Preservatives

Preservatives or antimicrobial agents play an important role in today's supply of safe and stable foods. Increasing demand for convenience foods and reasonably long shelf life of processed foods make the use of chemical food preservatives imperative. Some of the commonly used preservatives—such as sulfites, nitrate, and salt—have been used for centuries in processed meats and wine. The choice of an antimicrobial agent has to be based on a knowledge of the antimicrobial spectrum of the preservative, the chemical and physical properties of both food and preservative, the conditions of storage and handling, and the assurance of a high initial quality of the food to be preserved (Davidson and Juneja 1990).

Benzoic Acid

Benzoic acid occurs naturally in many types of berries, plums, prunes, and some spices. As an additive, it is used as benzoic acid or as benzoate. The latter is used more often because benzoic acid is sparsely soluble in water (0.27 percent at 18°C) and sodium benzoate is more soluble (66.0 g/100 mL at 20°C). The undissociated form of benzoic acid is the most effective antimicrobial agent. With a pKa of 4.2, the optimum pH range is from 2.5 to 4.0. This makes it an effective antimicrobial agent in high-acid foods, fruit drinks, cider, carbonated beverages, and pickles. It is also used in margarines, salad dressings, soy sauce, and jams.

Parabens

Parabens are alkyl esters of p-hydroxyben-zoic acid. The alkyl groups may be one of the following: methyl, ethyl, propyl, butyl, or heptyl. Parabens are colorless, tasteless, and odorless (except the methyl paraben). They are nonvolatile and nonhygroscopic. Their solubility in water depends on the nature of the alkyl group; the longer the alkyl chain length, the lower the solubility. They differ from benzoic acid in that they have antimicrobial activity in both acid and alkaline pH regions.

The antimicrobial activity of parabens is proportional to the chain length of the alkyl group. Parabens are more active against molds and yeasts than against bacteria, and more active against gram-positive than gramnegative bacteria. They are used in fruitcakes, pastries, and fruit fillings. Methyl and propyl parabens can be used in soft drinks. Combinations of several parabens are often used in applications such as fish products, flavor extracts, and salad dressings.

Sorbic Acid

Sorbic acid is a straight-chain, trans-trans unsaturated fatty acid, 2,4-hexadienoic acid. As an acid, it has low solubility (0.15 g/100 mL) in water at room temperature. The salts, sodium, or potassium are more soluble in water. Sorbates are stable in the dry form; they are unstable in aqueous solutions because they decompose through oxidation. The rate of oxidation is increased at low pH, by increased temperature, and by light exposure.

Sorbic acid and sorbates are effective against yeasts and molds. Sorbates inhibit yeast growth in a variety of foods including wine, fruit juice, dried fruit, cottage cheese, meat, and fish products. Sorbates are most effective in products of low pH including salad dressings, tomato products, carbonated beverages, and a variety of other foods.

The effective level of sorbates in foods is in the range of 0.5 to 0.30 percent. Some of the common applications are shown in Table 11-1. Sorbates are generally used in sweetened wines or wines that contain residual sugars to prevent refermentation. At the levels generally used, sorbates do not affect food flavor. However, when used at higher levels, they may be detected by some people as an unpleasant flavor. Sorbate can be degraded by certain microorganisms to produce off-flavors. Molds can metabolize sorbate to produce 1,3 pentadiene, a volatile compound with an odor like kerosene. High levels of microorganisms can result in the

Table 11-1 Applications of Sorbates as Antimicrobial Agents

Products

Levels (%)

Dairy products: aged cheeses, processed cheeses, cottage cheese, cheese

0.05-0.30

spreads, cheese dips, sour cream, yogurt

Bakery products: cakes, cake mixes, pies, fillings, mixes, icings, fudges, toppings,

0.03-0.30

doughnuts

Vegetable products: fermented vegetables, pickles, olives, relishes, fresh salads

0.02-0.20

Fruit products: dried fruit, jams, jellies, juices, fruit salads, syrups, purees, concen

0.02-0.25

trates

Beverages: still wines, carbonated and noncarbonated beverages, fruit drinks, low-

0.02-0.10

calorie drinks

Food emulsions: mayonnaise, margarine, salad dressings

0.05-0.10

Meat and fish products: smoked and salted fish, dry sausages

0.05-0.30

Miscellaneous: dry sausage casings, semimoist pet foods, confectionery

0.05-0.30

Source. Reprinted with permission from J.N. Sofos and F.F. Busta, Sorbic Acid and Sorbates, in Antimicrobials in Foods, P.M. Davidson and A.L. Branen, eds., p. 62,1993, by courtesy of Marcel Dekker, Inc.

Source. Reprinted with permission from J.N. Sofos and F.F. Busta, Sorbic Acid and Sorbates, in Antimicrobials in Foods, P.M. Davidson and A.L. Branen, eds., p. 62,1993, by courtesy of Marcel Dekker, Inc.

degradation of sorbate in wine and result in the off-flavor known as geranium off-odor (Edinger and Splittstoesser 1986). The compounds responsible for the flavor defect are ethyl sorbate, 4-hexenoic acid, 1-ethoxy-hexa-2,4-diene, and 2-ethoxyhexa-3,5-diene. The same problem may occur in fermented vegetables treated with sorbate.

Sulfites

Sulfur dioxide and sulfites have long been used as preservatives, serving both as antimicrobial substance and as antioxidant. Their use as preservatives in wine dates back to Roman times. Sulfur dioxide is a gas that can be used in compressed form in cylinders. It is liquid under pressure of 3.4 atm and can be injected directly in liquids. It can also be used to prepare solutions in ice cold water. It dissolves to form sulfurous acid. Instead of sulfur dioxide solutions, a number of sulfites can be used (Table 11-2) because, when dissolved in water, they all yield active S02.

The most widely used of these sulfites is potassium metabisulfite. In practice, a value of 50 percent of active S02 is used. When sulfur dioxide is dissolved in water, the following ions are formed:

S02 (gas) -> S02 (aq) S02 (aq)+ -» H20 H2S03 H2S03 H+ + HSOf (K, = 1.7 x 10"2) HSO-f —> H+ + S0321 (K2 = 5 x 10-6)

2HS63-->S2O52-+H2O

All of these forms of sulfur are known as free sulfur dioxide. The bisulfite ion (HS03~) can react with aldehydes, dextrins, pectic substances, proteins, ketones, and certain sugars to form addition compounds.

I II

Table 11-2 Sources of SOz and Their Content of Active S02

Chemical Formula Content of Active S02

Sulfur dioxide S02 100.00%

Sodium sulfite, anhydrous Na2S03 50.82%

Sodium sulfite, heptahydrate Na2S03-7 H20 25.41%

Sodium hydrogen sulfite NaHS03 61.56%

Sodium metabisulfite Na2S205 67.39%

Potassium metabisulfite K2S2Os 57.63%

Calcium sulfite CaS03 64.00%

The addition compounds are known as bound sulfur dioxide. Sulfur dioxide is used extensively in wine making, and in wine acet-aldehyde reacts preferentially with bisulfite. Excess bisulfite reacts with sugars. It is possible to classify bound S02 into three forms: aldehyde sulfurous acid, glucose sulfurous acid, and rest sulfurous acid. The latter holds the S02 in a less tightly bound form. Sulfites in wines serve a dual purpose: (1) antiseptic or bacteriostatic and (2) antioxidant. These activities are dependent on the form of S02 present. The various forms of S02 in wine are represented schematically in Figure 11-1. The free S02 includes the water-soluble S02 and the undissociated H2S03 and constitutes about 2.8 percent of the total. The bisulfite form constitutes 96.3 percent and the sulfite form 0.9 percent (all at pH 3.3 and 20°C). The bound S02 is mostly (80 percent) present as acetaldehyde S02, 1 percent as glucose S02, and 10 to 20 percent as rest S02. The various forms of sulfite have different activities. The two free forms are the only ones with antiseptic activity. The antioxidant activity is limited to the S032- ion (Figure 11-1). The antiseptic activity of S02 is highly dependent on the pH, as indicated in Table 11-3. The lower the pH the greater the antiseptic action of S02. The effect of pH on the various forms of sulfur dioxide is shown in Figure 11-2.

Sulfurous acid inhibits molds and bacteria and to a lesser extent yeasts. For this reason, S02 can be used to control undesirable bacteria and wild yeast in fermentations without affecting the S02-tolerant cultured yeasts. According to Chichester and Tanner (1968), the undissociated acid is 1,000 times more active than HS03~ for Escherichia coli, 100 to 500 times for Saccharomyces cerevisiae, and 100 times for Aspergillus niger.

The amount of S02 added to foods is self-limiting because at levels from 200 to 500 ppm the product may develop an unpleasant off-flavor. The acceptable daily intake (ADI) is set at 1.5 mg/kg body weight. Because large intakes can result from consumption of wine, there have been many studies on reducing the use of S02 in wine making. Although some other compounds (such as sorbic acid and ascorbic acid) may partially replace S02, there is no satisfactory replacement for S02 in wine making.

The use of S02 is not permitted in foods that contain significant quantities of thiamine, because this vitamin is destroyed by S02. In the United States, the maximum per

TOTAL SO.

H-bound S02-

so?

I active 1 antiseptic

h2s03

hso3

1 antioxidant

rest S02 QNtött

Figure 11-1 The Various Forms of S02 in Wine and Their Activity. Source: Reprinted with permission from J.M. deMan, 500 Years of Sulfite Use in Winemaking, Am. Wine Soc. J., Vol. 20, pp. 44-46, © 1988, American Wine Society.

acetaldehyde S02

rest S02 QNtött

Figure 11-1 The Various Forms of S02 in Wine and Their Activity. Source: Reprinted with permission from J.M. deMan, 500 Years of Sulfite Use in Winemaking, Am. Wine Soc. J., Vol. 20, pp. 44-46, © 1988, American Wine Society.

mitted level of S02 in wine is 350 ppm. Modern practices have resulted in much lower levels of SOz. In some countries S02 is used in meat products; such use is not permitted in North America on the grounds that this would result in consumer deception. S02 is also widely used in dried fruits, where levels may be up to 2,000 ppm. Other applications are in dried vegetables and dried potato

Table 11-3 Effect of pH on the Proportion of Active Antiseptic S02 of Wine Containing 100 mg/L Free S02

pH Active S02 (mg/L)

Ü2 37X) 2.8 8.0 3.0 5.0 3.3 3.0 3.5 1.8 3.7 1.2 4.0 0.8

products. Because S02 is volatile and easily lost to the atmosphere, the residual levels may be much lower than the amounts originally applied.

Nitrates and Nitrites

Curing salts, which produce the characteristic color and flavor of products such as bacon and ham, have been used throughout history. Curing salts have traditionally contained nitrate and nitrite; the discovery that nitrite was the active compound was made in about 1890. Currently, nitrate is not considered to be an essential component in curing mixtures; it is sometimes suggested that nitrate may be transformed into nitrite, thus forming a reservoir for the production of nitrite. Both nitrates and nitrites are thought to have antimicrobial action. Nitrate is used in the production of Gouda cheese to prevent gas formation by butyric acid-forming bacteria. The action of nitrite in meat curing is

Figure 11-2 Effect of pH on the Ionization of Sulfurous Acid in Water

considered to involve inhibition of toxin formation by Clostridium botulinum, an important factor in establishing safety of cured meat products. Major concern about the use of nitrite was generated by the realization that secondary amines in foods may react to form nitrosamines, as follows:

The nitrosamines are powerful carcinogens, and they may be mutagenic and teratogenic as well. It appears that very small amounts of nitrosamines can be formed in certain cured meat products. These levels are in the ppm or the ppb range and, because analytical procedures are difficult, there is as yet no clear picture of the occurrence of nitrosamines. The nitrosamines may be either volatile or nonvolatile, and only the latter are usually included in analysis of foods. Nitrosamines, especially dimethyl-nitrosamine, have been found in a number of cases when cured meats were surveyed at concentrations of a few |Xg/kg (ppb). Nitrosamines are usually present in foods as the result of processing methods that promote their formation (Hav-ery and Fazio 1985). An example is the spray drying of milk. Suitable modifications of these process conditions can drastically reduce the nitrosamine levels. Considerable further research is necessary to establish why nitrosamines are present only in some samples and what the toxicological importance of nitrosamines is at these levels. There appears to be no suitable replacement for nitrite in the production of cured meats such as ham and bacon. The ADI of nitrite has been set at 60 mg per person per day. It is estimated that the daily intake per person in Canada is about 10 mg.

Cassens (1997) has reported a dramatic decline in the residual nitrite levels in cured meat products in the United States. The current residual nitrite content of cured meat products is about 10 ppm. In 1975 an average residual nitrite content in cured meats was reported as 52.5 ppm. This reduction of nitrite levels by about 80 percent has been attributed to lower ingoing nitrite, increased use of ascorbates, improved process control, and altered formulations.

The nitrate-nitrite intake from natural sources is much higher than that from processed foods. Fassett (1977) estimated that the nitrate intake from 100 g of processed meat might be 50 mg and from 100 g of high-nitrate spinach, 200 mg. Wagner and Tannenbaum (1985) reported that nitrate in cured meats is insignificant compared to nitrite produced endogenously. Nitrate is produced in the body and recirculated to the oral cavity, where it is reduced to nitrite by bacterial action.

Hydrogen Peroxide

Hydrogen peroxide is a strong oxidizing agent and is also useful as a bleaching agent. It is used for the bleaching of crude soya lecithin. The antimicrobial action of hydrogen peroxide is used for the preservation of cheese milk. Hydrogen peroxide decomposes slowly into water and oxygen; this process is accelerated by increased temperature and the presence of catalysts such as catalase, lacto-peroxidase and heavy metals. Its antimicrobial action increases with temperature. When hydrogen peroxide is used for cheese making, the milk is treated with 0.02

percent hydrogen peroxide followed by catalase to remove the hydrogen peroxide. Hydrogen peroxide can be used for sterilizing food processing equipment and for sterilizing packaging material used in aseptic food packaging systems.

Sodium Chloride

Sodium chloride has been used for centuries to prevent spoilage of foods. Fish, meats, and vegetables have been preserved with salt. Today, salt is used mainly in combination with other processing methods. The antimicrobial activity of salt is related to its ability to reduce the water activity (aw), thereby influencing microbial growth. Salt has the following characteristics: it produces an osmotic effect, it limits oxygen solubility, it changes pH, sodium and chloride ions are toxic, and salt contributes to loss of magnesium ions (Banwart 1979). The use of sodium chloride is self-limiting because of its effect on taste.

Bacteriocins

Nisin is an antibacterial polypeptide produced by some strains of Lactococcus lactis. Nisin-like substances are widely produced by lactic acid bacteria. These inhibitory substances are known as bacteriocins. Nisin has been called an antibiotic, but this term is avoided because nisin is not used for therapeutic purposes in humans or animals. Nisin-producing organisms occur naturally in milk. Nisin can be used as a processing aid against gram-positive organisms. Because its effectiveness decreases as the bacterial load increases, it is unlikely to be used to cover up unhygienic practices.

Nisin is a polypeptide with a molecular weight of 3,500, which is present as a dimer of molecular weight 7,000. It contains some unusual sulfur amino acids, lanthionine and P-methyl lanthionine. It contains no aromatic amino acids and is stable to heat.

The use of nisin as a food preservative has been approved in many countries. It has been used effectively in preservation of processed cheese. It is also used in the heat treatment of nonacid foods and in extending the shelf life of sterilized milk.

A related antibacterial substance is nata-mycin, identical to pimaricin. Natamycin is effective in controlling the growth of fungi but has no effect on bacteria or viruses. In fermentation industries, natamycin can be used to control mold or yeast growth. It has a low solubility and therefore can be used as a surface treatment on foods. Natamycin is used in the production of many varieties of cheese.

Acids

Acids as food additives serve a dual purpose, as acidulants and as preservatives. Phosphoric acid is used in cola soft drinks to reduce the pH. Acetic acid is used to provide tartness in mayonnaise and salad dressings. A similar function in a variety of other foods is served by organic acids such as citric, tartaric, malic, lactic, succinic, adipic, and fu-maric acid. The properties of some of the common food acids are listed in Table 11-4 (Peterson and Johnson 1978). Members of the straight-chain carboxylic acids, propionic and sorbic acids, are used for their antimicrobial properties. Propionic acid is mainly used for its antifungal properties. Propionic acid applied as a 10 percent solution to the surface of cheese and butter retards the growth of molds. The fungistatic effect is higher at pH 4 than at pH 5. A 5 percent solution of calcium propionate acidified with lactic acid to pH 5.5 is as effective as a 10 percent una-cidified solution of propionic acid. The sodium salts of propionic acid also have antimicrobial properties.

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