Food acidulants are categorized either as general purpose acids or as specialty acids (1). General purpose acids are those that have a broad range of functions and can be used in most foods where acidity is desired or necessary. Specialty acids are those that are limited in their functionality and/or range of application.
Citric acid and malic acid are the predominant general purpose acidulants with tartaric and fumaric acids. Fumaric acid is included in this category even though its low solubility limits its potential range of application. How-ever, it is used in popular and widely consumed food products.
All other acidulants fall into the specialty acid category. The most commonly used specialty acids are acetic acid (vinegar), cream of tartar (potassium acid tartrate), phosphoric acid, glucono-delta-lactone, acid phosphate salts, lactic acid, and adipic acid.
Citric acid is the premier acid for the food and beverage industry because it offers a unique combination of desirable properties, ready availability in commercial quantities, and competitive pricing. It is estimated that citric acid worldwide accounts for more than 80% of general purpose acidulants used. It is found naturally in almost all living things, both plant and animal. It is the predominant acid, in substantial quantity, in citrus fruits (oranges, lemons, limes, etc), in berries (strawberries, raspberries, currants) and in pineapples. Citric acid is also the predominant acid in many vegetables, such as potatoes, tomatoes, asparagus, turnips, and peas, but in lower concentrations. The citrate ion occurs in all animal tissues and fluids. The total circulating citric acid in the serum of man is approximately 1 mg/kg of body weight (2).
Citric acid is manufactured by fermentation, a natural process using living organisms. The acid is recovered in pure crystalline form either as the anhydrous or mono-hydrate crystal depending on the temperature of crystallization. The transition temperature is 36.68C. Crystallization above this temperature produces an anhydrous product while the monohydrate forms at lower temperatures. With an occasional exception for technological reasons, the anhydrous form is preferred for its physical stability and is the more widely available commercial form.
The salts of citric acid that are used in the food industry are sodium citrate dihydrate and anhydrous, monosodium citrate, potassium citrate monohydrate, calcium citrate tetrahydrate, and ferric ammonium citrate, in both brown and green powder. These salts are used either for functional purposes such as buffering or emulsification, as a source of cation for technological purposes, such as calcium to aid the gelation of low methoxyl pectin, or as a mineral source for food supplementation.
Citric acid is a hydroxy tribasic acid that is a white granule or powder. It is odorless with no characteristic taste other than tartness. Its physical and chemical properties are listed in Table 1. Citric acid and its salts are used across the broad spectrum of food and beverage products:
This is the second most popular general purpose food acid although less than one-tenth the quantity of citric acid used. Malic acid is a white, odorless, crystalline powder or granule with a clean tart taste with no characterizing flavor of its own. The properties of malic acid are listed in Table 1.
Malic acid is made through chemical synthesis by the hydration of maleic acid. An inspection of its structure in Table 1 reveals that malic acid has an asymmetric carbon which provides for the existence of isomeric forms. The synthetic procedure for malic acid produces a mixture of the D and L isomers. The item of commerce, racemic D, L-malic acid does not occur in nature (3) although the acid is affirmed as GRAS for use in foods.
L-Malic acid is the isomeric form that is found in nature. It is the predominant acid in substantial quantity in apples and cherries and in lesser quantities in prunes, water-melon, squash, quince, plums, and mushrooms. Like citric acid, L-malic acid plays an essential part in carbohydrate metabolism in man and other animals.
There are no salts of malic acid that are items of commerce for the food and beverage industries. The application for malic and citric acid cover the same broad range of food categories.
While tartaric acid has the characteristics of a general purpose acid, fluctuating availability and price have caused users to reformulate where possible. Tartaric acid is a dibasic dihydroxy acid. The product is a white odorless granule or powder that has a tart taste and a slight characteristic flavor of its own. The properties of tartaric acid are listed in Table 1.
Tartaric acid has two asymmetric carbons which permit the formation of a dextro (+) rotatory form, a levo (-) rotatory form, and a meso form which is inactive due to internal compensation. A racemic mixture of dextro- and levo-rotatory forms is also a possibility.
Apart from very limited synthetic production in South Africa, tartaric acid is extracted from the residues of the wine industry. Therefore the natural form, L-(+)tartaric acid, is the article of commerce. The structure in Table 1 is L-(+)tartaric acid.
Potassium acid tartrate, also known as cream of tartar, is the major salt used in the food industry. It is occasionally used as the acid portion of a chemical leavening system for baked goods. It is also used as a doctor in candy making to prevent sugar crystallization by inverting a portion of the sucrose.
Tartaric acid can be used in most food categories and functions that require acidification but in practice it is limited to grape flavored products, particularly beverages and candies. It is also used where high tartness is desired in a highly soluble acid.
Fumaric acid is also a naturally occurring, organic, general purpose food acid. Although not found ubiquitously or in the concentrations of citric acid, fumaric nevertheless is found in all mammals as well as rice, sugar cane, wine, plant leaves, bean spouts, and edible mushrooms.
Fumaric acid is made synthetically by the isomerization of maleic acid. It is also produced by fermentation of glucose or molasses with Rhizopus spp. (4). It is a white crystalline powder that has the clean tartness necessary for a food acidulant. Table 1 shows that fumaric acid is the strongest of the food acids and is also the least soluble. The low solubility of fumaric acid limits its usefulness in foods. In processes where 50% stock solutions are mandatory such as carbonated beverages and jams and jellies, fumaric acid cannot be used.
Fumaric acid is extensively used in noncarbonated fruit juice drinks. Its greater acid strength allows lower use levels than citric acid and its solubility and rate of solution are sufficient for this process. Fumaric is also used in consumer packed gelatin desserts because of its low hygroscopicity. The only salt of fumaric acid that has had any application in the food industry is ferrous fumarate for iron fortification.
FUNCTIONS OF FOOD ACIDS
Food acidulants and their salts perform a variety of functions. These functions are as antioxidants, curing and pick-ling agents, flavor enhancers, flavoring agents and adjuvants, leavening agents, pH control agents, sequestrants, and synergists. The definitions for these functions are contained in the U.S. Code of Federal Regulations (5). Some of the functions overlap and in any given application an acidulant will often perform two or more functions.
Flavor Enhancer and Flavor Adjuvant
These functions are performed by a majority of the acidulants consumed by the food and beverage industries. Acids provide a tang or tartness that compliments and enhances many flavors but do not impart a characteristic flavor of their own. The acid itself should have a clean taste and be free of off notes that are foreign to foods. Some acids, such as succinic acid, have a distinctive taste which is incompatible with most food products; and hence, these acids have achieved very little use.
The need for tartness is obvious. Citrus and berry flavors would be flat and lifeless without at least a touch of acidity. However, not all fruit flavors require the same degree of tartness. Lemon candies and beverages are traditionally very sour, while orange and cherry are a little less tart. Flavors like strawberry, watermelon, and tropical fruits require only a trace of acidity for flavor enhancement.
In noncola carbonated beverages, beverage mixes, candies and confections, syrups and toppings, and any application where high solubility is required, citric and malic acids are used extensively. Fumaric acid is used in all ready constituted still beverages for economic reasons. Several acids are suitable for some flavors but not for others. For example, phosphoric acid is used in cola beverages but not in fruit flavored ones. Tartaric acid presents still another category. It has traditionally been used in grape flavored products even though it is suitable for other flavors.
The general purpose acids impart different degrees of tartness that are in part a result of their different acid strengths. Table 2 summarizes the tartness equivalence of the general purpose acids. The relationship shown is based only on tartness intensity and not character of flavor. This relationship can change depending on the formulation ingredients and the particular flavor system being studied. Malic acid, for example, has been claimed to be 10-15% more tart than citric acid in juice based, fruit flavored still beverages. In fruit and berry carbonated beverages, both acids have been perceived as being of equal tartness. Tartness is a difficult property to measure precisely and it must be determined by a trained and experienced taste panel.
Acids have also been used for their effects on masking undesired flavors in foods and food ingredients. Both citric and malic acids and citrate salts are known for their ability to mitigate the unpleasant aftertaste of saccharin. Gluconate salts and glucono-delta-lactone (GDL) have been patented for this function (6,7). Claims of enhanced benefits for malic acid over citric acid when used with the new intense sweeteners have been made but definitive advantages have not yet been demonstrated.
Control of acidity in many food products is important for a variety of reasons. Precise pH control is important in the manufacture of jams, jellies, gelatin desserts, and pectin jellied candies in order to achieve optimum development of gel character and strength. Precise pH control is also important in the direct acidification of dairy products to achieve a smooth texture and proper curd formation. Increasing acidity enhances the activity of antimicrobial food preservatives, decreases the heat energy required for sterilization, inactivates enzymes, aids the development of cure color in processed meats, and aids the peelability of frankfurters.
Gelatin desserts are generally adjusted to an average pH of 3.5 for proper flavor and good gel strength. However, the pH can range from 3.0-4.0. Adipic and fumaric acids are used in gelatin desserts that are packaged for retail sales. Their low hygroscopicity allows use of packaging materials that are less moisture resistance and less expensive.
In jams and jellies, the firmness of pectin gel is dependent on rigid pH control. Slow set pectin attains maximum firmness at pH 3.05-3.15 while rapid set pectin reaches maximum firmness at pH 3.35-3.45 (8). The addition of buffer salts such as sodium citrate and sodium phosphate assist in maintaining the pH within the critical pH range for the pectin type. These salts also delay the onset of gelation by lowering the gelation temperature. The acid should be added as late as possible in the process. Premature acid addition will result in some pectin hydrolysis and weakening of the gel in the finished product. The acid is added as a 50% stock solution and thus soluble acids are required. Citric is generally used in this application but malic and tartaric are also satisfactory.
The United States Federal Standards of Identity (9) provide for the direct acidification of cottage cheese by the addition of phosphoric, lactic, citric, or hydrochloric acid as an alternate procedure to production with lactic acid producing bacteria. Milk is acidified to a pH of 4.5-4.7 without coagulation, and then after mixing, is heated to a maximum of 1208F without agitation to form a curd. Glucono-delta-lactone is also permitted for this application. It is added in such amounts as to reach a final pH value of 4.5-4.8 and is held until it becomes coagulated. GDL is preferred for this application because it must undergo hydrolysis to gluconic acid before it can lower pH. Thus the rate at which the pH is lowered is slowed, avoiding local denaturation.
The activity of antimicrobial agents (benzoic acid, sorbic acid, propionic acid) is due primarily to the undissociated acid molecule (10). On the basis of undissociated acid concentration, Giannuzzi et al. (11) have shown that citric acid is more effective than ascorbic or lactic acids in inhibiting Listeria monocytogenes in a trypticase soya broth containing yeast extract. Under refrigerated temperatures, higher inhibition indices were obtained in the presence of lower concentrations of citric acid. Activity is therefore pH dependent and theoretical activity at any pH can be calculated. Table 3 shows the effect of pH on dissociation. It can be seen why acidification improves preservative performance and why benzoates are not generally recommended above pH 4.5.
The use of acid to make heat preservation more effective, especially against spore-forming food spoilage organisms, is an established part of food technology. Under U.S. Federal Standards of Identity (12), the addition of a suitable organic acid or vinegar is required in the canning of artichokes (to reduce the pH to 4.5 or below) and is optional in the canning of the vegetables listed in Table 4. Vinegar is not permitted in mushrooms. Citric acid is specifically permitted as an optional ingredient in canned corn and canned field corn.
The advantage of acidification is especially well illustrated in the canning of whole tomatoes. When the pH of these is greater than 4.5, there is increased incidence of spoilage in the cans. When tomatoes of pH 3.9 are processed at 2128F, only 34 min are required to kill a normal or high spore load without decreases in color and flavor and deterioration of structure. In contrast, at pH 4.8 the cooking must be 110 min (13).
In the processing of fruits and vegetables, whether for canning, freezing, or dehydration, the prevention of discoloration in the fresh cut tissue is a major concern. Reactions in which polyphenolic compounds are changed by oxidation into colored materials play an important part in this discoloration which may be accompanied by undesirable flavors. The ascorbic acid naturally present in many fruits and vegetables offers some protection, but this is of relatively short duration because of destruction of ascorbic acid by natural enzymes and air. Heating, as applied in blanching, destroys the oxidative enzymes which cause discoloration but may alter flavor and texture if continued sufficiently to completely inactivate oxidative enzymes.
Lowering pH by addition of acid substantially decreases the activity of natural color producing enzymes in fruits. Citric acid also sequesters traces of metals which may accelerate oxidation. Even greater protection is obtained by using citric acid in conjunction with a reducing agent such as ascorbic or erythorbic acid. Some processors have found that a combination of sodium erythorbate and citric acid best serves their needs.
The basis for the formulation of effervescent beverage powders, effervescent compressed tablet products, and chemically leavened baked goods is the reaction of an acidulant with a carbonate or bicarbonate resulting in the generation of carbon dioxide. The physical state of some food acids as dry solids is a property appropriate for beverage mixes and chemical leavening systems. In the absence of water, there is essentially no interaction between such acids and sodium bicarbonate. Thus these dry mixes can be stored for long periods.
A desired property for an acidulant in a chemical leavening system is that it react smoothly with the sodium bicarbonate to assure desirable volume, texture, and taste. Leavening acids and acid salts vary quantitatively in their neutralizing capacity. This relationship is shown in Table 5. A new heat-activated leavening agent, dimagnesium phosphate, was recently reported for use in finished baked products (14).
The various acids differ in their rate of reaction in response to elevation of temperature. This must be taken into consideration in selecting an acidulant for a particular condition. Under some conditions, a mixture of acidulants may be most suitable to achieve desired reaction times. Table 6 compares the reaction times of GDL and cream of tartar.
Glucono-delta-lactone is an inner ester of gluconic acid that is produced commercially by fermentation involving Aspergillus niger or A. suboxydans. When it hydrolyzes, gluconic acid forms and this reacts with sodium bicarbonate. Although GDL is relatively expensive, there are certain specialized types of products such as pizza dough and cake doughnuts for which it is eminently suited as an acid component of the leavening system. Cream of tartar (potassium acid tartrate) has limited solubility at lower temperatures. There is a limited evolution of gas during the initial stages of mixing in reduced temperature batters. At room temperature and above, the rate of reaction increases. Because of these characteristics, and its pleasant taste, cream of tartar is used in some baking powders and in the leavening systems of a number of baked goods and dry mixes.
Antioxidants, Sequestrants, and Synergists
Oxidation is promoted by the catalytic action of certain metallic ions present in many foods in trace quantities. If not naturally present in a food, minute quantities of these metals, particularly iron and copper, can be picked up from processing equipment. Oxidation is the cause of rancidity, an off flavor development in fat. It is also responsible for off color development that renders a food unappetizing in appearance. Hydroxy-polycarboxylic acids such as citric acid sequester these trace metals and render them unavailable for reaction. In this regard the acids function as antioxidants.
Hydroxy-polycarboxylic acids are often used in combination with antioxidants such as ascorbates or erythorbates to inhibit color and flavor deterioration caused by trace metal catalyzed oxidation. The ascorbates and erythorbates as well as BHA, BHT, and other approved antioxidants and reducing agents are oxygen scavengers and are effective when used alone. The effect of the combination of a sequestrant, such as a hydroxy-polycarboxylic acid, and an antioxidant is synergistically greater than the additive effect of either component used alone.
Citric acid is the most prominent antioxidant synergist although malic and tartaric acid have been used. In meat products, U.S. Department of Agriculture regulations permit citric acid in dry sausage (0.003%), fresh pork sausage (0.01%), and dried meats (0.01%). A short dip in a bath containing 0.25% citric and 0.25% erythorbic acid improves quality retention in frozen fish. This treatment is also applicable to shellfish to sequester iron and copper that catalyze complex blueing and darkening reactions.
Untreated fats and oils, both animal and vegetable, are likely to become rancid in storage. Oxidation is promoted by the catalytic action of certain metallic ions such as iron, nickel, manganese, cobalt, chromium, copper, and tin. Minute quantities of these metals are picked up from processing equipment. Adding citric acid to the oil sequesters these trace ions, thereby assisting antioxidants to prevent development of off flavors. Although the oil solubility of citric acid is limited, this can be overcome by first dissolving it in propylene glycol. The antioxidant can be dissolved in the same solvent so that the two can be added in combination.
The acids approved by the U.S. Department of Agriculture for this function in meat products (1) must be used only in combination with curing agents. In addition to ascorbates and erythorbates, the approved acids are:
In conjunction with sodium erythorbate or related reducing compounds, GDL accelerates the rate of development of cure color in frankfurters during smoking. This permits shortening smokehouse time by one half or more and products have less shrinkage and better shelf life.
The special property of GDL upon which these advantages depend is its lactone structure at room temperature. In this form there is no free acid group and the GDL can thus be safely added during the emulsifying stage of sausage making without fear of shorting out the emulsion. Under the influence of heat in the smoking process, the ester hydrolyzes rapidly and is converted in part to gluconic acid. This lowers the pH of the emulsion during smoking, providing conditions under which sodium erythorbate or other reducing compounds (erythorbic acid, ascorbic acid, and sodium ascorbate) react with greater speed to convert the nitrite of the cure mixture into nitric oxide. The nitric oxide, in turn, acts upon the meat pigment to form the desired red nitrosomyoglobin.
The many functions and broad range of applications of food acidulants makes the selection of the most suitable acid for a food product a matter of serious concern. The physical and chemical properties of food approved acidulants must be an essential part of the knowledge of those food technologists who make the decision.
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