Sulfites, reaction with quinones

Possible toxic reactions of sulfur dioxide are also indicated in Table I. The reaction of bisulfite with aldehydes has a classic position in biochemistry since Neuberg demonstrated in 1918 that the products of fermentation by yeast were altered by the addition of sodium sulfite, which caused the production of equal amounts of the bisulfite addition compound of acetaldehyde and of glycerol. This was concomitant with the blockage of conversion of acetaldehyde to ethanol. Addition compounds can also be formed with quinones and with ,/ -unsaturated compounds. None of these reactions has been adequately assessed as a possible contributor to toxicity. 

SC, sulfite, bisulfite, metabisulfite), which inhibit PPO and may combine with quinones or reduce quinones to phenols, thereby preventing pigment formation (4, 9-10). Sulfites can produce acute allergic reactions in some asthmatics, however, with serious if not lethal consequences (11-12). Therefore, the Food and Drug Administration has banned the use of sulfites in fruits and vegetables served or sold raw to consumers (13) and has proposed restrictions on the use of sulfites in certain fresh potato products (14). 

In its catalyzed form [catalyzed with hydroquinone (HQ), benzo-quinone, or copper], DEHA has a very fast reaction rate, almost as fast as catalyzed sulfite. Hydroquinone is the most popular catalyst for DEHA, and it is likely that the rapid reaction rate is, in part at least, due to the catalyst simply acting as an oxygen scavenger in its own right. 

There are indications that another type of catalysis is present in the reaction between hydroquinone and silver ions in alkaline solution. The increase of rate with increasing hydroquinone concentration is greater than direct proportionality. This situation is similar to that observed in the oxygen oxidation of durohydroquinone (tetramethylhydroquinone) (James and Weissberger, 16) where the quinone formed in the reaction catalyzes subsequent oxidation. A direct check on quinone catalysis of the hydroquinone-silver ion reaction was not made, since quinone is unstable in alkaline solution, particularly in the presence of sulfite which reacts with it. Experiments were made, however, on the reaction between durohydroquinone and silver ion. This reaction shows the same dependence of rate upon the square root of the silver ion concentration as the hydroquinone reaction does. Addition of duroquinone to the reaction mixture produces a definite acceleration, as shown in Table II. 

Neutral and Alkaline Sulfite Pulping In neutral sulfite pulping the most important reactions of lignin are restricted to phenolic lignin units only. The first stage always proceeds via the formation of a quinone methide with simultaneous cleavage of an a-hydroxyl or an a-ether group (Fig. 7-12). At least in noncyclic structures, the quinone methide is readily attacked by a... 

Fig. 7-12. Reactions of phenolic /8-aryl ether and a-ether structures (1) during neutral sulfite pulping (Gierer, 1970). R = H, alkyl, or aryl group. The quinone methide intermediate (2) is sulfonated to structure (3). The negative charge of the a-sulfonic acid group facilitates the nucleophilic attack of the sulfite ion, resulting in /8-aryl ether bond cleavage and sulfonation. Structure (4) reacts further with elimination of the sulfonic acid group from a-position to form intermediate (5) which finally after abstraction of proton from /8-position is stabilized to a styrene-/8-sulfonic acid structure (6). Note that only the free phenolic structures are cleaved, whereas the nonphenolic units remain essentially unaffected.

Reaction of amino groups of amino acids, peptides, and proteins with carbonyl groups of carbohydrates or with tyrosine-derived quinone groups encompass changes respectively called nonenzymatic and enzymatic browning reactions (57-75). Such changes may adversely affect the appearance, taste, nutritional quality, and safety of many foods. Until recently, sodium sulfite has been used to inhibit browning... 

Substances with reducing effects are sulfur dioxide and sulfites, which are also used as preservatives, inhibitors of enzymatic browning reactions and inhibitors of non-enzymatic browning reactions. The bleaching activity is mainly based on their ability to reduce the primary products of enzymatic browning reactions - quinones, whose subsequent reactions would otherwise lead to undesirable discoloration in dried fruits, vegetables, potatoes and other products. Furthermore, these compounds are used for the bleaching of hops, lecithin concentrates, mushrooms, nuts and fish products. 

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