PRESERVATIVES | Permitted Preservatives – Sulfur Dioxide

PRESERVATIVES | Permitted Preservatives – Sulfur Dioxide

Permitted Preservatives – Sulfur Dioxide K Prabhakar, Sri Venkateswara Veterinary University, Tirupati, India EN Mallika, NTR College of Veterinary Sc...

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Permitted Preservatives – Sulfur Dioxide K Prabhakar, Sri Venkateswara Veterinary University, Tirupati, India EN Mallika, NTR College of Veterinary Science, Gannavaram, India Ó 2014 Elsevier Ltd. All rights reserved. This article is a revision of the previous edition article by K Prabhakar, K S Reddy, volume 3, pp 1750–1754, Ó 1999, Elsevier Ltd.

Introduction Sulfur dioxide is an important chemical extensively used in the processing and preservation of foods of both plant and animal origin. It has been known since ancient times as a sanitizing agent or antiseptic. It gained popularity as a preservative owing to its apparent lack of toxicity in mammals. Its use was widespread in the United States and other countries in the Western hemisphere until the early part of the twentieth century when incidents of abuses like masking the initial stage of spoilage in foods led to legislation to check indiscriminate and fraudulent commercial applications. Sulfur dioxide is a colorless gas with a characteristic odor. It is highly soluble in water and liquefies at 10  C. It is used in gaseous or liquefied form, or as its neutral and acid salts.

very effective for purposes of disinfection. Grapes and cut fruits are exposed to fumes of burning sulfur before dehydration or transportation.

Salts of Sulfurous Acid Sulfite, bisulfate, and metabisulfite are extensively used in foods and beverages. They can be easily applied in dry form or as solutions. They are stable, economical, and comparatively free from heavy metal impurities. Sulfite solutions are easily absorbed by fruits, which are dipped in the solution before freezing or dehydration.

Liquid Sulfur Dioxide Sulfur Compounds The sulfur dioxide-generating compounds with application in the food industry are as follows: l

Sulfur dioxide as a gas Sulfurous acid l Salts of sulfurous acid, such as sodium sulfite, sodium bisulfite, and potassium sulfite l Hydrosulfurous acid and its salt, sodium hydrosulfite l Pyrosulfurous acid and its salt, sodium pyrosulfite or metabisulfite.

Liquid SO2 is free from impurities and is commonly used in wineries. Accurately measured quantities can be incorporated. Special steel containers are required for storage and transportation, making it a costly source of SO2.

l

The sulfur dioxide content of these compounds is listed in Table 1.

Sulfur Dioxide Gas The gas is obtained directly by burning sulfur from natural sources. It is the cheapest of all the sources of sulfur dioxide and

Table 1 Approximate theoretical available sulfur dioxide content of various sources Compounds

Formula

Availability (%)

Liquid sulfur dioxide Sulfurous acid (6%) Potassium sulfite Sodium sulfite Potassium bisulfite Sodium bisulfite Potassium metabisulfite Sodium metabisulfite

SO2 H2SO3 K2SO3 Na2SO3 KHSO3 NaHSO3 K2S2O5 Na2S2O5

100.00 6.00 33.00 50.8 53.3 61.6 67.4 57.7

From Joslyn, M.A., Braverman, J.B.S., 1954. The chemistry and technology of the pretreatment and preservation of fruit and vegetable products with sulphur dioxide and sulfites. Adv. Food Res. 5, 97–154.

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Range of Foods to Which Sulfites May Be Added The range of foods into which sulfur dioxide is incorporated includes fruits, vegetables, fruit juices and concentrates, syrups, wines and jams, and to a lesser extent prawns, fish, minced meats, sausages, and mushrooms. The maximum permissible levels of SO2 in some important foods as specified by the Preservatives in Food Regulation 1979 for the United Kingdom are listed in Table 2. Only slight variations exist between the maximum levels permitted in various products in different countries, because of universal concern for consumer protection.

Antimicrobial Action of Sulfur Dioxide Sulfur dioxide is highly soluble in water and forms sulfurous acid, which dissociates into bisulfite or sulfite depending on the pH. Undissociated sulfurous acid is claimed to be the main antimicrobial agent inhibiting bacteria, yeasts, and molds. The possible mechanisms of inhibition by sulfurous acid are attributed to the following: l

Reaction of bisulfite with acetaldehyde in the cell Reduction of essential disulfide linkages in enzymes l Formation of bisulfite addition compounds that interfere with respiratory reactions involving nicotinamide adenine dinucleotide (NAD). l

Encyclopedia of Food Microbiology, Volume 3

http://dx.doi.org/10.1016/B978-0-12-384730-0.00264-0

PRESERVATIVES j Permitted Preservatives – Sulfur Dioxide Table 2

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Composition of the Food and Food Products

Maximum UK permitted levels of SO2

Food product

Maximum SO2 level (mg kg1)

Fruits, fruit pulp, tomato pulp Fruit spread Grape juice products Jams Mushrooms, frozen Pickles Raw peeled potatoes Salad dressing Sauces Soft drinks for consumption without dilution Dehydrated potatoes Dehydrated cabbage Yogurt Beer Wine Flour for biscuits Desserts, fruit-based milk and cream Sausages or sausage meat Hamburgers or similar products

350 100 70 100 50 100 50 100 100 70 550 2500 60 70 450 200 100 450 450

Foods containing higher levels of components that form inert complexes on reaction with SO2 cannot be effectively preserved with SO2 alone, especially at room temperature.

Influence of pH The antimicrobial action of SO2 is more effective in foods with acidic pH. Two to four times as much SO2 is required to inhibit growth at pH 3.5 compared with pH 2.5. At higher pH values like 7, sulfites do not appear to have significant inhibitory action on yeasts and molds and very high levels are required to control growth of bacteria. Acid is commonly added to lower the pH of foods, enabling preservation with lower levels of SO2. Sulfites are being used in antimicrobial edible coatings.

Effect of Heat Heating to high temperatures drives off SO2 from foods and considerably reduces the antimicrobial effects. On heating, the sulfur compound decomposes and the free component escapes by volatilization. At pasteurization temperatures, it is reported to increase the thermal death rate of microorganisms present and enables more rapid destruction of microbes.

Factors Influencing Antimicrobial Action Initial Microbial Population and the Stage of Growth

Temperature of Storage

The initial level of bacterial contamination affects the preservative efficacy of SO2. Minced meat samples containing 300 ppm of sulfur dioxide during refrigerated storage revealed spoilage on the 6th day for samples with an initial contamination level of 7.6  107 cfu g1, compared with spoilage on the 13th day for samples with an initial microbial load of 6.9  105 cfu g1.

A synergistic action of lower temperatures and SO2 addition is claimed by some investigators, as more pronounced bacteriostatic effects were observed in minced meat samples stored at lower temperatures than at higher temperatures (Table 3). It is generally assumed that sulfite preservation of foods at room temperature competes with refrigerated storage of foods without any additives.

Type of Microorganisms Present Strains like acetic acid bacteria, yeasts, and molds are effectively eliminated through incorporation of SO2. The inhibitory effect also depends on the levels of SO2 incorporated and maintained. Coliaerogenous bacteria were not affected by 150 ppm, but at 450 ppm, their multiplication was totally inhibited. Cyclopiazonic acid produced by Aspergillus species and Penicillium was inhibited by potassium metabisulfite.

Table 3 Approximate shelf life of minced meats at different storage temperatures, with or without SO2 Preservation storage temperatures (  C)

Without SO2

With SO2

7 15 22

3–5 days 1–2 days <20 h

13 days 6–7 days 1–2 days

Sulfur Dioxide-Producing Sources Equilibrium between various forms of SO2-undissociated sulfurous acid, free sulfite, or bisulfite ions and hydroxysulfonates is determined by pH, temperature, composition, and storage condition of foods.

Free and Bound Components of Added Sulfur Dioxide The free or unbound component of added SO2 has the significant antimicrobial action. It is claimed that the inhibition power of the free component of added SO2 is 30–60 times more effective than that of the bound component.

Behavior of Sulfur Dioxide in Foods Several reaction products are formed through reversible and irreversible reactions in SO2-treated foods. The amounts of interaction products vary in different foods depending on the processing and storage conditions. Because of these reactions, SO2 has multifarious functions in addition to its antimicrobial effects. It can act as an antioxidant, as a bleaching agent, as a color fixative, and as an inhibitor of enzymic discolorations and nonenzymic browning. The interaction products of reversible reactions of sulfites do not pose serious problems as most of them are unstable. Addition of SO2 to menadione,

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PRESERVATIVES j Permitted Preservatives – Sulfur Dioxide

a water-soluble synthetic form of vitamin K, is reported to result in the formation of a reversible sulfonate adduct, which readily dissociates in animals to become a source of vitamin K. Irreversible reactions, however, like cleavage of thiamin have nutritional significance.

Inhibition of Enzymic Discoloration Enzymic browning is a result of processes involved in the production of pigments from enzymically oxidized phenolic compounds of natural origin. Sulfites form inactive complexes with enzymes or combine with breakdown products to form stable complexes, thus inhibiting enzymeinduced formation of abnormal colors in fruits and vegetables during processing and storage. Sulfite dips are used to control discoloration due to enzymic browning in frozen stored fruits and vegetables, as food enzymes are not destroyed by freezing. The development of white specks during storage in prawns can be controlled with the use of 10% salt and 0.04% sodium metabisulfite solution without loss of nutrients. The formulations based on 4-hexyl-resorcinol and sulfides can delay the appearance of melanosis in prawns by inhibiting polyphenoloxidase activity. Commercial sulfides can inhibit luminescent bacterial growth.

Inhibition of Nonenzymic Browning Nonenzymic browning involves reactions between amino groups and carbonyl groups leading to the formation of insoluble, dark-colored compounds with a bitter taste. Sulfur dioxide is the most commonly used chemical to inhibit nonenzymic browning in foods. Inhibition of browning reactions by SO2 is attributed to the stabilization of the intermediate compounds formed. It combines reversibly with reducing sugars and aldehyde intermediates and irreversibly with certain unsaturated aldehyde intermediates. The appearance of heat-processed and canned vegetables, fruits, fish, and comminuted meat products like sausages can be improved through the inhibition of nonenzymic browning. White wines are treated with SO2 gas or metabisulfite to inhibit nonenzymic brown discoloration during storage. Sulfur dioxide also inhibits nonenzymic browning in dehydrated fruits and vegetables during storage at ambient temperatures.

Antioxidant Properties Sulfur dioxide in the form of a gas or a sulfite dip during processing and storage of dehydrated vegetables, fruits, and grape juice prevents loss of ascorbic acid. It is used in canned tomato sauce to prevent carotenoid oxidation and to preserve the bright color. It is added to beer as a solution in water to inhibit adverse changes in flavor due to oxidation by dissolved oxygen. Lipids in sausages and comminuted meat products are protected from oxidation changes if sulfite or metabisulfite is included. It also prevents the oxidation of the essential oils and carotenoids and inhibits development of abnormal color and flavor in citrus juices.

Reducing and Bleaching Actions Sulfurous acid and the acid sulfites reduce many colored compounds to colorless derivatives. Dried cut fruits with slight darkening can be almost completely restored to their original color by treating with SO2 probably owing to the formation of colorless compounds. In sugar processing, SO2 bleaches the naturally occurring pigments such as anthocyanins and other colored nonsugars and also reduces darkening during evaporation and crystallization owing to its combination with reducing sugars. As a reducing agent, it keeps reductones in the inactive reduced form rather than in the active dehydro form. The attractive bright pink color of sulfited minced meat samples is maintained until spoilage during storage at 7–15  C. This color fixation property of SO2 is attributed to its ability to maintain heme iron in the reduced state. Studies revealed increased consumer preference for cooked sulfited minced meat samples. Sulfites also prevent gray discoloration in minced meats and raw sausages when they are exposed to air. Sodium metabisulfite has been used extensively in the mushroom industry as a whitening agent.

Losses from Binding to Food Constituents Sulfur dioxide is highly reactive with other components in foods; hence it does not persist for long periods. A large part of the SO2 added to foods remains fixed or bound. Glucose, aldehydes, ketonelike substances, pectin, and so on present in foods determine the extent of binding of added SO2 in foods. However, 0.2% potassium metabisulfite with 2% citric acid can extend the shelf life of tofu without disturbing its sensory properties and without losses. Glucose binds SO2 in a reversible manner. The extent of binding is reported to be related to the total concentration of soluble solids in the food. Combination of bisulfites with sugars is much slower than with aldehydes and ketones and the products formed are relatively less stable. Sulfur dioxide after combination with sugars or aldehydes exercises very little antimicrobial action. When increased levels of SO2 are added to foods, the proportion of the free component increases. At low pH, the combination of SO2 with glucose is delayed, ensuring that more time is available for the SO2 to act on the microorganisms present. Levels of SO2 decrease considerably during storage. Loss of SO2 in sealed bottles of wine initially containing up to 400 ppm ranges between 20 and 50%. In minced meat samples incorporating 450 ppm of SO2, levels started to decrease within a few hours. During storage at 7  C, levels of SO2 decreased to around 295 ppm after the first day, to 270 ppm on the third day, to 240 ppm on the fifth day, and stabilized at 200 ppm on days 7–13, after which spoilage was observed. In samples stored at 15  C, residual SO2 levels decreased to 350 ppm on the first day, 280 ppm on the third day, and 220 ppm on the fifth day. Spoilage was noticed on the sixth day of storage when the residual level was 120 ppm. Reduction in the concentration of SO2 is faster at higher temperatures and it also coincides with increased microbial loads.

PRESERVATIVES j Permitted Preservatives – Sulfur Dioxide

Importance of Species and Strain Tolerance Sulfur dioxide is reported to have selective antiseptic action. Acetic acid bacteria, lactic acid bacteria, and coliaerogenous bacteria are more sensitive than others. This compound is most effective against Gram-negative bacteria. Several studies indicate a general decline in the growth of spoilage organisms and also of added cultures of Clostridium botulinum, Clostridium sporogenes, Clostridium perfringens, and Salmonella typhimurium in minced meats with SO2 levels of 450 ppm. Bactericidal effect was found to be significant within 3 h of the addition of Salmonella enteritidis and Yersinia enterocolitica. Germination of bacterial spores also was found to be affected. In minced meats without preservative, all groups of bacteria multiply throughout the storage period, whereas in sulfited samples only a portion of the microflora causes spoilage. During storage of minced meat samples with 450 ppm of SO2 at 7  C, coliforms, salt-tolerant bacteria and streptococci did not reveal significant changes in their numbers. Lactobacilli, however, were significantly inhibited by day 9 when spoilage was noticed. These organisms play a major role in the spoilage of vacuum-packaged meats during refrigerated storage. It is to be explored whether extension of refrigerated storage life of vacuum-packaged meats is possible with the addition of SO2 or sulfites in a safe way. In a minced meat sample with 450 ppm of SO2 stored at 15  C, lactobacilli, salt-tolerant bacteria, and enterococci showed significant increases after a lag phase of 4–5 days. A combination of 0.6% chitosan with 170 ppm of sulfite retarded growth of spoilage organisms for 24 days. Among yeasts, fermentative types are more resistant than true aerobic species. Certain desirable strains of yeasts required for fermentation are made sulfite resistant through gradual sensitization. Such resistant yeasts are utilized for fermentation in winemaking at levels of SO2 at which other undesirable strains of yeasts and molds do not develop.

Toxic Effects in Humans The extensive use of SO2 in the form of sulfites, bisulfites and metabisulfites in foods and beverages the world over indicates that allergic reactions and residual toxicity problems in consumers are almost nil in the normal pattern of human exposure. In spite of its high reactivity with biologically important molecules, SO2 is oxidized to sulfate by sulfite oxidase enzyme and excreted in urine safely. The enzyme sulfite oxidase is reported to be present at higher than adequate levels in liver and other tissues of the human body. The capacity of the mammalian sulfite oxidase for sulfite oxidation is reported to be extremely high in relation to the normal sulfite load expected from both endogenous and exogenous sources. Sulfites are known to destroy thiamin (vitamin B1) in foods by cleavage of thiamin into 4-methyl-5-hydroxyethyl thiazole and the sulfonic acid of 2, 5-dimethyl-4-aminopyrimidine. This cleavage is completed within 24–48 h at a pH of 5.0 and at room temperatures. Hence, sulfites are not used in foods that are major sources of thiamin. Studies

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have revealed, however, that humans consuming up to 200 mg of SO2 per day showed no signs of thiamin deficiency. This reaction need not be taken as a serious disadvantage since some nutrient losses are expected in almost all popular commercial methods of food preservation. Adverse effects were not observed even with chronic sulfite administration. Chronically ingested sulfite does not accumulate in the tissues or reach levels hazardous to human health because of its rapid metabolic removal. However, problems may occur in humans affected with sulfite oxidase deficiency disease. The possibilities of undesirable interactions between SO2 and other dietary components or cellular constituents leading to interference in metabolic processes or damage to the structural integrity of proteins have not been evidenced in human systems; hence, SO2 is considered to be a safe preservative if used in permitted levels. A few cases of allergic reactions observed in asthma patients after consumption of sulfited foods such as pickled onions were found to be due to the presence of very high levels of SO2. If foods are processed at permitted levels of SO2, such problems may not arise.

Conclusion The rapid strides made by the processed and convenience food industry would not have been possible without the use of traditional and chemical preservatives. In view of concerns about potential toxicity to the consumers in the long run, the worldwide trend is to restrict the use of these preservatives to well below their legally permitted levels. No single permitted preservative fulfills the needed requirements of effectiveness and absolute safety. Sulfur dioxide is no exception to this, in spite of its proven effectiveness and safety as indicated by its continued usage in a wide range of foods. Future development will lead to the optimum utilization of combinations of permitted preservatives so that their individual levels of incorporation can be greatly reduced without compromising the safety and stability of food products. A combination of 50 ppm of sorbate and 50 ppm of SO2 is reported to have inactivated yeasts such as Saccharomyces cerevisiae during heating, even in the presence of glucose. The food industry requires the continued use of preservatives like SO2 in traditional ways until synergistic combinations have undergone detailed investigations on enhanced safety.

See also: Preservatives: Classification and Properties; Preservatives: Traditional Preservatives – Organic Acids; Preservatives: Traditional Preservatives – Wood Smoke; Preservatives: Permitted Preservatives – Benzoic Acid; Permitted Preservatives – Hydroxybenzoic Acid; Permitted Preservatives: Nitrites and Nitrates; Preservatives: Permitted Preservatives – Sorbic Acid; Spoilage of Animal Products: Seafood; Wines: Microbiology of Winemaking; Production of Special Wines; Wine Spoilage Yeasts and Bacteria; Advances in Processing Technologies to Preserve and Enhance the Safety of Fresh and Fresh-Cut Fruits and Vegetables.

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Further Reading Alvarez, O.M., Caballero, M.E.L., Montero, P., Guillen, M.G., 2005. A 4-hexyl resorcinol- based formulation to prevent melanosis and microbial growth in chilled Tiger prawn (Marsupenaeus japonicus) from aqua culture. Journal of Food Science 70 (9), M 415–M 422. Austin, R.K., Clay, W., Phimphivong, S., Smilanick, J.L., Henson, D.J., 1997. Patterns of sulfite residue persistence in seedless grapes during three months of repeated sulfur dioxide fumigations. American Journal of Enology and Viticulture 48 (1), 121–124. Burke, C.S., 1980. International legislation. In: Tilbury, R.H. (Ed.), Developments in Food Preservatives, vol. 1. Applied Science Publishers, London, p. 25. Cerrutti, P., Alzamora, S.M., Chirife, J., 1988. Effect of potassium sorbate and sodium bisulfite on thermal inactivation of Saccharomyces cerevisiae in media of lowered water activity. Journal of Food Science 53 (6), 1911–1912. Chauhan, S.K., Tyagi, S.M., Chauhan, G.S., 1998. Effect of various preservatives on the shelf life of Tofu. Journal of Food Science and Technology 35, 72–73. Duvenhage, J.A., 1994. Control of post-harvest decay and browning of litchi fruit by sodium metabisulphite and low pH dips – an update. In: Litchi Year Book, vol. 6. South African Litchi Growers Association. 36–38. Gray, T.J.B., 1980. Toxicology. In: Tilbury, R.H. (Ed.), Developments in Food Preservatives, vol. 1. Applied Science Publishers, London, p. 53. Gunnison, A.F., 1981. Sulphite toxicity: a critical review of in vitro and in vivo data. Food Cosmet Toxicology 19, 667–682. Joslyn, M.A., Braverman, J.B.S., 1954. The chemistry and technology of the pretreatment and preservation of fruit and vegetable products with sulphur dioxide and sulfites. Advances in Food Research 5, 97–154.

Krishna Reddy, V., Reddy, S.M., 1990. Efficacy of food preservation in the control of cyclopiazine acid production by penicillium griseofulvum. Journal of Food and Science Technology 27 (3), 180–181. Premi, B.R., Sethi, V., Maini, S.B., 1999. Effects of steeping preservatives on the Aonia (Emblica officinalis Gaerln) fruits during storage. Journal of Food Science and Technology 36, 244–247. Roberts, A.C., McVeeny, D.J., 1972. The uses of sulphur dioxide in the food industry. A review. Journal of Food Technology 7, 221–238. Roller, S., Sagoo, S., Board, R., Mahony, T.O., Caplice, E., Fitzgerald, G., Fogden, M., Owen, M., Fletcher, M., 2002. Novel combination of chitosan, carnocin and sulphite for preservation of chilled pork sausage. Meat Science. 62 (2), 165–177. Sinskey, A.J., 1980. Mode of action and effective application, pp. 111–136. In: Tilbury, R.H. (Ed.), Developments in Food Preservatives, vol. 1. Applied Science Publishers, London, p. 111. Stammati, A., Zanetti, C., Pizzoferrato, L., Quattrucci, E., Tranquilli, G.B., 1992. In vitro model for the evaluation of toxicity and anti nutritional effects of sulphites. Food Additives and Contaminants 9 (5), 551–560. Studdert, V.P., Labuc, R.H., 1991. Thiamin deficiency in cats and associated with feeding meat preserved with sulphur dioxide. Australian Veterinary Journal 68 (2), 54–57. Taylor, S.L., Bush, R.K., 1986. Sulfides as food ingredients. Food Technology 40 (6), 47. Taylor, S.L., Higley, N.A., Bush, R.K., 1986. Sulfite in foods, uses, analytical methods, residues, fate, exposure assessment, metabolism, toxicity and hypersensitivity. Advances in Food Research 30, 1. Trenerry, 1996. The determination of the sulphite content of some foods and beverages by capillary electrophoresis. Food Chemistry 55 (3), 299–303. Usseglio-Tomasset, L., 1992. Properties and use of sulphur dioxide. Food Additives and Contaminants 9 (5), 399–404.