transformed to cytidine by bisulfite. There is no evidence that these reactions occur in vivo.

Damant et al. (1989) found a food-coloring dye, sunset yellow, reacted with bisulfite to form a lemon yellow compound. The addition product attached at the carbon 4 of the sunset yellow molecule. Traces of the compound could be found in stored commercial soft drinks that had been heated and contained sulfite.

Sulfur dioxide can loosely bind to anthocyanins. Jurd (1972) suggested the binding site for HSO₃^- is on the 4-position rather than the 2-position (e.g., malvidin monoglucoside). This contrib-utes to the difficulty in the measurement of free sulfur dioxide in highly colored wines. Glories (1984) studied the equilibrium between SO₂ and the anthocyanin–bisulfite complex. In model solutions in the range of 30 to 50 mg/L of SO₂ added, the following formula was used to calculate the equilibrium constant K_s.

Ks = [AHSO₃]/([A⁺][(S) – (AHSO₃^–)]) = 10⁵M^–1 where:

A = total anthocyanins

A⁺ = anthocyanins in the ionized form (colored) AHSO₃ = anthocyanin–bisulfite complex

S = HSO₃^–added (calculated from Henderson-Hasselbach equation)

At 10 mg/L of SO₂ added, the color is reduced by about 25%, and at 50 mg/L of SO₂ added, it is reduced by 80%.

Heintz (1976) reported the occurrence of an addition product of sorbic acid and bisulfite. After approximately 100 days of storage, a potassium bisulfite solution (148 mg/L) with sorbic acid (200 mg/L) contained 62% less sulfur dioxide compared to a standard solution of potassium bisulfite with no added sorbic acid. This could be significant in wine because the reduction of free sulfur dioxide could result in the malolactic bacteria being available to act on the remaining sorbic acid to form 2-ethoxyhexa-3,5-diene (Crowell and Guymon, 1975).

Allyl isothiocyanate in mustard was shown to react with sulfites used as antioxidants to form allylaminothiocarbonyl sulfonate (Cejpek et al., 1998). The reaction affected flavor of the mustard by reducing pungency.

Undoubtedly other reactions with sulfur dioxide take place that could reduce the amounts of available sulfur dioxide, but these have been reported as the primary reactions.

Margheri et al. (1986) and Bach and Hess (1983) could not find any correlation between amino acid levels in the medium and the accumulation of SO₂ binding compounds. Dittrich and Barth (1984) did find correlations between the SO₂ binding substances and wineries and grape source.

They analyzed 544 German wines. Of the three major SO₂ binding components, pyruvate was always found in the smallest amounts. Somers and Wescombe (1987) noted that wines that had undergone malolactic fermentation decreased significantly in the SO₂ binding components, with a corresponding increase in free sulfite.