364 Journal of Food Protection, Vol. 48, No. 4, Pages 364-375 (April 1985) Copyright5 International Association of Milk, Food, and Environmental Sanitarians Growth and Inhibition of Microorganisms in the Presence of Sorbic Acid: A Review MICHAEL B. LIE WEN and ELMER H. MARTH* Department of Food Science and The Food Research Institute, University of Wisconsin-Madison, Madison, Wisconsin 53706 Downloaded from http://meridian.allenpress.com/jfp/article-pdf/48/4/364/1657271/0362-028x-48_4_364.pdf by guest on 29 September 2021 (Received for publication July 9, 1984) ABSTRACT food product to which preservatives may be added. No Sorbate (sorbic acid) generally is an effective inhibitor of upper limits are imposed for foods not covered by Fed­ most molds and yeasts and some bacteria. Environmental fac­ eral Standards of Identity. Thus, in natural cheese, the tors such as pH, water activity, temperature, atmosphere, maximum quantity may not exceed 0.3% by weight, cal­ microbial load, microbial flora and certain food components can culated as sorbic acid, and the maximum is set at 0.2% influence the effectiveness of sorbate. Strains of microor­ in pasteurized blended cheese, pasteurized cheese spread, ganisms resistant to sorbate exist and therefore are common cold pack cheese and cheese food and spread (39). causes of food spoilage. Some molds and bacteria are able to degrade sorbate. This paper reviews the factors that affect the antimicrobial effectiveness of sorbate in foods. APPLICATIONS Sorbic acid is a straight chain a, p-unsaturated monocarboxylic acid and has the structure Sorbic acid was first isolated from the pressed un- CH3CH = CHCH = CHCOOH (179). The carboxyl group ripened berry of the rowan or mountain ash tree in 1859 reacts readily and forms salts and esters. The salts are by the German chemist A. W. Hoffmann (6). It was not important in food applications because of their high solu­ until 1939, however, that E. Miiller and C. M. Gooding, bility in water. Potassium sorbate was specifically de­ working independently in Germany and the United States, veloped to prepare the aqueous stock solutions needed for respectively, discovered the antimicrobial activity of sor­ dip, spray and metering applications (36, 65). Sorbic acid bic acid (95). In 1945, Gooding was awarded a U.S. pa­ is only slightly soluble in water; however, it is more sol­ tent for use of the antifungal properties of sorbic acid uble in fats than is potassium sorbate. Sorbic acid can (64), and since the mid-1950s it has been used to a grow­ be incorporated with dry materials by mixing with salt, ing extent throughout the world to protect a variety of flour or corn starch. It can be solubilized with sodium foods against deterioration by microorganisms. or potassium hydroxide, or it may be dissolved in prop­ ylene glycol or ethanol for use in dips or sprays (36). REGULATORY STATUS Since sorbic acid sublimes upon heating, it should be added to foods only after any prolonged boiling or heat­ Because sorbic acid was not used as a food preserva­ ing that may be employed in food processing (6, 36). tive until a time when extensive toxicological investiga­ Sorbic acid and its potassium salt are the most widely tions were required for new food additives, it is among used forms of the compound and are collectively known the most thoroughly investigated of all preservatives. It as sorbates. Their most common use is preservation of was found harmless in numerous acute, subchronic and food, animal feed, cosmetic and pharmaceutical products chronic toxicity tests (52, 61, 94). Sorbic acid is as well as technical preparations that come in contact metabolized in the body like other fatty acids. In humans with food or the human body. Methods of application in­ and animals, it is subject to B-oxidation typical of fatty clude: direct addition into the product; dipping, spraying, acid degradation. The half-life in the body is 40-110 min, or dusting of the product; or incorporation into the wrap­ depending on the dosage, and under normal conditions per (6, 7, 158). it is completely oxidized to C02 and H20 (53, 95). Typical use levels in foods range from 0.02% in wine Use of sorbic acid in foods is permitted in most coun­ and dried fruits to 0.3% in some cheeses (Table 1). tries which regulate their food supply (95). The Foods in which sorbate has commercially useful antimic­ maximum permissible level, other than in exceptional robial activity include baked goods (cakes and cake situations, is between 0.1 and 0.2%. In the U.S., sorbic mixes, pies and pie fillings, doughnuts, baking mixes, acid is a GRAS substance and its use is permitted in any fudges, icings), dairy products (natural and processed JOURNAL OF FOOD PROTECTION, VOL. 48, APRIL 1985 SORBIC ACID INHIBITS MICROBIAL GROWTH 365 TABLE 1. Typical concentration (%) of sorbic acid used in proaches its dissociation constant (pKa), which is 4.75. various food products. At this pH value, 50% of sorbic acid is in the effective Cheeses 0.2-0.30 undissociated form (161) (Table 2). Therefore, sorbate is Beverages 0.03-0.10 more effective in foods with low rather than high pH Cakes and pies 0.05-0.10 values (6, 7, 11, 14, 95, 161). The upper pH limit for Dried fruits 0.02-0.05 activity of sorbate is 6.0-6.5, while those for propionate Margarine (unsalted) 0.05-0.10 and benzoate are 5.0-5.5 and 4.0-4.5, respectively. Many Mayonnaise 0.10 investigators have demonstrated the importance of using Fermented vegetables 0.05-0.20 sorbate at proper pH values. For example, Park and Jams and jellies 0.05 Marth (119) showed that Salmonella typhimurium would Fish 0.03-0.15 grow in nutrient broth (pH 6.7) or skimmilk (pH 6.4) Semi-moist pet food 0.1-0.3 fortified with 0.3% sorbic acid when the media were not Wine 0.02-0.04 acidified. However, when the pH was reduced to 5.0, Fruit juices 0.05-0.20 growth did not occur in either medium and, in time, a Downloaded from http://meridian.allenpress.com/jfp/article-pdf/48/4/364/1657271/0362-028x-48_4_364.pdf by guest on 29 September 2021 major portion of the population was inactivated. cheeses, cottage cheese, sour cream, yogurt), fruit and berry products (artificially sweetened confections, dried TABLE 2. Effect of pH on sorbic acid dissociation" fruits, fruit drinks, jams, jellies, wine), vegetable prod­ pH Undissociated (%) ucts (olives, pickles, relishes, salads) and other miscel­ 7.00 0.6 laneous food products (certain fish and meat products, 6.00 6.0 mayonnaise, margarine, salad dressings) (6, 108). 5.80 7.0 5.00 37.0 4.75 (pKa) 50.0 ENVIRONMENTAL FACTORS AFFECTING EFFEC­ 4.40 70.0 TIVENESS OF SORBATE 4.00 86.0 3.70 93.0 Environmental factors such as pH, water activity (aw), 3.00 98.0 temperature, atmosphere, initial microbial load, type of "From Sofos and Busta (148). microbial flora, and certain food components, singly or in combination, can influence the antimicrobial activity of sorbate. Together with preservatives such as sorbic acid, they often act to broaden antimicrobial action or Water activity increase it synergistically. Use of other preservatives in Addition of substances to food that reduce its aw have combination with sorbate can also broaden or intensify a beneficial effect on the action of antimicrobial preserva­ antimicrobial action. The length and temperature of stor­ tives (10). The most important substances to consider are age are other important considerations. If growth of spoil­ salt and sugar (87). Both intensify the action of sorbate age or pathogenic microorganisms is inhibited, but the primarily by reducing aw. They may also induce cellular microorganisms are not killed, growth will eventually re­ swelling, which makes many microorganisms more sus­ sume under proper conditions. The length of inhibition ceptible to action of preservatives (95). Costilow et al. will vary with storage temperature as well as with any (42-44) noted that salt (NaCl) increased the effectiveness of the other factors discussed. of sorbate in inhibiting spoilage yeasts in cucumber fer­ mentations. Sheneman and Costilow (155) saw the same Effects ofpH effect with sucrose in sweet cucumber pickles. Robach Organic acids such as sorbic acid dissociate in aqueous and Stateler (148) found that certain combinations of solutions and release hydrogen ions. Some food preserva­ NaCl and sorbate resulted in a synergistic inhibition of tives such as acetic acid act mainly by lowering the pH Staphylococcus aureus. Acott et al. (1) showed that suc­ of the environment to the point where microorganisms rose (aw =0.85) and sorbate in combination inhibited can no longer grow. Although the undissociated form of growth of Aspergillus glaucus and Aspergillus niger and acetic acid has antimicrobial action, it is primarily the slowed the growth of Staphylococcus epidermidis. At an hydrogen ion and the resulting decrease in pH that in­ aw of 0.88, all three organisms were able to grow, but hibits microorganisms (45, 91). at a slower rate than in the absence of sorbate. Gooding With sorbic acid and other organic acids, it is the un­ et al. (65) indicated that salt and glucose have a marked dissociated molecule that provides antimicrobial activity synergistic effect on fungistasis by sorbic acid. In high (45, 95, 99). The amount of the molecule in the undis­ sugar systems, increased antimicrobial activity occurred sociated form is determined by pH. This, along with sol­ even at pH values above 6.5. In contrast, Beuchat (16) ubility properties, determines the foods in which organic reported that the presence of sucrose or NaCl can, in acids may effectively be used (135).