Chemistry

Dihydroxyacetone (C3H6O3) is a white, crystalline, hygroscopic powder. This 3-carbon sugar forms a dimer in freshly prepared aqueous solution (Fig. 1). With heating to effect a solution in alcohol, ether, or acetone, it reverts to the monomer. The monomeric form is more important in the browning reaction, which leads to the skincolor change [4]. DHA

ce2oh c - o ch2oh

Figure 1 The chemical structure of DHA: (a) monomer, (b) dimer.

is stable between pH 4 to 6, but above pH 7 efficacy is lost with the formation of brown-colored compounds. A buffered mixture at pH 5 is most stable. Heating above 38°C for long periods of time will also effect stability. DHA needs to be stored in a cool, dry place. Glyceraldehyde, the isomer of DHA, is also present in solution [4]. Glyceraldehyde may degrade into formaldehyde and formic acid. In acidic solution (pH 4), this isomerization and consequently these latter undesirable ingredients are minimized.

The Maillard or browning reaction has been defined as the reaction of an amino group of amino acids, peptides, or proteins with the glycosidic hydroxyl group of sugars. DHA in the context of this reaction may be considered a 3-carbon sugar, reacting with free amino groups available as amino acids, peptides, and proteins supplied by the keratin to form products or chromophores referred to as melanoidins [5]. Melanoidins have some physicochemical properties similar to naturally occurring melanin [6].

Figure 1 The chemical structure of DHA: (a) monomer, (b) dimer.

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