The Committee evaluated a group of 15 furfuryl derivatives (Table 1) by the Procedure for the Safety Evaluation of Flavouring agents. The group comprises the parent furfuryl alcohol, the corresponding aldehyde, furfural, five esters formed from furfuryl alcohol and simple aliphatic carboxylic acids, five esters formed from simple aliphatic alcohols and furoic acid, and three structurally related furfuryl derivatives (5-methylfurfural, 2-benzofurancarboxaldehyde, and 2-phenyl-3-carbethoxyfuran). These flavouring agents were grouped on the basis of the criterion that all are hydrolysed and/or metabolized to furoic acid or a substituted furoic acid.

The Committee has evaluated only one member of this group previously, namely furfural (No. 450). Furfural was considered by the Committee at its thirty-ninth and fifty-first meetings (Annex 1, references 101 and 137), but no ADI was established. At its present meeting, the Committee established a group ADI of 0–0.5 mg/kg bw for furfural, furfuryl alcohol, and a number of derivatives of furfuryl alcohol and furoic acid on the basis of a NOEL of 53 mg/kg bw per day in a 13-week study in rats given furfural, with a safety factor of 100 (see monograph addendum on furfural, p. 3).

Seven of the 15 substances have been detected as natural components of foods (Maarse et al., 1994, 1996) including roasted coffee, beer, milk, roasted almonds, white bread, and whisky.

The total annual volume of production of the 15 substances in this group was reported to be 6600 kg in Europe (International Organization of the Flavor Industry, 1995) and 4500 kg in the USA National Academy of Science, 1989; (Lucas et al., 1999). These values are equivalent to total daily per capita intakes of 940 µg in Europe and 590 µg in the USA. Furfural accounted for approximately 55% of the total per capita intake in Europe (520 µg/day) and 77% of that in the USA (460 µg/day). The annual volumes of use and estimated per capita intake of furfuryl alcohol and 14 related substances are shown in Table 2.

Table 2. Annual volume and estimated per capita intake of furfuryl alcohol and related substances in Europe and the United States^a

N/D, no intake data reported; –, no quantitative information available on natural occurrence in foods

Furfuryl esters are hydrolysed to furfuryl alcohol and the corresponding carboxylic acid. Furfuryl alcohol (No. 451) is subsequently oxidized to furfural (No. 450), which is then oxidized to 2-furoic acid. Furoate esters (Nos 746–750) are or are predicted to be hydrolysed directly to 2-furoic acid and the corresponding alcohol. Furoic acid forms a coenzyme A (CoA) thioester, which may be either metabolized to a glycine conjugate that is excreted in urine or condensed with acetyl CoA to form 2-furanacryloyl CoA, which is converted to a glycine conjugate and excreted in urine. The three remaining furfuryl derivatives (Nos 745, 751, and 752) are expected to follow similar metabolic pathways, i.e. ester hydrolysis, oxidation, and glycine conjugation, with side-chain oxidation (No. 745) or aromatic oxidation (Nos 751 and 752). In rodents, a minor pathway has been identified which involves furan ring oxidation to produce carbon dioxide and as yet unidentified metabolites.

Table 1 summarizes the evaluation of furfuryl alcohol and 14 related substances used as flavouring agents.

In the unlikely event that all foods containing the nine substances in structural class II (together with allyl 2-furoate evaluated previously by the Committee) were consumed concurrently on a daily basis, the estimated combined intake would exceed the threshold for human intake for class II. In the unlikely event that all foods containing the six substances in structural class III were consumed concurrently on a daily basis, the estimated combined intake would exceed the threshold for human intake for class III. Given the wide margin of safety between the level of intake and the NOEL for furfural and the fact that the available detoxication pathways (glycine conjugation or condensation followed by glycine conjugation) would not be saturated at the current levels of intake, the Committee concluded that the combined intake would not raise concern about safety.

On the basis of the predicted metabolism of these substances and data on their toxicity, the Committee concluded that consumption of furfuryl alcohol and the 14 related substances in this group at the current levels of intake would not raise concern for safety. In applying the Procedure, the Committee noted that all of the available data on toxicity are consistent with the results of the safety evaluation.

This monograph summaries key data relevant to the evaluation of the 15 flavouring substances in this group, which includes the parent alcohol, furfuryl alcohol, the corresponding aldehyde furfural, five esters formed from furfuryl alcohol and simple aliphatic carboxylic acids, five esters formed from simple aliphatic alcohols and furoic acid, and three structurally related furfuryl derivatives (5-methylfurfural, 2-benzofurancarboxyaldehyde, and 2-phenyl-3-carbethoxyfuran).

Seven of the 15 substances have been detected as natural components of foods (Maarse et al., 1994, 1996). Quantitative data on natural occurrence and consumption ratios have been reported for five substances in the group, and these indicate that they are consumed predominantly from traditional foods (i.e. consumption ratio >1) (Stofberg & Kirschman, 1985; Stofberg & Grundschober, 1987; see Table 2).

The daily per capita intake of this group of flavouring agents is 940 µg/day in Europe (16 µg/kg bw per day) and 600 µg/day in the USA (10 µg/kg bw per day). Furfural accounted for approximately 55% of the total per capita intake in Europe (520 µg/day) and 77% in the USA (460 µg/day).

A recent study showed that furfural is formed in vivo during oxidative damage to DNA by hydroxylation of the C5’ repair of deoxyribose (Dix et al., 1996). The oxidized deoxyribose fragment forms furfural, which may react subsequently with adenine bases to produce kinetin (N⁶-furfuryladenine), which has been found in human DNA (Barciszewski et al., 1996, 1997a,b). The amount of furfural formed by this mechanism would be expected to be extremely small and the substance would be short-lived.

Substances were considered to be members of this group on the basis that all furfuryl alcohol derivatives are hydrolysed and/or metabolized to furoic acid or a substituted furoic acid.

2.3.1 Absorption and transformation

Furfural is converted to furfuryl alcohol by enteric bacteria under both aerobic and anaerobic conditions (Boopathy et al., 1993). Both substances are therefore anticipated to be present in the gastrointestinal tract of animals given furfural. Furfuryl alcohol and furfural are rapidly absorbed in the gastrointestinal tract of rodents (Nomeir et al., 1992). At doses of 0.1–200 mg/kg bw, both are absorbed, metabolized, and excreted, primarily in the urine (Rice, 1972; Nomeir et al., 1992; Parkash & Caldwell, 1994). The results of the studies described below support this conclusion.

When [¹⁴C]furfuryl alcohol at doses up to 27.5 mg/kg bw or [¹⁴C]furfural at doses up to 12.5 mg/kg bw were administered to rats by gavage in corn oil, 83–89% of the radiolabel was excreted in the urine and 2–4% in the faeces, and 7% was exhaled as ¹⁴C-carbon dioxide within 24 h. Residual radiolabel was distributed primarily to the liver and kidneys, the amount generally being proportional to the dose (Nomeir et al., 1992). Similar results were obtained by Parkash & Caldwell (1994) after oral administration of [¹⁴C]furfural at doses up to 60 mg/kg bw in rats and up to 200 mg/kg bw in mice.

A similar pattern of absorption, distribution, and excretion was reported for alkyl-substituted furfural derivatives. In rats given [¹⁴C]5-methylfurfural at doses up to 500 mg/kg bw by gavage, 70–82% of the administered dose was excreted in urine and 8–13% in faeces within 48 h. In mice given the same dose, a similar pattern of excretion were observed (Godfrey et al., 1999).

2.3.2 Metabolism

In general, furfuryl esters (furfuryl acetate, furfuryl propionate, furfuryl pentanoate, furfuryl octanoate, and furfuryl 3-methylbutanoate) are expected to be hydrolysed to furfuryl alcohol and the corresponding saturated aliphatic carboxylic acid, while furoate esters (methyl 2-furoate, propyl 2-furoate, amyl 2-furoate, hexyl 2-furoate, and octyl 2-furoate) are expected to be hydrolysed to 2-furoic acid and the corresponding saturated aliphatic alcohol. Hydrolysis is catalysed by classes of enzymes recognized as carboxylesterases or esterases (Heymann, 1980), the most important of which are the A-esterases. In mammals, A-esterases occur in most tissues of the body (Heymann, 1980; Anders, 1989), but they predominate in the hepatocytes (Heymann, 1980).

Evidence that hydrolysis may occur in the gastrointestinal tract comes from experiments by Grundschober (1977), who demonstrated that the structurally related substances isoamyl furylpropionate and ethyl furylpropionate were completely hydrolysed within 2 h by pancreatin. Similarly, Paul et al. (1948) showed that the glycine conjugate of furoic acid is the major metabolite in the urine of rats given furfuryl diacetate, furfuryl propionate, or methyl furylacrylate orally. This experiment demonstrates that these esters are hydrolysed in vivo to furfuryl alcohol, the metabolic precursor of furoic acid. The esters of furfuryl and furoate can be predicted to be hydrolysed to the parent alcohol and acid, respectively. Buck (2000) demonstrated rapid hydrolysis of furfuryl alcohol esters by preparations of rat liver, rat intestinal mucosa, and rat blood. While the rate of hydrolysis of these esters was slower in a human blood preparation, it was faster than those for other compounds with sterically hindered ester bonds, such as cinnamyl and linalyl esters.

After hydrolysis of the furfuryl esters, furfuryl alcohol is oxidized to furfural, which is subsequently oxidized to 2-furoic acid (Paul et al., 1948; ; Rice, 1972; Nomeir et al., 1992; Parkash & Caldwell, 1994). The major metabolic detoxication pathway for furoic acid involves formation of a CoA thioester, which is then either conjugated with glycine and excreted in the urine or condensed with acetyl CoA to form the CoA thioester of 2-furanacrylic acid. 2-Furanacryloyl CoA is also conjugated with glycine and excreted primarily in the urine (see Figure 1). A minor metabolic pathway has been reported in rodents whereby carbon dioxide is produced, presumably by furan-ring oxidation (Nomeir et al., 1992; Parkash & Caldwell, 1994).

A number of experiments provide evidence that these pathways exist. In rats, the glycine conjugate of furoic acid was detected in urine collected for 6 h after a single oral dose of 20 mg of furfuryl alcohol, furfural, furoic acid, furfuryl diacetate, furfuryl propionate, furanacylic acid, or methyl furanacrylate (Paul et al., 1948).

Furfuryl alcohol and furfural have been shown to participate in common pathways of detoxication in rodents. In groups of four male Fischer 344 rats, [¹⁴C]furfuryl alcohol at oral doses up to 27.5 mg/kg bw or [¹⁴C]furfural at 12.5 mg/kg bw were oxidized to furoic acid, which was excreted mainly in the urine, either as the glycine conjugate (73–80%) or free (1–6%). Furoic acid was also condensed with acetyl CoA and excreted in the urine as the glycine conjugate of furanacrylic acid (3–8%). In rats given furfural at 12.5 mg/kg bw, 7% was exhaled as carbon dioxide. Over the dose ranges studied, the extent of metabolism, the relative amounts of metabolites, and the rates of excretion were linear (Nomeir et al., 1992).

Single oral doses of [¹⁴C]furfural (0.1, 10, or 60 mg/kg bw) given to male and female Fischer 344 rats or [¹⁴C]furfural (1, 20, or 200 mg /kg bw) given to male and female CD-1 mice were metabolized to the glycine conjugates of furoic acid (76–84% in rats and 65–89% in mice) and furanacrylic acid (16–24% in rats and 10–17% in mice) within 24 h. Male mice at the high dose expired < 5% of the radiolabel as [¹⁴C]carbon dioxide, but expired radiolabel was not measured in mice at other doses. The absorption, tissue distribution, extent of metabolism, relative amounts of metabolites, including carbon dioxide, and the rates of excretion in rodents were linear over the range of doses investigated (0.1–200 mg/kg bw) (Parkash & Caldwell, 1994).

The condensation of furoic acid with acetyl CoA to yield furanacrylic acid appears to be a dynamic equilibrium favouring the CoA thioester of furoic acid (Parkash & Caldwell, 1994), as suggested by the observation that furoic acid was excreted in the urine of dogs given furanacrylic acid (Friedmann, 1911). An analogous equilibrium was established between other aromatic carboxylic acids (e.g. benzoic acid and cinnamic acid) (Nutley et al., 1994). Excretion of free furoic acid and furanacrylic acid in animals given higher doses suggests that the capacity for glycine conjugation may be limited, probably by the supply of endogenous glycine (Gregus et al., 1993).

In rats, the metabolic fate of 5-methylfurfural is similar to that of furfural. Two urinary metabolites were identified when rats were given an oral dose of an aqueous solution containing 5-methylfurfural at 80, 120, or 160 mg/kg bw. The principal metabolite was the glycine conjugate of 5-methylfuroic acid (> 40%); 5-methylfuryl methyl ketone was a minor metabolite (6.6–7.8%). 5-Methylfuryl methyl ketone was also a urinary metabolite of 5-methylfuroic acid, suggesting that the ketone is formed directly from the acid (Liebert, 1988). In a manner similar to the formation of furan-acrylic acid from furfural, the CoA thioester of 5-methylfuroic acid condenses with acetyl CoA to form an alpha-ketoester which sequentially undergoes hydrolysis and decarboxylation to form 5-methylfuryl methyl ketone. 5-Methylfuryl methyl ketone may be excreted in the urine or reduced to the corresponding alcohol before excretion, in a manner similar to that of structurally related furyl ketones (Boyd et al., 1975).

Groups of male Fischer 344 rats and male B6C3F₁ mice were given [¹⁴C]-5-methylfurfural at a dose of 5, 10, 100, or 500 mg/kg bw by oral gavage. 5-Hydroxymethyl-2-furoic acid was the major urinary metabolite in both species and accounted for 77–90% of the recovered radiolabel at each dose. The quantity of the glycine conjugate N-(5-hydroxymethyl-2-furoyl)-glycine excreted by rats was inversely proportional to the dose, possibly due to glycine depletion at higher doses. 2,5-Furandicarboxylic acid accounted for only 2–4% of the recovered radiolabel in rats and 4–6% in mice (Godfrey et al., 1999). Similarly, 5-hydroxymethylfurfural administered to rats as single oral doses of 0.08–330 mg/kg bw was excreted mainly as 5-hydroxymethyl-2-furoic acid and to a lesser extent as N-(5-hydroxymethyl-2-furoyl)glycine (Germond et al., 1987).

The formation of labelled carbon dioxide in rats and mice presumably occurs via decarboxylation of furoic acid. No evidence of decarboxylation has been observed in humans (Flek & Sedivec, 1978). Since heteroaromatic and aromatic carboxylic acids do not normally decarboxylate in vivo (Caldwell, 1982), it can be assumed that oxidation of furoic acid in rodents precedes the loss of carbon dioxide. Epoxidation (Ramsdell & Eaton, 1990) or hydroxylation (Ravindranath & Boyd, 1985; Koenig & Andreesen, 1990) of the furan ring may yield reactive intermediates such as furfural-2,3-epoxide, acetylacrolein or alpha-ketoglutaric acid, which are readily decarboxylated. Biochemical changes in the lungs and livers of animals exposed to furfural indicate that ring oxidation may be catalysed by a cytochrome P450b isoenzyme to yield a reactive intermediate such as an epoxide or acylacrolein, which is subsequently conjugated with glutathione (Gupta et al., 1991; Mishra et al., 1991). There is no evidence of ring oxidation in humans at low levels of exposure. Essentially all the furfural absorbed (2 mg/kg bw) by inhalation in a study in humans was excreted in the urine as furoylglycine and furanacrylic acid, and no free furoic acid was found (Flek & Sedivec, 1978).

In summary, the component linear saturated alcohols and carboxylic acids (C1–C8) formed by ester hydrolysis of furfuryl esters and furoic acid esters participate in fatty acid beta-oxidation and the citric acid cycle or in the C1 tetrahydrofolate pathway, eventually to yield carbon dioxide and water (Voet & Voet, 1990). The available data indicate that most of the 15 furfuryl derivatives would be hydrolysed and/or oxidized to yield furoic acid. In humans, furoic acid would be efficiently excreted as either 2-furoylglycine or 2-furanacryloylglycine (2-furanacryluric acid).

2.3.2 Toxicological studies

2.3.2.1 Acute toxicity

LD₅₀ values after oral administration are available for four of the 15 furfuryl derivatives. The values reported in rodents range from 50 mg/kg bw for furfural (Fassett, 1963) to 2200 mg/kg bw for 5-methylfurfural (Moreno, 1978). Most of the values fall between 100 and 250 mg/kg bw (Boyland, 1940; Gajewski & Alsdorf, 1949; Haag et al., 1959; Jenner et al., 1964). In rodents, signs of acute toxicity are seen mainly in the liver, and high doses are associated with hepatic cirrhosis accompanied by sporadic eosinophilic degeneration of hepatic cells. Nuclear pyknosis, eosinophilic necrosis, and increased hepatocyte mitosis have also been reported (Shimizu & Kanisawa, 1986). The changes observed in cytosolic and mitochondrial enzyme activities are indicative of increased hepatocellular catabolism (Jonek et al., 1975; Kaminska, 1977).

2.3.2.2 Short-term and long-term studies of toxicity and carcinogenicity

The results of short-term and long-term studies of the toxicity of the substances in this group are shown in Table 3. Details of the studies that were critical to evaluating the safety of furfuryl alcohol, furfural, 2-benzofurancarboxaldehyde, and 2-phenyl-3-carbethoxy furan are given below.

M, male; F, female

    See Also:
       Toxicological Abbreviations