Prepared by:
          The forty-ninth meeting of the Joint FAO/WHO Expert
          Committee on Food Additives (JECFA)

        World Health Organization, Geneva 1998



        Six groups of flavouring agents were evaluated using the Procedure
    for the Safety Evaluation of Flavouring Agents as modified at the
    present meeting (Figure 1).

        The Committee noted that in applying the Procedure, the substance
    is first assigned to a structural class as identified at the
    forty-sixth meeting of the Committee (Annex 1, reference 122). The
    structural classes are as follows:

    -   Class I.  Substances with simple chemical structures and efficient
        modes of metabolism which would suggest a low order of toxicity by
        the oral route.

    -   Class II.  Substances with structural features that are less
        innocuous than those of substances in Class I but are not
        suggestive of toxicity. Substances in this class may contain
        reactive functional groups.

    -   Class III.  Substances with structural features that permit no
        strong initial presumption of safety, or may even suggest
        significant toxicity.

        A key element of the procedure involves determining whether a
    flavouring agent and the product(s) of its metabolism are innocuous
    and/or endogenous substances. For the purpose of the evaluations, the
    following definitions were used, which have been adapted from the
    report of the forty-sixth meeting of the Committee:

        Innocuous metabolic products are defined as products that are
    known or readily predicted to be harmless to humans at the estimated
    intake of the flavouring agent.

        Endogenous substances are intermediary metabolites normally
    present in human tissues and fluids, whether free or conjugated;
    hormones and other substances with biochemical or physiological
    regulatory functions are not included; the estimated intake should be
    judged not to give rise to perturbations outside the physiological

        The Committee first considered the metabolic pathways common to
    the flavouring agents evaluated at the present meeting.

     a) Hydrolysis of esters 

        Linear alkyl esters are hydrolysed rapidly to their component
    alcohols and carboxylic acids in the intestinal tract, blood and liver
    and most tissues throughout the body. Hydrolysis is catalysed by

    classes of enzymes recognized as carboxylesterases or esterases. For
    simple linear esters, as considered at this meeting, the rate of
    hydrolysis increases with increase in chain length of either the acid
    or alcohol component. The rate of hydrolysis of straight-chain esters
    is approximately 100 times faster than the rate of hydrolysis of
    branched-chain esters. The rates of hydrolysis of the alkenyl esters
    citronellyl acetate and citronellyl phenylacetate, by artificial
    pancreatic juice, were similar to the rates for simple branched-chain

     b) Oxidation of alkyl primary alcohols and aldehydes

        Most linear and branched-chain, saturated and unsaturated primary
    alcohols are oxidized rapidly to their corresponding aldehydes by
    alcohol dehydrogenase. The rate of oxidation increases with increase
    in chain length and with the presence of a double bond.

        The subsequent oxidation of the aldehydes to the corresponding
    acid is catalysed by dehydrogenase and oxidase enzymes. Most active is
    a NAD+/NADH-dependent aldehyde dehydrogenase present in the cytosol,
    the activity of which increases with increasing relative molecular
    mass of the aldehyde substrate. Aldehydes may also be reduced to
    alcohols or conjugated with sulfhydryl-containing substances, such as
    glutathione. The oxidation of aldehydes with low relative molecular
    mass by aldehyde dehydrogenase requires glutathione, which suggests
    that the free aldehyde may be conjugated rapidly with glutathione 
     in vivo to form a thiohemiacetal that is subsequently oxidized to
    the corresponding acid. Branched-chain aldehydes are excellent
    substrates for aldehyde dehydrogenase, and the rate of oxidation of
    2-methylpropanal is similar to that of acetaldehyde.

     c) Oxidation of linear saturated carboxylic acids

        Aliphatic linear saturated carboxylic acids are metabolized in the
    fatty acid -oxidation pathway, the tricarboxylic acid cycle, or the
    C1-tetrahydrofolate pathway. Oxidation of formic acid to carbon
    dioxide and water occurs primarily in the liver and is catalysed by
    tetrahydrofolate in humans and other primates.

        Other carboxylic acids are condensed with coenzyme A (CoA) to
    yield thioesters that undergo -cleavage to acetyl CoA. Even-numbered
    carboxylic acids give acetyl CoA, whereas odd-numbered carboxylic
    acids yield acetyl CoA and propionyl CoA. Acetyl CoA enters the citric
    acid cycle directly. Propionyl CoA is converted to succinyl CoA before
    entering the citric acid cycle.

     d) Oxidation of branched saturated carboxylic acids

        Short-chain (< C6) branched aliphatic acids undergo
    -oxidation preferentially in the longer chain followed by cleavage to
    yield linear acid fragments that are metabolized via the fatty acid
    pathway or tricarboxylic acid cycle. Isobutyric acid, isovaleric acid

    and 2-methylbutyric are formed during the oxidative deamination of
    endogenous branched-chain amino acids and are metabolized by normal
    pathways of intermediary metabolism. At high dose levels, longer
    branched-chain acids may undergo omega-oxidation to yield diacids that
    undergo further oxidation and cleavage.

        Acids with a methyl substituent are extensively metabolized to
    CO2 via -oxidation, unless the methyl group is located at the
    -position (e.g., 3-methyl-pentanoic acid), in which case
    alpha-oxidation occurs, yielding short-chain acid fragments capable of
    being completely metabolized.

        The presence of a 2-ethyl substituent prevents the -oxidation of
    aliphatic carboxylic acids, and these compounds undergo omega-and
    omega-1 oxidation to yield polar metabolites that are excreted
    primarily in the urine. Saturation of this omega-oxidation pathway may
    lead to formation of the 2-substituted carboxylic acid that may be
    excreted as the glucuronic acid conjugate.

    FIGURE 1


    See Also:
       Toxicological Abbreviations