Sulfites, thiamin

Sulfur Dioxide and Sulfites. Sulfur dioxide [7446-09-5], SO2, sodium bisulfite [15181-46-1], NaHSO, and sodium metabisulfite [23134-05-6] ate effective against molds, bacteria, and certain strains of yeast. The wine industry represents the largest user of sulfites, because the compounds do not affect the yeast needed for fermentation. Other appHcations include dehydrated fmits and vegetables, fmit juices, symps and concentrates, and fresh shrimp (79). Sulfites ate destmctive to thiamin, and cannot be used in foods, such as certain baked goods, that ate important sources of this vitamin. 

In a solution of sodium sulfite at pH 5, thiamin is cleaved by what appears to be a nucleophilic displacement reaction on the methylene group to give the free thiazole and a sulfonic acid. 

SULFUR (In Biological Systems). Sulfur, in some form, is required by all living organisms. It is utilized in various oxidation stales, including sulfide, elemental sulfur, sulfite, sulfate, and thiosulfate by lower forms and in organic combinations by all. The more important sulfur-containing organic compounds include the amino acids (cysteine, cystine, and methionine, which are components of proteins) the vitamins thiamine and... 

However, this reaction cannot possibly account for the inhibitory effect of sulfite. It is thought that sulfur dioxide reacts with the degradation products of the amino sugars, thus preventing these compounds from condensing into melanoidins. A serious drawback of the use of sulfur dioxide is that it reacts with thiamine and proteins, thereby reducing the nutritional value of foods. Sulfur dioxide destroys thiamine and is therefore not permitted for use in foods containing this vitamin. 

Thiamin is labile to sulfite, which cleaves the methylene bridge. The reaction is slow at acid pH, but rapid above pH 6. Sulfite treatment of dried fruit and other foods results in more or less complete loss of thiamin. 

Thiaminolytic enzymes are found in a variety of microorganisms and foods, and a number of thermostable compounds present in foods (especially polyphenols) cause oxidative cleavage of thiamin, as does sulfite, which is widely used in food processing. The products of thiamin cleavage by sulfite and thiaminases are shown in Figure 6.1. 

Beside the conversion of endogenic sulfur compounds the addition of S-compounds like sulfite, as an antimicrobial agent, antioxidant and enzyme inhibitor (75) or like thiamine (vitamin Bl), as a nutrient for yeasts, are allowed in the EEC (within defined maximum values). Furthermore chemical reactions like sulfite addition to aldehydes, Maillard reaction or Strecker degradation play an important role with regard to the sulfur chemistry of wines. 

A few aspects of the chemistry of the vitamin should be noted. Thiamin is stable at mildly acidic pH but unstable and easily destroyed at pH 8.0 or higher, especially with heating. Thiamin is also destroyed by sulfite, especially at pH 6.0 or higher. 

In an acidic environment, it is protonated, and occurs mainly as sulfurous acid. In an alkaline environment, the protons dissociate, and it occurs mainly as bisulfite. Sulfurous acid is in an equilibrium with sulfur dioxide, which can leave a solution of water to enter atmosphere. The toxic effects of sulfite arise from its reactions with sulfhydryl groups, aldehyde groups, and ketones. Sulfite can also react with enz5nne-bound NAD and FAD. It is well known that the sulfite added to foods can react with the thiamin in the food, destroying this vitamin. The reaction of sulfite with sulfhydryl groups (R— SH) results in its conversion to an S-sulfonate group (R—S—SO3-). 

Like all sulfites, potassium metabisulfite is not recommended for use in foods that are a source of thiamin, owing to the instability of the vitamin in their presence. Such foods include meat, raw fruits and vegetables, fresh potatoes, and foods that are a source of vitamin B12. A specification for potassium metabisulfite is contained in the Food Chemicals Codex (FCC). 

An undesirable reaction of sulfite in food is the cleavage of thiamin by means of an attack at the pyrimidin moiety (Zoltewicz et al., 1984). This was one of the reasons for a ban, in many countries, on the use of sulfite in meat. Another reason is the preserving effect on the meat color, which makes stale meat look as if it were fresh. Sulfite, however, is unable to reduce metmyoglobin back to myoglobin (Wedzicha and Mountfort, 1991). 

Food additives can enhance the safety and nutritional quality of a food or vice versa. By preventing oxidation of fat and easily oxidized vitamins, antioxidants ensure that safety is enhanced and the intended nutritional value of the food is delivered. Antibrowning agents such as sulfite retain phytochemicals and vitamins A and C but lower the amount of thiamine, folate, and pyridoxal. Sorbic acid can prevent... 

Potassium Bisulfate Potassium Sulfate Saccharin Silver(I) Sulfide Sodium Cyclamate Sodium Sulfite Sodium Thiosulfate Sulfur Dioxide Sulfuric Acid Thiamine... 

Figure 5.3 Alkali-induced transformations of thiamin and cleavage by sulfite. (A) In alkaline medium, thiamin (1) can be slowly transformed to the so-called yellow form (4). This transformation involves the formation of a pseudobase (2) and opening of the thiazolium ring to form a thiolate (3).

Thiamin is unstable in alkaline solutions and is cleaved by sulfite. 

In the elucidation of the structure of thiamine (1934-1936) Williams and collaborators have had a major share, but important contributions also came from the laboratories of Todd and Bergel in Great Britain and Windaus and I. G. Farben in Germany. Williams et al. made the important observation that thiamine is quantitatively split by neutral sulfite solution at room temperature as follows ... 

The point of attachment on the pyrimidine fragment was of course already marked by the position of the sulfonic group introduced during the sulfite cleavage of the vitamin. Thus, in 1936, the structure I could be assigned to thiamine, ten years after the crystalline vitamin was first obtained. 

Sulfite at room temperature cleaves the molecule into its pyrimidine and thiazole parts. Ferricyanide in alkedine solution oxidizes thiamine to thio-chrome, a compound with intense blue fluorescence. 

Indeed both -lactylthiamine pyrophosphate (XX) and a-hydroxyethyl-thiamine pyrophosphate (XXI) have been isolated and identified as products after incubation of pyruvate with a purified carboxylase preparation " . When [2- - C]pyruvate is used, the radioactivity is found in the thiazole part of the molecule after sulfite cleavage of XXL Acetaldehyde is formed from pyruvic acid by yeast carboxylase by enzymic cleavage of intermediate XXI, Uberating thiamine pyTophosphate . XXI has also been identified as intermediate in the formation of acetyl-coenzyme A from pyruvic acid by p3u uvic oxidase . The transketolase reaction has been shown to proceed via a gly-colaldehyde-enzyme intermediate here one may expect to find dihydroxy-ethylthiamine pyrophosphate as active glycol-aldehyde . Such experiments strongly support Breslow s concept of the reaction mechanism. 

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