Chemistry of Food Additives Direct and Indirect Effects George H. Paull Food and Drug Administration, Division of Food and Color Additives, 200 C St., S.W., Washington, DC 20204 The papers in this symposium are a reminder of the intractable complexity of the chemistry of foods. In this larger context, I am tempted to think that the chemistry of food additives is a much simpler subject than we at the FDA normally believe i t to he. In general we know far more about the chemistry of what are commonly called "food additives" than we know about the chemistry of foods themselves. On the other hand, one cannot really distinguish between food additives and foods, and U S . law does not distinguish them. Food additives are ingested as components of foods: they may be natural or synthetic: they may be deliberately added to the various foods in amounts raneine from a few nounds to millions of pounds per year; and tceyhay even be kintentionally added through migration from packaging materials. I will omit the bewildering fine points of the legal defmition of the term "food additive." I t is i m ~ o r t a n tto recoenize. - . however, that what one person means by the term may he different from what others mean. I will discuss food additives in the larger sense as including substances that are exempt from the food additive ~rovisions of the law by virtue of their being generally recognized as safe by qualified experts or by being specifically approved by the U.S. government prior to 1958. Food additives, in this context, mav be considered substances whose deliberate use bv the f 1 d industry causes the fwd ro he changed in some way.l will try to show how knowledge of their chrm~stryhelps us place our dafety concern at an appropriate lrvrl and illustrate by historical examples how such knowledre has contributed to aeencv ~ ~ w decisions. ~ ~ ~ A Although many aspects of food additive chemistry can he scientificdly interesting or important as clues to helpus focus on essential issues, as a regulatory agency responsible for assurine a safe food suonlv .. . our ultimate ouestions have to be: (1) what new .iubsttlnces will be found in food as aresult of an additive; (2) what will be their concentration in 1'ood;and (3) how much of these substances one is likely to consume. The t w e s of thinking characteristic of oreanic. inoreanic. ohvsical. hiochemistr; will help guide o& understkdi& o i wh; additives are used. what their fate is likelv to he. and what questions should bk asked to assure that thkir use is safe, hut to a reeulatorv chemist. analvtical chemistrv. nrovides the . most useful answers to essential concerns. T o answer the above auestions. we first must know the identity of an additive, that is, the primary component(s), impurities, and degradation products tbat are likely to be found under its conditions of use. The first of these, the primary component or components, usually can be established readily but in many cases identity can be defined only by a manufacturing process and physical and chemical properties. The composition of an additive of natural origin can vary from season to season and certainly can vary with the natural species of origin. Analytical verifiable specifications may he needed to control impurities and products of degradation. Products resulting from reaction with food can also he of significant health concern, as I will illustrate later. The amount of an additive in a food. and hence in the diet. will vary with the technical effect of the additive. A color ad: ditive will generally he used in much smaller concentrations ~~~~~~~~~~~~
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Journal of Chemical Education
than an additive affecting texture. Packaging materials also become components of food in differing amounts depending on the composition of the package, type of food, and storage conditions. Because of the principles of diffusion, molecules of lower mass are more likely to migrate to fwd than are larger molecules. The morphological properties of a polymer (glassy or crystalline, oriented or non-oriented) will affect the degree to which it leaches from packaging material and enters foods. Catalysts, used in small amounts, are much less likely to hecome components of foods than are the plasticizers which are used a t the 20-35 percent level to make plastics flexible. Because of the ereater likelihood of extraction of chemical components frim packaging materials intended for unrefrigerated foods. for moist foods. or for foods that remain packaged for proionged periods of time, such packaging materials are of ereater concern than are those used onlv with refrieerated fiods, with dry foods, or with foods thacwill remain in the package - onlv.for short ueriods. The imoortant ooint is that the purpose for which an additive is usedand the conditions under which it is used affect the amount of substance which becomes part of the diet. Therefore, we do not say that certain additives are safe, hut rather that they can he used safely under certain conditions. Analytical chemistry is essential for establishing that an additive can be used safely. One usually cannot measure minor components of a package migrating into a complex food matrix. Therefore, we categorize foods according to properties that might affect packaging material so tbat pure solvents may be used to simulate foods in tests. Foods that may be aqueous, acidic, alcohol-based, or fatty are simulated by water, 3% acetic acid, 8%or 50% ethanol or heptane. We require that migration from new packaging materials into simulating solvents be determined under worst-case storaee conditions before the substance is authorized for use. We $ten find that we need to set specifications for the purity of an additive or for the amount of an additive to be used. In such cases wereauire that reliable analvtical methods be uuhliclv available .. so that we can assure tbat manufacturers comply with reouired restrictions. If s~ecificationsor restrictions are necessary to assure that anAadditivecan be used safely, then we will conclude that the use of the additive is safe only if reliable methods are available to enforce those restrictions. The following examples illustrate some of the principles that I have described. Most of them deal with identity, since qualitative or descriptive chemistry will best fit the available space. Also, the chemical issues regarding additives deliheratelv added to food are more clearly defined than are the chemical issues of packaging materi&, so I will concentrate on the former. Polysorbate 60 The first example, polysorhate 60, is a memher of the class of non-ionic emulsifiers, which is a condensation product of sorhitol anhydride (sorbitan),stearic acid, and ethylene oxide. Roughly 1.5 million lbs of polysorbate 60 are added annually to the nation's food supply. Its regulatory name provides little chemical information, but the common synonym polyoxyethylene (20) sorbitan monostearate presents a mental picture
of a unique molecular structure. However, when forming the anhydride, sorbitol can cyclize and dehydrate to form three independent structures with different numbers of hydroxyl groups for further reaction.
Further, not all the sorhitol necessarily will react. Now, to prepare the monostearate, sorhitan and residual sorbitol must he esterified with stearic acid, but which hydroxyl group will be esterified? All of them, of course, and none of them, because while some molecules receive one, two or three stearates, some will receive none. Actually, to assure the appropriate emulsifying effert, the material contains an average of 1.3 moles of fatty acids per mole of sorbitan, so mono means 1.3. Also since jtearic acid is of natural origin the commercial product containsa mixtureofassociated fatty acids,chiefly palmitic. 'I'he polyoxyethylene (20) means that an averageof20 moles of ethvlene oxide are condensed onto the remaining hydroxyl to form ether linkages. Because of the structure of ~~
/O\ CH2-CH2
CI-CH-CH,OH
a
~
ethylene oxide ethylene ehlorohydrin
to human health was prepared for FDA and USDA by the National Research Council 13). .. From ancient times, salts have been used t o cure meat. In regions where these salts contained a large fraction of nitrate and nitrite impurities, the salts' effect of fixing a pink color in the meat was recoenized. Nitrates and nitrites can he converted through enzymatic actions in many bacterial, fungal, and ~ l a nsvstems t to nroduce reactive nitroeen oxides. Onlv recently was it discovered that nitrite, rather than nitrate, oroduces the desired effects. Nitrite d t n react with mvoelobin " " in red meat to form the more stable nitric oxide myoglobin which forms, upon heating, the pink nitrosyl hemochrome that holds its color under conditions in which the red oxymyoglohin from uncured meat is converted to brown metmyoglobin. This is the basis of some of the earlier interest in fixing color with curing salts. For reasons still not completely determined, nitrite also contributes significantly to the flavor of cured meats. Most important, through mechanisms still unclear, nitrite retards microbial spoilage by inhibiting the growth of a variety of microorganisms, the most significant of which is C. botulinum, the pruducerdthe deadly toxin responsihle for hotulism. N~tritealso serves as an antioxidant to retard lipid oxidation and the corresponding development of rancidity in meat. While some scientists continue to investigate the mechanisms whereby nitrite, possibly in combination with other comoonents of meat. inhibits C. botulinum erowth. others havestudied the important, unintended reaction withamines present in food. Nitrous acid, of course, is a good reagent for classifying amines. Because it is a good source of nitrosonium ion (NO+). it readilv combines with secondarv m i n e s to form nitrosamines; many nitrosamines are known to be potent carcinogens. ~
1.4dioxane
ethylene oxide, i t can form ether linkages with itself as well as with sorbitan (otherwise a 20:l ratiocouldnot beachieved) or form an ester with unreacted fatty acid. Therefore, i t can also form polyethylene glycol or polyoxyethylene stearate. Given this background, i t no longer seems easy to draw a structure of polysorbate 60. Clearly this additive is defined by how i t is made and by physical properties. I t is indeed a mixture of all the imaeinable comoonents. Most of these ingredients or their mekbolites are Ather common components of food or, as u,ith oolvethvlene elvcol. are well.tested substances. SO far, hoke;er, i haveUl;een describing only the identity of the primary product. The major concerns about impurities result from degradation produds or side reactions of ethylene oxide and are relevant to all ethoxylated compounds. Ethylene oxide itself is toxic and is used as a microbial sterilizer. When reacted with water in a 1:l ratio it forms ethylene glycol. When hydrated in the presence of a chloride ion it forms ethylene chlorohydrin (Fig. 2). It can also react with itself and water to form diethylene glycolor cyclize to form 1,4-dioxane (Fig. 2). All of these lntter compounds are of toxic cuncern, and the chemical Industry has developed manufacturing rworesses and clean-UD procedures to minimize their ~ e c e n t lthe ~ , mou& of dioxane, which ranged up to several hundred parts per million (ppm), raised significant concern. FDA developed a method to measure dioxane in polysorbates a t the ppm level (I),and a specification was established in the Food Chemicals Codex limiting dioxane in food-grade ethoxylates to 10 ppm (2).
NRrate and Nltrlte Salts The second example deals less with identity of the additive than with identity of active components and with different chemical properties that can produce different chemical, physical, and biological effects. Nitrate and nitrite salts nrovide interesting examples for several reasons, both chemical and legal. They are sometimes, hut not alwavs. exemnt from fadadditive &IS under the law becauseof.U.S.Department of Agriculture (USDAI approvals granted 1)efore 1958.Thev have multiple effectsin meats. An rxcellent, thorough review ofall aspectsof nitratesand nitritesas they relate to fad and
RzNH f HONO
-
R2NN=0
~
+ HzO
.
The formation of nitrosamines aonears . to be inevitable whenever nitrites, nitrates, or nitrogen oxides are present with amines. (This is true even of maltine-. nrocesses where flames used to dry the malt cause nitrogen and oxygen in air to produce nitrogen oxides which react with amines in the malt. As a result, nitrosamines have been found in scotch and beer.) Because of the concern about nitrosamines. sensitive analytical methods have been developed that can determine many nitrosamines in the low parts-per-billion level. For example, low levels of N-nitrosopyrrolidine are routinely determined in cooked bacon by USDA. Also, manufacturing practices have been changed to minimize the production of nitrosamines. Because the desired effects appear to be due to reaction products of nitrite rather than residual nitrite or nitrate, the amounts of nitrite added to food have been reduced and nitrate salts are no longer added to many foods in which they were previously used. Also, as the labels on packages of cured meats indicate, salts of ascorbic acid (vitamin C) or isoascorbic acid (erythorbic acid) are frequently added. These and other antioxidants inhibit nitrosation by reducing nitrous acid to nitric oxide. This example illustrates the complexity of food systems in that a simple salt, sodium or potassium nitrite, originally present accidentally as a reduction product of an i m.~ u r i.t vin common salt. is now used intentionallvas a color fixer, flavor, antimicrobial agent, and antioxidant but may react with comoonents of food to form toxic bv-oroducts. These reactions; in turn, are inhibited by other abt'ioxidants which may he nutrients (vitamins C and E).
.~
~~~~
Diethylpyrocarbonate (DEPC) As with nitrite, competing reactions with components of food can pose problems with other additives. Years ago, a new antimicrobial agent was developed which seemed ideal. Diethylpyrocarbonate (DEPC), or diethyldicarbonate, acted as a "cold sterilizer" in beverages such as beer, wines, and fruit juices. Not only was i t effective, but also it degraded rapidly Volume 61
Number 4
April 1984
333
in water to products which were normally present in much greater amounts, ethanol and carbon dioxide. C,H3-O--CO--04M42H5 (DEPC)
+ H20
-
2C1H,0H + 2C0,
In view of the concern about the use of anv antimicrobial agent (a substance used for its toxic effect onmicroorganisms-but potentiallv toxic to higher organisms as well), a sterilizer that hydrolyzes to ethanoiand carbon dioxide seems to be a bit of good luck. Again, however, chemicals may react with anythin% iiven the riiht kinetics and thermodynamics, and beer, wine, and fruit juices contain more than water. Consequently, the use of LWI'C produced complicated side effects. Less than ten years after it was approved for use in the U.S., Swedish srialso reacts with ammonia to enrists renorted . - ~ ~ ~(41 ,, that ~ DEPC produce ethyl carbamate, or urethane,
-~.~ ~
~~
~~
(urethane) and that ammonium ion is present in such foods in significant amounts. Urethane was another of the substances known to be carcinogens, whose number a t that time was small but rapidly growing. A research program was quickly developed to test alternative hypotheses and to determine urethane in beverages sterilized with DEPC. FDA scientists found low f in grapefruit juice, and industrial scientists levels ~ ; urethane found i t in heer and wine when these heverages were treated in the laboraturv with legallv authorized amounts of 1)F:I'C. The approval f i r use of thismaterial was revoked (5). Chick-Edema Factor Even additives that are a normal component of the diet can pose safety problems when their method of production changes. In 1958. the Commissioner of Food and Drugs. .. . testifyi& before Congress on the need for legislation regulating food additives, described a seriuus probirm with an additive in poultry feed: Fat added to chicken feed made chickens grow faster, and i t appeared to be a safe additive. Feed producers looking for cheap sources of fat obtained by-product fatty acid from a firm engaged in separating glycerine from fat. Millions of chickens in the eastern US. died from ingesting this feed, and a search was begun to isolate the toxin, which became known as the chick-edema factor. Other fatty acids were also found to contain the toxin, which was still unidentified but thought to consist of chlorinated aromatic hydrmarbons. FDA required that batches of fatty acid be tested in a bioassay.
334
Journal of Chemical Education
I.ater, gas chromatographic methods were developed which showed characteristic peaks that ruuld be used to srreen for chick-edema factor. Industrial scientists investigated individual components of the toxin and showed them to be chlorinated dibenzo-p-dioxins, Pyrolysis of commercially available chloropbenols, including pentachlorophenol, produced chromatographic peaks with retention times identical to those of the cblorodioxins in the chick-edema factor. Chlorophenols were often used in pretanuing hide-stripping operations and by-product fats were salvaged from the bides. These fats were therefore contaminated with chlorophenols, providing a source for dioxin formation (6). Thus, impurities from a manufacturing process can pose serious problems with "natural" food additives as well as with "synthetic" additives. Much has been nublished about the chemistrv of dioxins. so 1will not dwell on that aspect, but I du want toemphasize the fact that in the 1950's and 1960's there were toxins of unknown identity sometimes found in fatty acids. Summary Excellent books have been written about food additives and their chemical effects, including a handbook (7). The interesting chemistrv of these substances can varv as widely as the imaginatiun of enterprising chemists: from the use of immobilized mrvmes, to indirestihle polvmeric hulkinr agents or antioxidants, tobutadiene-styrene rubber for usein ;hewing gum base, or to asweetener made from amino acids. As these examples show, however, when we at FDA look at the chemistry of food additives we are often concerned with unintended components found in low concentration. Because we are concerned primarily with safety evaluation, we are more interested in the components that pose safety problems than in the mechanisms that explain why an additive is effective. With readilv available knowledee of toxic chemicals and accepted practices of resting, it is unlikely that a manufacturer tudav would deliheratelv add a harmful substance to food. ~ h u i we , look for likeiy impurities from manufacturing, degradation ~roducts.reaction nroducts with food. and subs t k c e s migrating from food. A11 of these are likely to be comnonents of food onlv at low concentrations. The areas of chemical knowledge that traditionally have been essential in decision-making are the results of analvses that show which of the many possil~ilit~rs are trur and u hirh arr merely speculative, and thedata that show whethera particular hatch of additive continues to meet acceptable standards. Literature Clted (1) Birkel,T.J., Wamer,C.R.,andFaeio,T., J. Assoe.
ON
A n d Chem., 62,931
(1979).
(21 Committee on Coder Specifications, "Fmd Chemicsls Coder," 3rd ed., National Academy Press,Washington. DC, 1981,pp. 235,477. (3) Committee on Nitrite and Alternative Curing Agents in Fmd. "The Health Elf& of Nitrato, Nitrite, and N-NitrosoCompounds." National Academy Prers, Washington. DC. 1981. (4) L4froth. G.,and Gejvsll, T., Science, 174.1248 (1971). (51 '"37 Federal Register."p. 15426 (Aug. 2,1972). (6) Firestone, D.,"Etiology of Chick Edema Disease," Enuir. H d l h Perspeef.. 5. 59
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