Sulfur dioxide (S02) and its salts (potassium sulfite [K2S03], sodium sulfite [Na2S03], potassium bisulfite [KHS03], sodium bisulfite [NaHS03], potassium metabisulfite [K2S205], and sodium metabisulfite [NaS205]) have been used as disinfectants since the time of the ancient Greeks and Romans (37). Although sulfites now have multiple uses as food additives, their original purpose was as an antimicrobial. Sulfites are used primarily in fruit and vegetable products to control spoilage and fermentative yeasts, and molds on fruits and fruit products (eg, wine), acetic acid bacteria, and malolactic bacteria (37). In addition to use as antimicrobials, sulfites act as antioxidants and inhibit enzymatic and nonenzymatic browning in a variety of foods. Their primary application is in fruits and vegetable products, but they are also used to a limited extent in meats.
The most important factor impacting the antimicrobial activity of sulfites is pH. The inhibitory effect of sulfites is most pronounced when the acid or S02 • H20 is in the un-dissociated form. As the pH decreases, the proportion of S02 • H20 increases and the bisulfite (hso3 ) ion concentration decreases. Therefore, their most effective pH range is less than 4.0. Sulfites show increased effectiveness at low pH because the unionized sulfur dioxide can pass across the cell membrane in this form.
Sulfites, especially as the bisulfite ion, are very reactive. These reactions not only determine the mechanism of action of the compounds, they also influence antimicrobial activity. For example, sulfites form addition compounds (a-hydroxysulfonates) with aldehydes and ketones.
Sulfur dioxide is fungicidal even in low concentrations against yeasts and molds. The inhibitory concentration range of sulfur dioxide is 0.1 to 20.2 /xg/ml for Saccharo-myces, Zygosaccharomyces, Pichia, Hansenula, and Candida species (37). Sulfur dioxide at 25 to 100 //g/ml inhibits Byssochlamys nivea growth and patulin production in grape and apple juices (38). Against bacteria, sulfur dioxide is more inhibitory to Gram-negative rods than to Grampositive rods. Gram-negative bacteria susceptible to sul fites include Salmonella, E. coli, Citrobacter, Yersinia enterocolitica, Enterobacter, Serratia marcescens, andHaf-nia (39).
Sulfur dioxide is used to control the growth of undesirable microorganisms in fruits, fruit juices, wines, sausages, fresh shrimp, and acid pickles and during extraction of starches (37). It is added at 50 to 100 mg/L to expressed grape juices used for making wines to inhibit molds, bacteria, and undesirable yeasts. At appropriate concentrations, sulfur dioxide does not interfere with wine yeasts or with the flavor of wine. The optimum level of sulfur dioxide (50-75 mg/L) is maintained to prevent postfermentation changes by microorganisms. In a few countries, sulfites may be used to inhibit the growth of microorganisms on fresh meat and meat products. Sulfite or metabisulfite added in sausages is effective in delaying the growth of molds, yeast, and salmonellae during storage at refrigerated or room temperature (39). Sulfur dioxide restores a bright color but may give a false impression of freshness.
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