NATURAL ACIDS AND ACIDULANTS

 

Background

Acids, or acidulants as they are also called, are commonly used in food processing as flavor intensifiers, preservatives, buffers, meat-curing agents, viscosity modifiers, and leavening agents. This article discusses the functions that acidulants have in food systems and reviews the more commonly used food acidulants.

Functions of Acidulants

The reasons for using acidulants in foods are numerous and depend on what the food processor hopes to accomplish. As outlined above, the principal reasons for incorporating an acidulant into a food system are flavor modification, microbial inhibition, and chelation.

Flavor Modification

Sourness or tartness is one of the five major taste sensations: sour, salty, sweet, bitter, and umami (the most recently determined). Unlike the sensations of sweetness and bitterness, which can be developed by a variety of molecular structures, sourness is evoked only by the hydronium ion of acidic compounds. Each acid has a particular set of taste characteristics, which include the time of perceived onset of sourness, the intensity of sourness, and any lingering of aftertaste. Some acids impart a stronger sour note than others at the same pH. As a general rule, weak acids have a stronger sour taste than strong acids at the same pH because they exist primarily in the undissociated state. As the small amount of hydronium ions is neutralized in the mouth, more undissociated acid (HA) molecules ionize to replace the hydronium ions lost from equilibrium. The newly released hydronium ions are then neutralized until no acid remains. Taste characteristics of the acid are an important factor in the development of flavor systems.

As pH decreases, the acid becomes more undissociated and imparts more of a sour taste. For example, the intense sour notes of lactic acid at pH 3.5 may be explained by the fact that 70% of the acid is undissociated at this pH, compared with 30% for citric acid. In addition to sourness, acids have nonsour characteristics such as bitterness and astringency, though these are less perceptible. At pH values between 3.5 and 4.5, lactic acid is the most astringent. Acids also have the ability to modify or intensify the taste sensations of other flavor compounds, to blend unrelated taste characteristics, and to mask undesirable aftertastes by prolonging a tartness sensation. For example, in fruit drinks formulated with low-caloric sweeteners, acids mask the aftertaste of the sweetener and impart the tartness that is characteristic of the natural juice. In another example, in substitutes for table salt, acids remove the bitterness from potassium chloride and provide the salty taste of sodium chloride. Other acids, such as glutamic and succinic acids, possess flavor-enhancement properties. 

Because acids are rarely found in nature as a single acid, the combined use of acids simulates a more natural flavor. Two acids that are frequently blended together are lactic and acetic.

Microbial Inhibition

Acidulants act as preservatives by retarding the growth of microorganisms and the germination of microbial spores which lead to food spoilage. The effect is attributed to both the pH and the concentration of the acid in its undissociated state. It is primarily the undissociated form of the acid which carries the antimicrobial activity: as the pH is lowered, this helps shift the equilibrium in favor of the undissociated form of the acid, thereby leading to more effective antimicrobial activity. The nature of the acid is also an important factor in microbial inhibition: weak acids are more effective at the same pH in controlling microbial growth. Acids affect primarily bacteria because many of these organisms do not grow well below about pH 5; yeasts and molds, in comparison, are usually acid-tolerant. 

In fruit- and vegetable-canning operations, the combined use of heat and acidity permits sterilization and spore inactivation to be achieved at lower temperatures; this minimizes the degradation of flavor and structure that generally results from processing.

Acidification also improves the effectiveness of antimicrobial agents such as benzoates, sorbates, and propionates. For example, sodium benzoate – an effective inhibitor of bacteria and yeasts – does not exert its antimicrobial activity until the pH is reduced to about 4.5. Blends of acids act synergistically to inhibit microbial growth. For example, lactic and acetic acids have been found to inhibit the outgrowth of heterofermentative lactobacilli.

Chelation

Oxidative reactions occur naturally in foods. They are responsible for many undesirable effects in the product, including discoloration, rancidity, turbidity, and degradation of flavor and nutrients. As catalysts to these reactions, metal ions such as copper, iron, manganese, nickel, tin, and zinc need to be present in only trace quantities in the product or on the processing machinery. 

Many acids chelate the metal ions so as to render them unavailable; the unshared pair of electrons inthe molecular structure of acids promotes the complexing action. When used in combination with antioxidants such as butylated hydroxyanisole, butylated hydroxytoluene, or tertiary butylhydroquinone, acids have a synergistic effect on product stability.

Citric acid and its salts are the most widely used chelating agents. 

Other Functions

One of the most common reasons for adding acids is to control pH. This is usually done as a means to retard enzymatic reactions, to control the gelation of certain hydrocolloids and proteins, and to standardize pH in fermentation processes. In the first example, the lowering of pH inactivates many natural enzymes which promote product discoloration and development of off-flavors. Polyphenol oxidase, for example, oxidizes phenols to quinones, which subsequently polymerize, forming brown melanin pigments that discolor the cut surfaces of fruits and vegetables.

The enzyme is active between pH 5 and 7 and is irreversibly inactivated at a pH of 3 or lower. In the second example, acidification to 2.5–3 is required for high-methoxyl pectins to form gels. Because pH influences the gel-setting properties and the gel strength obtained, proper pH control is critical in the production of pectin- and gelatin-based desserts, jams, jellies, preserves, and other products. In the final example, standardization of pH is done routinely in fermentation processes, such as wine-making, to ensure optimum microbial activity and to discourage growth of undesirable microbes. Acids are also added post-fermentation to stabilize the finished wine.

Acid salts function as buffers in various systems. For example, in confectionery products, acid salts are used to control the inversion of sucrose into its constituents, glucose and fructose, the latter being hygroscopic. The resulting lower concentration of fructose yields a less hygroscopic food system and a longer shelf-life.

Acids are a major component of chemical leavening systems, where they remain nonreactive until the proper temperature and moisture conditions are attained. The gas evolved by reaction of the acid with bicarbonate produces the aerated texture that is characteristic of baked products such as cakes, biscuits, doughnuts, pancakes, and waffles. The onset and the rate of reaction of these compounds are controlled by such factors as the solubility of the acid, the mixing conditions for preparing the batter, and the temperature and moisture of the batter. Many chemical leavening systems are based on salts of phosphoric and tartaric acids.

Acids have also been used for other purposes. For example, they are added to chewing gum to stabilize aspartame and to cheese to impart favorable textural properties and sensory attributes.

Commonly Used Acidulants

Among the most widely used acids are acetic, adipic, citric, fumaric, lactic, malic, phosphoric, and tartaric acids. Glucono-d-lactone, though not itself an acid, is regarded as an acidulant because it converts to gluconic acid under high temperatures. 

Acetic Acid

Acetic acid is the major characterizing component of vinegar. Its concentration determines the strength of the vinegar, a value termed ‘grain strength,’ which is equal to 10 times the acetic acid concentration. Vinegar containing, for example, 6% acetic acid has a grain strength of 60 and is called 60-grain. Distillation can be used to concentrate vinegar to the desired strength.

Fermentation conducted under controlled conditions is the commercial method for vinegar production.

Bacterial strains of the genera Acetobacter and Acetomonas produce acetic acid from alcohol which has been obtained from a previous fermentation involving a variety of substrates such as grain and apples. Vinegar functions in pH reduction, control of microbial growth, and enhancement of flavor. It has found use in a variety of products, including condiments such as ketchup, mustard, mayonnaise, and relish, salad dressings, marinades for meat, poultry, and fish, bakery products, soups, and cheeses. Pure (100%) acetic acid is called glacial acetic acid because it freezes to an ice-like solid at 16.6ºC. Though not widely used in food, glacial acetic acid provides acidification and flavoring in sliced, canned fruits and vegetables, sausage, and salad dressings.

Adipic Acid

Adipic acid, a white, crystalline powder, is characterized by low hygroscopicity and a lingering, high tartness that complements grape-flavored products and those with delicate flavors. The acid is slightly more tart than citric acid at any pH. Aqueous solutions of the acid are the least acidic of all food acidulants, and have a strong buffering capacity in the pH range 2.5–3.0.

Adipic acid functions primarily as an acidifier,buffer, gelling aid, and sequestrant. It is used in confectionery, cheese analogs, fats, and flavoring extracts. Because of its low rate of moisture absorption, it is especially useful in dry products such as powdered fruit-flavored beverage mixes, leavening systems of cake mixes, gelatin desserts, evaporated milk, and instant puddings.

Citric Acid

The most widely used organic acid in the food industry, citric acid, accounts for more than 60% of all acidulants consumed. It is the standard for evaluating the effects of other acidulants. Its major advantages include its high solubility in water; appealing effects on flavor, particularly its ability to deliver a ‘burst’ of tartness; strong metal chelation properties; and the widest buffer range of the food acids (2.5–6.5).

Citric acid is naturally present in animal and plant tissues and is most abundantly found in citrus fruits including the lemon (4–8%), grapefruit (1.2–2.1%), tangerine (0.9–1.2%) and orange (0.6–1.0%).  pineapple juices. Citric acid is available in a liquid form, which solves processing problems related to incorporating the acid into a food system, such as predissolving citric acid crystals and caking or crystallate deposits on processing equipment. Also available are granulated forms which allow the particle size to be customized to meet the particular need.

Citric acid has numerous applications. It is commonly added to non-alcoholic beverages where it complements fruit flavors, contributes tartness, chelates metal ions, acts as a preservative, and controls pH so that the desired sweetness characteristics can be achieved. Sodium citrate subdues the sharp acid notes in highly acidified carbonated beverages; in club soda, it imparts a cool, saline taste and helps retain carbonation. The acid is also used in wine production both prior to and after fermentation for adjustment of pH; in addition, because of its metal-chelating action, the acid prevents haze or turbidity caused by the binding of metals with tannin or phosphate.

The calcium salt of citric acid is used as an anticaking agent in fructose-sweetened, powdered soft drinks, where it neutralizes the alkalinity of other ingredients that support browning, such as magnesium oxide and tricalcium phosphate.

Citric acid has also found use in confectionery and desserts. In hard confectionery, buffered citric acid imparts a pleasant tart taste; it is added to the molten mass after cooking, as this prevents sucrose inversion and browning. Citric acid is used in gelatin desserts because it imparts tartness, acts as a buffering agent, and increases the pH for optimum gel strength.

Low levels of the acid, ranging from 0.001 to 0.01%, work with antioxidants to retard oxidative rancidity in dry sausage, fresh pork sausage, and dried meats. Citric acid is also used in the production of frankfurters: 3–5% solutions are sprayed on the casings after stuffing and prior to smoking to aid in their removal from the finished product. Used at 0.2% in livestock blood, sodium citrate and citric acid act as anticoagulants, sequestering the calcium required for clot formation so that the blood may be used as a binder in pet foods.

In seafood processing, citric acid inactivates endogenous enzymes and promotes the action of antioxidants, resulting in an increased shelf-life. Citric acid also chelates copper and iron ions that catalyze the oxidative formation of off-flavors and fishy odors associated with dimethylamine. In processed cheese and cheese foods, citric acid and sodium citrate function in emulsification, buffering, flavor enhancement, and texture development. Sodium citrate is also combined with sodium phosphate as a customized emulsification salt for processed cheese. Cogranulation of citric acid with malic and fumaric acids yields new tart flavor profiles.

Fumaric Acid

The extremely low rate of moisture absorption of this acid makes it an important ingredient for extending the shelf-life of powdered food products such as gelatin desserts and pie fillings. Fumaric acid can be used in smaller quantities than citric, malic, and lactic acids to achieve similar taste effects.

Fermentation of glucose or molasses by certain Rhizopus spp. is the method used to produce fumaric acid commercially. The acid is also made by isomerization of maleic acid with heat or a catalyst, and is a byproduct of the production of phthalic and maleic anhydrides. Fumaric acid is also made in particulate form, where the acid makes up about 5–95% of the particulate, with the remainder being other acids such as malic, tartaric, citric, lactic, ascorbic, and related mixtures.

Applications of fumaric acid include rye bread, jellies, jams, juice drinks, candy, water-in-oil emulsifying agents, reconstituted fats, and dough conditioners. In refrigerated biscuit doughs, the acid eliminates crystal formations that may occur in all-purpose leavening systems. In wine, it functions as both an acidulant and a clarifying aid, although it does not chelate copper or iron.

Glucono-d-Iactone (GDL)

A natural constituent of fruits and honey, GDL is an inner ester of d-gluconic acid. Unlike other acidulants, it is neutral and gives a slow rate of acidification. When added to water, it hydrolyzes to form an equilibrium mixture of gluconic acid and its d- and g-lactones. The acid formation takes place slowly when cold and accelerates when heated. As GDL converts to gluconic acid, its taste characteristics change from sweet to neutral with a slight acidic afteraste.

GDL is produced commercially from glucose by a fermentation process that uses enzymes or pure cultures of microorganisms such as Aspergillus niger or Acetobacter suboxydans to oxidize glucose to gluconic acid. GDL is extracted by crystallization from the fermentation product, an aqueous solution of gluconic acid and GDL.

Because of its gradual acidification, bland taste, and metal-chelating action, GDL has found application in mild-flavored products such as chocolate products, tofu, milk puddings, and creamy salad dressings. In cottage cheese prepared by the direct-set method, GDL ensures development of a finer-textured finished product, void of localized denaturation. It also shortens production time and increases yields. In cured-meat products, GDL reduces cure time, inhibits growth of undesirable microorganisms, promotes color development, and reduces nitrate and nitrite requirements.

Lactic Acid

Lactic acid is one of the earliest acids to be used in foods. It was first commercially produced about 60 years ago, and only within the past two decades has it become an important ingredient. The mild taste characteristics of the acid do not mask weaker aromatic flavors. Lactic acid functions in pH reduction, flavor enhancement, and microbial inhibition. Two methods are used commercially to produce the acid: fermentation and chemical synthesis. Most manufacturers using fermentation are in Europe.

Confectionery, bakery products, beer, wine, beverages, dairy products, dried egg whites, and meat products are examples of the types of products in which lactic acid is used. The acid is used in packaged Spanish olives where it inhibits spoilage and further fermentation. In cheese production, it is added to adjust pH and as a flavoring agent.

Malic Acid

This general-purpose acidulant imparts a smooth, tart taste which lingers in the mouth, helping to mask the aftertastes of low- or non-caloric sweeteners. It has taste-blending and flavor-fixative characteristics and a relatively low melting point with respect to other solid acidulants. The low melting point allows it be homogeneously distributed into food systems.

Compared with citric acid, malic acid has a much stronger apparent acidic taste. As dl-malic acid is the most hygroscopic of the acids, resulting in lumping and browning in dry mixes, the encapsulated form of this acid is preferred for dry mixes.

Malic acid occurs naturally in many fruits and vegetables, and is the second most predominant acid in citrus fruits, many berries, and figs. Unlike the natural acid, which is levorotatory, the commercial product is a racemic mixture of d- and l-isomers. It is manufactured during catalytic hydration of maleic and fumaric acids, and is recovered from the equilibrium product mixture.

The acid has been used in carbonated beverages, powdered juice drinks, jams, jellies, canned fruits and vegetables, and confectionery. Its lingering profile enhances fruit flavors such as strawberry and cherry. In aspartame-sweetened beverages, malic acid acts synergistically with aspartame so that the combined use of malic and citric acids permits a 10% reduction in the level of aspartame. In frozen pizza, malic acid is used to lower the pH of the tomato paste without chelating the calcium in the cheese, as would citric and fumaric acids. This application improves the texture of the frozen pizza.

Phosphoric Acid

The second most widely used acidulant in food, phosphoric acid, is the only inorganic acid to be used extensively for food purposes. It produces the lowest pH of all food acidulants. Phosphoric acid is produced from elemental phosphorus recovered from phosphate rock.

The primary use of the acid is in cola, root beer, and other similar-flavored carbonated beverages. The acid and its salts are also used during production of natural cheese for adjustment of pH; phosphates chelate the calcium required by bacteriophages, which can destroy bacteria responsible for ripening. As chemical leavening agents, phosphates release gas upon neutralizing alkaline sodium bicarbonate; this creates a porous, cellular structure in baked products.

The main reason for incorporating phosphates into cured meats such as hams and corned beef is to increase retention of natural juices; the salts are dissolved in the brine and incorporated into the meat by injection of brine, massaging, or tumbling.

When used in jams and jellies, phosphoric acid acts as a buffering agent to ensure a strong gel strength; it also prevents dulling of the gel color by sequestering prooxidative metal ions.

Tartaric Acid

Tartaric acid is the most water-soluble of the solid acidulants. It contributes a strong tart taste which enhances fruit flavors, particularly grape and lime. This dibasic acid is produced from potassium acid tartrate which has been recovered from various byproducts of the wine industry, including press cakes from fermented and partially fermented grape juice, less (the dried, slimy sediments in wine fermentation vats), and argols (the crystalline crusts formed in vats during the second fermentation step of wine-making). The major European wine-producing countries, Spain, Germany, Italy, and France, use more of the acid than the USA.

Tartaric acid is often used as an acidulant in grape and lime-flavored beverages, gelatin desserts, jams, jellies, and hard sour confectionery. The acidic monopotassium salt, more commonly known as ‘cream of tartar,’ is used in baking powders and leavening systems. Because it has limited solubility at lower temperatures, cream of tartar does not react with bicarbonate until the baking temperatures are reached; this ensures maximum development of volume in the finished product.

Written by J D Dziezak, Dziezak & Associates, Ltd., Hoffman Estates, IL, USA in "Enciclopedia of Food Sciences and Nutrition", Academic Press, USA, Editor-in-chief Benjamin Caballero, excerpts pp.12-17. Digitized, adapted and illustrated to be posted by Leopoldo Costa.