Quality Factors of Fruits and Vegetables - American Chemical Society

Chapter 17

Chemistry and Safety of Acidified Vegetables Barry G. Swanson and William R. Bonorden

Downloaded by UNIV OF TENNESSEE KNOXVILLE on October 2, 2015 | http://pubs.acs.org Publication Date: September 7, 1989 | doi: 10.1021/bk-1989-0405.ch017

Department of Food Science and Human Nutrition, Washington State University, Pullman, WA 99164-6330

Acidified vegetables means low acid vegetables to which acid(s) or acid food(s) are added; these vegetables include, but are not limited to, beans, cucumbers, cabbage, artichokes, cauliflower, peppers and tropical f r u i t s , singly or in any combination. Acidified vegetables have a water activity greater than 0.85 and a finished equilibrium pH equal to or smaller than 4.6. Acidified vegetables may be called, or may purport to be "pickles" or "pickled". Increasing the acidity of canned vegetables reduces required processing times and temperatures, thus contributing to enhancement of flavor, color, texture and nutritive quality. Addition of organic acids improves the microbiological, color, structural and flavor stability of select fresh, minimally processed or canned vegetables. One important step in the development of the canning process was the recognition that adding acid to the brine of canned vegetables to a pH of less than 4.6: 1) 2) 3) 4)

reduced the thermal death inhibited the germination inhibited enzyme activity quality; and prevented the germination botulinum spores.

time of vegetative c e l l s ; and growth of microbial spores; and stabilized color and flavor and outgrowth of Clostridium

Food and Drug Administration (FDA) Good Manufacturing Practices make significant distinctions between acid, low acid, and acidified foods based on a finished equilibrium pH of 4.6. Acidified Foods Acidified foods are defined as "low acid" foods to which acid(s) or acid food(s) are added; these foods include, but are not limited to 0097-6156/89/0405-0216$06.00/0 ο 1989 American Chemical Society

In Quality Factors of Fruits and Vegetables; Jen, J.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

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Chemistry and Safety ofAcidified Vegetables 217


beans, cucumbers, cabbage, artichokes, cauliflower, onions, peppers, tropical fruits and f i s h , singly or in any combination. Acidified foods have a water activity greater than 0.85 and a finished equilibrium pH of 4.6 or less (1). Spoilage Microorganisms. The spoilage organisms of concern in acidified foods are aciduric bacteria with no special heat-resistant qualities (2). Bacillus coagulans, an aciduric, anaerobic spore-former, is responsible for flat-sour spoilage of tomato products (3). Clostridium pasteurianum is also a troublesome aciduric, anaerobic sporeforming microorganism in acidified foods. Clostridium pasteurianum produces butyric acid and gas that will swell and spoil canned acid foods. Vegetative cells of Clostridium pasteurianum are heat labile in acidified vegetables. Clostridium botulinum growth normally does not occur in foods with a pH less than 4.8, yet the FDA and the food processing industry historically recognized the discriminatory pH inhibiting Clostridium botulinum growth as 4.6. The discriminatory pH of 4.6 established by regulation is an equilibrated pH in the finished vegetable product as determined from a ground or pureed vegetable. Townsend et a l . (4) reported a minimum of ten days are required for C. botulinum growth at a pH of 5.0 in vegetable products. However, pH is not the overriding factor for growth of a l l microorganisms, especially in relation to vinegar and acetic acid (5). Lactobacilli, Leuconostoc, yeasts and molds may proliferate in acidified vegetables depending on pH, may spoil the vegetable food, but w i l l not endanger the health of the consumer. Vegetables have commonly been fermented to produce pickles or pickled foods, or acidified directly by the addition of lemon juice or vinegar. Recent economic instability in the Pacific Northwest, occurring simultaneously with excellent yields of many low acid vegetables, has resulted in entreprenurial development of many small regional vegetable producer/processors. Many of the entrepreneurs are marketing conveniently processed acidified vegetables asparagus, beans, onions, g a r l i c , peppers and/or tomatoes. Purposes. The primary purpose of acidifying low acid vegetables to a finished equilibrium pH of 4.6 or less is to prevent the growth of Clostridium botulinum and to reduce the thermal resistance of microorganisms which in turn reduces processing time and temperature. Acidifying vegetables permits thermal processing in a boiling water bath at atmospheric pressure rather than processing under steam pressure. The advantages of shortened processing times and reduced processing temperatures are exhibited as acceptable uniform texture, color, flavor and nutritive value of vegetables. Many of the advantages could not be achieved with thermal processing that accompanies application of steam pressure. Potential advantages other than quality and safety include reduced energy costs and increased production rates. Acidulants are added to food directly or indirectly for more than 25 separate purposes (6). Gardner (6) suggests the major functions of acidification are to enhance flavor and reduce thermal processing time. Tartness, i f not excessive, adds subtle character to the overall flavor of vegetables. Acids also intensify some



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QUALITY FACTORS OF FRUITS AND VEGETABLES

flavoring agents, enhance aromas, act as flavor fixatives, and provide seasoning or blending properties that result in uniformity of taste. Flavor enhancement characteristics of acids are often related to both the hydrogen ion and undissociated acid concentration in solution (7,8). Thermal resistance of microorganisms and other living organisms is reduced as acidity increases and pH decreases (4). Shillinglaw and Levine (9) demonstrated that acetic acid is more growth inhibitory and bacteriacidal than c i t r i c acid at comparable pH. Edible organic acids are more toxic to bacteria and yeasts than highly dissociated inorganic acids such as hydrochloric acid at the same pH. Acetic acid toxicity for various microorganisms is not confined to hydrogen ion concentration alone, but seems to be a function of the undissociated acetic acid molecule (5). Contamination of fruits and vegetables by vegetative microorganisms is often accompanied by spores. Microbial spores are very resistant to thermal inactivation. Acids contribute a preservative capacity in addition to reducing thermal resistance, inhibiting the germination and growth of spores. The addition of acids to vegetables is only an aid to thermal processing, and is not a solution to heavy microbial contamination or inadequate sanitation (6). Acidulants also act as synergists to antioxidant inhibition of rancidity, slowing catalysis of oxidation by chelating metals and providing a reducing environment of hydrogen ions which serve to partially regenerate antioxidants. Inhibition of oxidation results in stabilizing of food colors, reducing turbidity and stabilizing flavors (6). Acidulants inhibit enzymes by reducing pH below the optimum for activity of specific enzymes. Enzymatic discoloration can be controlled by the addition of acids to vegetables (10). Acidulants are also added to foods for several other specific applications. Acids protonate galacturonic acid to enhance gelling of pectin, fruit and sugar mixtures. Leavening acids promote the production of carbon dioxide from sodium bicarbonate. Acids promote crosslinking of cereal proteins to enhance the texture of baked goods. Ascorbic acid and phosphoric acid often provide nutrients for microbial growth during fermentations. Acids can act as emulsifying agents in combination with alcohols and/or monoglycerides. Acids modify the consistency or texture of foods by chelating minerals and enhancing crosslinking of proteins. Addition of acids to sucrose solutions results in production of invert sugar and increases the sweetness of the solution. Acidulant use. Acetic, c i t r i c and phosphoric acids are the most commonly used acidulants, although l a c t i c , fumaric, adipic, succinic, malic, benzoic, tartaric, ferulic and gluconic acids are sometimes used. Lemon juice, 5% c i t r i c acid, and vinegar, 5% acetic acid, are the most commonly used natural acidulants. The acidity of fermented vinegars is often questionable, and should be determined prior to use as an acidifying agent. The acidity of juice from fresh lemons may vary from 4.0 to 8.0% expressed as c i t r i c acid, and should also be used carefully. Comparison of the physical and general properties of commercially available food acids is often necessary. Acid


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SWANSON & BONORDEN


solubility in water (6), dissociation (Table I) and flavor are the most relevant properties to successful food acidification. Solubility of food constituents in acid solution may also be important. For example, the solubility of rutin is apparently less in solutions of acidified asparagus than in the brine of canned asparagus, so undesirable rutin precipitation in acidified asparagus is prevalent and d i f f i c u l t to control (11).


Table I.

Dissociation constants of organic acids

Citric Acid

_ K

K

K

Acetic Acid Ascorbic Acid Benzoic Lactic Malic Succinic Tartaric

1 2 3ο

K

-

l -

-

l ? 4-

K

A A K

7.1 1.7 6.4 1.8 7.9 1.6 6.5 8.4 3.9 7.8 6.9 2.5 1.0 4.6

X X X X X X X X X X X X X X

< 1 0

1 0

6 l5

1 0 10

J

-12

10.5

io 1 10 ! 10

-6 -5 6

10

1 0

5 ιο'ί

ίο

10"

5

Vegetables. Vegetables are generally low acid foods with a pH greater than 4.6. Vegetables are acidified to reduce the times and temperatures required to reach commercial s t e r i l i t y , to inhibit enzymatic and nonenzymatic discoloration, and to enhance flavor. Vegetables that are commonly acidified include asparagus, green beans, cabbage, artichokes, pimentos, cucumbers, peppers, onions, mushrooms and peppers. Vegetables are acidified by soaking in an acid solution prior to processing, bulk storage in acid solutions, blanching in acid solution or direct addition of an acid brine. Acidifying soak water with c i t r i c or malic acid enhances the quality of canned dry beans (12) and dry lima beans (13). The occurrence of s p l i t s , drained weights, shear values, color and taste of dry beans were improved. A 0.25% c i t r i c acid soaking solution improved the color of canned lima beans. Acidified soak water improves color by retarding the retention of Fe and Cu salts with sulfides or polyphenols that contribute to discoloration (13). Bulk storage of snap beans and peas at a pH of 1.2-1.6 in hydrochloric acid solution for 4 mo. resulted in quality vegetables (14,15). Acidified bulk storage was employed to lengthen the processing season and enhance quality. Following acidified bulk storage, blanched peas and snap beans had better quality than unblanched vegetables. Some cultivars of green beans changed color from green to red during acidification. The color change was attributed to changes in leucoanthyocyanins (anthocyanins). Blanched green beans stored in a hydrocholoric acid solution at a pH



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of 1.5 changed color from dark green to a light more acceptable green color (15). The color changes were attributed to changes in pheophytins and pheophorbides as described in native chlorophyll. Cauliflower sometimes becomes pink and garlic sometimes exhibits a blue-green color when acidified. Tomatoes were covered with a hydrochloric acid solution, pH of 1.2-1.6, to inactivate microorganisms and plant enzymes (16). Pre-peeled tomatoes were stored in acid solutions successfully. Fructose and carotene degradation in the presence of oxygen were practical disadvantages. The processing and acidified bulk storage of semifinished fruits and vegetables reduced costs and improved quality (17). Diffusion. Acid diffusion in pH modified foods was investigated with fluorescent dyes or color indicators (18). Colorimetric pH profiles compared well with electromagnetic measurements. Effective mass diffusivities for acetic, c i t r i c , gluconic and phosphoric acids in potatoes were on the order of 10 m /sec. Diffusion occurs mainly in an unsteady state in heterogeneous, multilayer, cellular systems (19). Diffusion can be seen mathematically predicted by one-dimensional solutions of the second Fick equation: ^

= div(D*grad C)

ci£t)

=

e r f c

1

—

2VD7E

where: Da = Apparent Diffusivity C = Concentration of tracer at height y and at time t Acidified Vegetables Acidified or fermented green beans were observed to have acceptable firmness, color and flavor (20). The acidified and fermented beans were much firmer with less sloughing than conventional canned beans. Acidified or fermented green beans are suitable for use in casseroles and salads. Palmito, the young tender leaves of select palm trees, often called heart-of-palm, are acidified with c i t r i c acid to inhibit discoloration of the naturally white leaves and inhibit C. botulinum growth (21). Diffusion of c i t r i c acid into palmito required more than 33 days; vacuum acidification and biological transport of acid into palmito were unsuccessful. Processing, type of acid, ripeness, post-harvest holding period, acidification technique and peeling method influence the quantity of acid required to acidify pimiento peppers to a given pH (22). Several organic acids have been evaluated as acidulants for pimientos. Fumaric acid was most effective for reducing the pH of pimientos (23) and tomatoes (24). Succinic acid imparted undesirable flavors when added in large quantitites. Powers et a l . (25) reported acidification did not affect the color of pimientos,



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SWANSON & BONORDEN


but did modify the flavor. Acidification increased the drained weight and firmness of canned pimientos (25,26). Addition of an acidulant and calcium chloride resulted in a highly significant synergistic increase in drained weight and firmness of canned pimientos. Addition of 0.05% monosodium glutamate (MSG) to acid-treated pimientos increased organoleptic acceptability slightly (25). Pimientos of different maturity exibit different pH after processing when acidified by immersion, whereas pimientos acidified in the jar exhibited no pH differences due to maturity (27). An equation predicting pimiento pH from acid bath concentration, temperature and duration of dip was used to illustrate that acid bath concentration was by far the most important determinant. Some garden grown peppers are more highly buffered than others, requiring additional acid to reach an acceptable pH for acidified home-canned peppers (28). The addition of no less than 1 tablespoon of vinegar (5% acetic acid) per pint (15 mL/473 mL)of canned peppers was recommended. Mushrooms, pearl onions and cherry peppers were packed with sufficient c i t r i c or acetic acid to reach an equilibrium pH of 4.8 (29). Equilibration with acetic acid required 1 day for mushrooms, 15 days for onions and 30 days for cherry peppers. Larger equilibration times were required for c i t r i c acid. The time required for the pH inside the vegetables to decrease to 4.8 or less was a function of acid type, i n i t i a l acid concentration and the vegetable being acidified. Crispness of cucumber pickles is an important quality characteristic (30), whether the pickles are derived from a genuine brine-stock fermentation or from the fresh-pack process. Enzymatic softening of cucumber pickles is inhibited by acidification, but nonenzymatic softening is enhanced by addition of acids. Oxalic acid reached an equilibrium very quickly in cucumbers, but was very damaging to texture. Acetic and lactic acid equilibrated more slowly than oxalic acid, but s t i l l reduced pickle firmness. Citric acid penetrated at the slowest rate and also reduced firmness. Enhanced softening at reduced pH was ascribed to hydrolytic cleavage of qlycosidic bonds in the neutral carbohydrates of the cell wall (31). Enhanced softening at neutral pH was ascribed to polymer cleavage, in this case through beta elimination reactions involving pectins (32). Mold-induced softening of cucumber pickles was inhibited in fermentations reaching an equilibrium pH of 4.0 or in fresh-pack pickles with addition of 0.16% acetic acid (33). Direct acidification of cabbage with acetic, c i t r i c , glucono-delta-lactone and lactic acids produced saurkraut-1ike products with diminished flavor and aroma (34). Acidification of canned tomatoes with c i t r i c , fumaric and malic acids reduced pH effectively (24). Fumaric acid was most effective. Adipic acid was least effective. Succinic acid was an effective acidulant, but induced an undesirable taste. Acidulation tended to reduce pH more when salt was added to tomatoes than when salt was omitted. Acids also tasted less sour in the presence of salts. Acidification of mung bean sprouts with acetic, c i t r i c , gluconic, lactic or malic acid enhanced texture (35). Treatment of mung bean sprouts with acetic acid was detrimental to flavor. The best acidified mung bean sprouts were obtained by acidification with gluconic acid (35).


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Addition of 1% or 3% ascorbic or c i t r i c acid to processed sweet potatoes did not improve the color (36). With a pH change from 8.0 to 3.0 elicited by a citrate buffer, sweet potatoes' color and attractiveness improved (37).


Summary Acidified vegetables provide the consumer with a safe, wholesome, quality food; and the entrepreneur producer/processor with a convenient way to market a regional low-acid vegetable. Standardized lemon juice, c i t r i c acid solutions or vinegar appear to be the most convenient and effective acidulants. Acidified vegetables equilibrated to a pH of 4.5, or preferably to a pH of 4.0 or less and adequately processed in a water bath w i l l exhibit a firmer texture, increased drained weight, and more stable color and flavor than adequately processed pressure canned low acid vegetables. Acknowledgements Presented at the American Chemical Society Meeting, Los Angeles, September, 1988. Supported in part by Washington State University, College of Agriculture and Home Economics, Agricultural Research Center. Project 10A-3057-0216. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

Code of Federal Regulations, 21CFR114, Food and Drug Administration, 1982. Schmidt, T.R. The Use of Citric Acid in the Canned Fruit and Vegetable Industry; Miles Centennial Series; Elkhart, IN, 1983; p 24. Fields, M.L. Adv. Food Res. 1970, 18, 163-217. Townsend, C.T.; Yee, L.,; Mercer, W.A. Food Res. 1954, 19, 536-542. Levine, A . S . ; Fellers, C.R. J. Bact. 1940, 39, 499-515. Gardner, W.H. Food Acidulants; Allied Chemical Corp: NY, 1966; p 185. Amerine, M.A.; Roessler, E.B.; and Ough, C.S. Am. J. Enol. Vitic. 1965, 16, 29-37. Plane, R.A.; Mattick, L.R.; and Weirs, L.D. Am. J. Enol. V i t i c . 1980, 31, 265-268. Schillinglaw, C.A.; Levine, M. Food Res. 1943, 8, 464-476. Joslyn, M.A.; Ponting, J.D. Adv. Food Res. 1951, 3, 8-17. G r i f f i t h , J.Q.; Krewson, C.F.; Naghski, J. Rutin and Related Flavonoids; Mack Publishing Co.: Easton, PA, 1955; p 275. Junek, J.J.; Sistrunk, W.A.; Neely, M.B. J. Food Sci. 1980, 45, 821-824. Luh, B.S.; Wang, C.; Daoud, H.H. J. Food Sci. 1975, 40, 557-561. Basel, R.M. J. Food S c i . 1982, 47, 2082-2083. Basel, R.M. J. Food Technol. 1983, 18, 797-799. Basel, R.M. Ph.D. Thesis, Ohio State University, OH, 1980.


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Chemistry and Safety ofAcidified Vegetables

Kafedshiev, I.; Kolev, D. Food Technol. 1980, 34(7), 56-60. McCarthy, M.J.; Heil, J.R. J. Food Sci. 1988, 53, 494-495, 499. Stahl, R.; Loncin, M. J. Food Proc. Pres. 1979, 3, 213-223. Kozup, J.; Sistrunk, W.A. J. Food Sci. 1982, 47, 1001-1005. Quast, D.G.; Bernhardt, L.W. J. Food Prot. 1978, 41(8), 667-674. Flora, L.F.; Heaton, E.K.; Shewfelt, A.L. J. Food Sci. 1978, 43, 415-419. Supran, M.K.; Powers, J.J.; Rao, P.V.; Dornseifer, P.T. Food Technol. 1966, 20, 215-220. Pray, L.W.; Powers, J.J. Food Technol. 1966, 20, 87-90. Powers, J.J., Morse, R.E., Sane, R.H.; Mills, R.C. Food Technol. 1950, 4, 485-488. Powers, J.J.; Pratt, D.E.; Downing, D.L.; Powers, I.T. Food Technol. 1961, 15, 67-74. Flora, L.F.; Heaton, E.A.; Shewfelt, A.L. J. Food Sci. 1978, 43, 415-419. Sapers, G.M., Carre, J.; Divito, A.M.; Panusiuk, O. J. Food Sci. 1980, 45, 726-729. Stroup, W.H.; Dickerson, R.W. J r . ; Johnston, M.R. J. Food Prot. 1985, 48, 590-594. Bell, T.A.; Turney, L.S.; Etchells, J.L. J. Food Sci. 1972, 37, 446-449. van Buren, J.P. J. Texture Studies 1979, 10, 1-23. Kiejberts, M.J.H.; Pilnik, W. Carb. Res. 1974, 33, 359-362. Potts, E.A.; Fleming, H.P. J. Food Sci. 1982, 47, 1723-1727. Lonergan, D.; Lindsay, R.C. J. Food Prot. 1979, 42(1), 38-42. Buescher, R.W.; Chang, J.S. J. Food Sci. 1983, 48, 1598-1599, 1603. Twigg, B.A.; Schott, L.E.; Bouwkamp, J.C. J. Food Sci. 1974, 39, 565-567. Sistrunk, W.A. J. Food Sci. 1971, 36, 39-42.

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Quality Factors of Fruits and Vegetables - American Chemical Society

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