"Sodium salt; 384 mg/L for FPA. 'Sodium salt; 5930 mg/L for FPA. c560 mg/L for FPA. d974 mg/L for FPA. "172 mg/L for FPA. '29 mg/L for FPA.
"Sodium salt; 384 mg/L for FPA. 'Sodium salt; 5930 mg/L for FPA. c560 mg/L for FPA. d974 mg/L for FPA. "172 mg/L for FPA. '29 mg/L for FPA.
safety of saccharin does not meet the standards for food additives established by Congress.
Acting in response to strong consumer and industry pressure to ensure the continued availability of saccharin, Congress placed a moratorium on the FDA ban pending additional research. This moratorium on the saccharin ban has since been extended several times, and the one presently in place is set to expire on May 1, 2002. During this period of moratorium, the regulatory climate has evolved considerably such that, at the present time, a policy that considers both risks and benefits is now considered as more appropriate than the absolute statements of the Delaney Clause. In addition, risk is now recognized to be more of a function of dose than an all-or-none phenomenon as was previously considered. Mechanism of action is also recognized as an important factor to consider. Since the 1970s, research has been conducted in effort to elucidate the mechanism whereby saccharin initiates bladder cancer in rats. Interestingly, in one important study, evidence was obtained that the effect, which could only be demonstrated in rats, is due to the sodium component of sodium saccharin and not due to saccharin, the sweetener (30). In this and related studies, sodium ascorbate (vitamin C), sodium chloride (salt), and other sodium salts were shown to promote similar effects on the rat bladder. Most scientists think that the FDA should withdraw its requirement of a cancer warning label for food products containing saccharin in the very near future.
Temporary U.S. regulatory restrictions regarding the use of saccharin have been defined (31). For example, in beverages, saccharin may be used up to a level of 12 mg/ oz (406 ppm). JECFA has established 5 mg/kg as an ADI for saccharin and stated that the rodent bladder tumors caused by sodium saccharin are not relevant to humans.
The physical properties of saccharin are ideal for use as a nonnutritive sweetener. First, it is extremely stable toward all the conditions to which it may be exposed in food applications. In studies conducted at the Sherwin Williams Company, it was determined that at pH 3.3 and 120°C, 18 and 69% loss occurred at 27 and 219 h, respectively (M. L. Mitchell, unpublished results, 1989). The exclusive product of this hydrolysis is 2-sulfobenzoic acid, which is also present as a low-level contaminant in commercial samples of saccharin. At pH 7.0 and 120°C, even greater stability was found with no significant degradation after 27 h and only 6% loss after 219 h. At pH 9.0 and 120°C, stability was also high with 2 and 12% losses, respectively, found after 27 and 219 h. The sole degradation product formed under neutral and alkaline pH conditions is 2-sulfonamidobenzoic acid, which is also a low-level contaminant in commercial samples of saccharin. As a consequence of this high stability, neither loss of sweetness during food product lifetime nor degradation product safety is a significant concern for saccharin. Second, saccharin is highly water soluble. As the sodium salt, 1200 g is soluble in 1 L of water, whereas in the acid form, a solution of 3.4 g/L may be obtained (32). If we consider saccharin Pw (8) to be 180, as an example for applications where an 8% sucrose level of sweetness intensity is required, it can be calculated that saccharin is either 2700
or 8 times as soluble as necessary, depending on whether the sodium salt or free-acid form is employed.
At the present time, no sweetener matches saccharin in terms of cost. As discussed earlier in the general discussion on sweetener cost, the current price of $3.90/lb (18) coupled with Pw(8) = 180 combine to result in a saccharin CSE of 2.2 cents/lb. Thus, saccharin is a very inexpensive sweetener.
Although many sweet-tasting organic compounds were found during the 59 years between the discovery of saccharin and 1937, none achieved significant usage in foods. Then, in 1937, Sveda, a graduate student in Audrieth's laboratory at the University of Illinois, discovered that metal salts of cyclohexylsulfamic acid are sweet (33). These salts have become known as cyclamate salts or, more commonly, cyclamate. Cyclamate, in the acid form, has the chemical structure 2 and was the first of the sulfamate structural class of sweeteners to be discovered. Sweeteners of this class exhibit the —NH-S03—moiety as the common structural feature. Cyclamate is generally used in foods as either the sodium or calcium salt. Since 1937, many sweet analogues of 2 have been synthesized, although none have been developed for use in foods.
Sensory panel studies on cyclamate salts demonstrate them to better reproduce the taste quality of sucrose than does saccharin. Nonetheless, a significant bitter taste attribute is noted for cyclamate. The flavor profile of sodium cyclamate is illustrated in Figure 1. Interestingly, a salty attribute is also present in the sodium cyclamate profile. This factor is almost certainly due to the high sodium ion concentration present in intensely sweet solutions. The C/R function for sodium cyclamate is illustrated in Figure 2. As is true for all high-potency sweeteners, the potency of sodium cyclamate is dependent on sucrose reference concentration. Thus, PJ2) = 42, Pw(8) = 23, and PJ10) = 17 are calculated from the equation given in Figure 2. The temporal profile of sodium cyclamate is very similar to that of sucrose. ATs of 4 and ETs of 14s have been determined for both substances (7). Thus, although sodium cyclamate exhibits a flavor profile with significant bitter and salty notes, it exhibits a sucrose-like temporal profile and, in the aggregate, mimics the taste of sucrose sufficiently to be generally useful in food applications.
The history of the evaluation of cyclamate safety has been reviewed comprehensively (34,35). Cyclamates were first approved in 1951 for use as drugs for use by people with diabetes and others who had to restrict their sugar intake. In 1958, they were listed by the FDA on the original GRAS list. As was discussed earlier, the use of cyclamate sweeteners experienced explosive growth during the 1960s after it was found that a 10:1 mixture of cyclamate and saccharin exhibits taste quality superior to either sweetener individually and approaches that of sucrose. In 1969, the FDA became concerned about cyclamate safety. At that time, the FDA was advised of a rat study in which the commonly used 10:1 cyclamate/saccharin mixture was shown to induce bladder tumors. These results were interpreted by the FDA to indicate that cyclamate salts (ie, not saccharin salts) are bladder carcinogens in rats. As a consequence, cyclamates were immediately removed from the GRAS list and, in 1970, were banned from use. At the same time, however, cyclamates have remained on the market in more than 50 countries.
Since the 1970 FDA action, Abbott Laboratories and the Calorie Control Council have maintained continuous efforts to have cyclamates reapproved. As a result of this effort, it is now generally accepted that cyclamates are not carcinogens (34). The principal remaining areas of concern are related to the biological activity of cyclohexylamine, the major cyclamate metabolite. In early studies, hypertensive activity and testicular atrophy effects were noted for this metabolite. However, in recent studies, no adverse hypertensive or reproductive effects have been observed in humans. Nonetheless, these weak biological effects for cyclohexylamine may limit the ADI to be granted by the FDA in the event that it is reapproved. The ADI established by JECFA for cyclamate is 11.0 mg/kg. In the European community, cyclamate levels in CSDs are restricted to a maximum of 400 ppm (36). From the equation given in Figure 2, it can be calculated that 400 ppm cyclamate is equivalent to 1.7% sucrose in sweetness intensity. Thus, although cyclamates may be reapproved in the United States, restrictions may limit their application to sweetener blend systems.
Cyclamates exhibit excellent solubility and stability characteristics for use in essentially all imaginable applications. The sodium and calcium salt forms are both commercially available. Although the acid form is sufficiently water soluble (13.3 g/100 mL), its high acidity results in preference for the very soluble sodium (20 g/100 mL) or calcium (25 g/100 mL) salts (37). Since Pw(8) = 23, as an example for applications where an 8% sucrose equivalent level of sweetness is desired, it can be calculated that sodium cyclamate is 570 times as soluble as necessary. Hy-drolytic degradation of cyclamate salts proceeds to yield cyclohexylamine and inorganic sulfate. As a consequence of the adverse biological activity of cyclohexylamine, FDA scientists conducted a comprehensive evaluation of cyclohexylamine levels in a spectrum of food products (38). Cyclohexylamine was found in the majority of these products albeit at low levels. Interestingly, even in the most acidic samples (cola CSDs), cyclohexylamine levels increased insignificantly during 4 months of ambient-temperature storage. Cyclohexylamine in food products appears to be substantially derived as a cyclamate sweetener contaminant. Data have been reported on the hydrolysis of cyclamate under extreme conditions (39). After 1 h at 100°C, 13.7, 8.1, 0.98, 0.10, 0.52, and 0.58% of cyclamate sweetener is lost at pH values of 0.9, 1.6, 2.5, 4.5, 5.3, and 6.5, respectively. In summary, cyclamate sweeteners are quite stable. No significant loss of sweetness or generation of unsafe degradation products is expected in any common applications.
Cyclamates are economical sweeteners. On the world market, sodium cyclamate is presently available for $1.04/ lb (40). On a CSE basis, this is equivalent to 4.5 cents/lb if one employs Pw(8) = 23 times that of sucrose as the relevant sweetness potency. Thus, although sodium cyclamate is approximately two times more expensive than saccharin, it still represents a very substantial economy over sucrose.
Aspartame l-Aspartyl-l-phenylalanine methyl ester (3) is an example of one of several novel structural types of sweet-tasting organic compounds identified in the 1960s. It is known under the generic name aspartame, is often abbreviated as APM, and is very well known to consumers as Nutra-Sweet®, a brand name of Monsanto. As was true for all the sweeteners described herein that are not of botanical origin, the sweetness of APM was discovered by accident. The discovery was made by Schlatter while working under the direction of Mazur of G.D. Searle & Company (3). APM was prepared by Schlatter as an intermediate in a drug discovery program aimed at finding new ulcer treatments. Interestingly, this compound had been prepared some years earlier by chemists at ICI in Great Britain. Its sweet taste had not been noted, however.
HOOC NH2 7=0
Aspartame was the first sweet-tasting member of a structural class of sweeteners generally known as the di-peptides. Peptides are oligomers of a-amino acids. Aspartame is simply derived from the two natural amino acids l-aspartic acid and l-phenylalanine. It is ironic that even though substantially more than 1000 sweet-tasting analogues of aspartame have been prepared since 1965, none are more advantageous than aspartame after consideration of all the properties requisite for commercial viability.
The phenomenal commercial success of aspartame is easily explained by its exceptional sucrose mimicry. As suggested by its flavor profile summarized in Table 1, it is essentially indistinguishable from sucrose; no nonsweet taste attributes are observed. Interestingly, however, the temporal profile of aspartame is different from sucrose. Aspartame exhibits both a slightly delayed AT [5 s (APM) vs. 4 s (sucrose)] and a protracted ET [19 s (APM) vs. 14 s (sucrose)] (7). These differences are not sufficient, however, to adversely affect acceptability in food products. An interesting and useful manner in which aspartame has an advantage over other high-potency sweeteners is that it enhances other flavor attributes. This effect was noted early on and has been systematically studied by Baldwin and Korschgen (41) and also by Wiseman and McDaniel (42). It has been hypothesized that this unique advantageous property of aspartame is due to its atypical temporal profile (43). More specifically, aspartame's flavor enhancement properties are suggested to be consequences of its slightly delayed AT relative to that of sucrose. Thus, greater temporal resolution of sweetness (taste) and flavor (olfaction) signals to the brain occurs for APM than is the case for sucrose. This increased resolution of neural olfactory and taste signals for APM over sucrose may result in the flavor intensity to be judged higher in the better-resolved aspartame-sweetened system. This effect is then interpreted as a flavor enhancement effect of aspartame.
The sweetness potency of aspartame is again dependent on the sucrose reference concentration, although somewhat less so than for the other sweeteners discussed in this section. Thus, from the APM C/R function given in Table 1, it is calculated that Pw(2) = 250, Pw(8) = 140, and Pw(10) = 107. Disparities between these potencies and literature potencies quoted earlier are a consequence of differences in methodology.
The safety of aspartame has been as extensively studied as any food additive. Most of the principal metabolism, preclinical, and clinical studies have been reviewed (44—46). On the basis of evaluation of all the safety assessment studies, aspartame was approved by the FDA in 1981 with an ADI of 20 mg/kg. This ADI is based on a NOAEL in preclinical studies of greater than 2,000-4,000 mg/kg. In 1983, based on clinical studies, the ADI for aspartame was raised by the FDA to 50 mg/kg. The ADI established by JECFA for aspartame is 40 mg/kg. No effects were noted in the clinical studies at doses many times greater than approved for human consumption. Consideration of aspartame metabolism explains why this is the case. On ingestion, aspartame is completely broken down to the two natural amino acid building blocks and methanol. The safety of the amino acids l-aspartic acid and l-phenylalanine is, of course, no surprise since normal dietary protein (from meat, milk, eggs, etc) provides substantially greater quantities of these nutrients. Methanol also is not a safety concern when exposure occurs as a metabolite of aspartame. Beverages formulated with aspartame to a sweetness level matching 10% sucrose contain the equivalent of 50 to 60 mg/L of methanol; this is substantially less than the average of 140 mg/L content for fruit juices. Thus, the safety of aspartame should be no surprise to anyone. Consumption of aspartame does not result in exposure of the internal body tissues to any novel substances. Natural subunit assembly and metabolic subunit disassembly is a unique high-potency sweetener concept presently exemplified only by aspartame. Despite the safety of aspartame, however, products containing it are required to carry an informational statement for people with a rare genetic disease known as phenylketonuria (PKU). Approximately 1 of 15,000 people has this disorder, which involves an inability to metabolize phenylalanine. Unchecked, PKU results in mental retardation. In the United States, this disorder is detected at birth, if present. The normal treatment is a phenylalanine-restricted diet from infancy through childhood and sometimes into adulthood. It should be emphasized, however, that phenylalanine consumption is only restricted, not eliminated, since it is an essential amino acid and, as such, is necessary for life. Persons with PKU must carefully monitor their consumption of phenylalanine from all dietary sources, including aspartame.
Aspartame exhibits sufficient solubility for all food applications. At ambient temperature in water, a solubility of approximately 1.0% can be attained at pH 4 and also at neutral pH; solubility reaches a minimum at pH 5.5, the isoelectric point (47). Thus, using Pw(8) = 140, it can be calculated that aspartame is greater than 70 times more soluble than necessary to provide an 8% sucrose level of sweetness intensity.
If aspartame has a drawback, it is hydrolytic stability. At 25°C (77°F), its hydrolytic half-lives in aqueous buffer are 116, 260, 242, 82, and 2 days for pHs of 3, 4, 5, 6 and 7, respectively (21). Clearly, maximum utility would be expected in the pH range 3 to 5. Fortuitously, this is a representative range for most food systems. It is particularly fortuitous that none of the three principal degradation products, l-aspartyl-l-phenylalanine (AP), 3-benzyl-6-carboxymethyl-2,5-diketopiperazine (DKP), and /?-l-aspar-tyl-l phenylalanine methyl ester (/i-APM) exhibit either off-tastes in aged food products or safety problems. All the degradation products have been tested and demonstrated to be safe. Interestingly, /?-l-aspartyl-l-phenylalanine (fi-AP), the product of ester hydrolysis of /?-APM, has been demonstrated to be naturally produced on metabolism of dietary protein (48). In summary, aspartame is sufficiently stable for use in acidic food products. On the other hand, the neutral pH range encountered in baked goods is problematic. An encapsulated form of aspartame has been developed, however, to address this limitation (49).
The discovery of acesulfame was accidental, just as was the case for saccharin, cyclamate, and aspartame. In 1967, Clauss and Jensen, while working in the laboratories of Hoechst AG on the reactions of fluorosulfonylisocyanate with acetylenes, obtained the product 5,6-dimethyl-l,2,3-oxathiazin-4(3H)-one-2,2-dioxide and noted it to be sweet (50). As a consequence of the 1969 ban on cyclamates in the United States, Hoechst initiated a systematic program to optimize the properties found in this compound. In the end, it was determined that the preferred product candidate was 6-methyl-l,2,3-oxathiazin-4(3ii)-one-2,2-dioxide, which has since been given the generic name acesulfame and has chemical structure 4. The potassium salt of acesulfame is known as acesulfame-K, is often referred to as ACE-K, and has been given the brand name Sunette®. Acesulfame is structurally related to saccharin in that both exhibit the essential-for-activity N-sulfonyl amide structural motif (ie, -C0NHS02-) and thus are both members of the N-sulfonyl amide sweetener class. The chemistry, safety assessment, sensory properties, and applications of acesulfame-K have been reviewed (51).
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