Page 43 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

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Encyclopedia of Physical Science and Technology EN006C-254 June 28, 2001 19:52

Food Colors 115

FIGURE 12 Conversion of tyrosine to melanin, catalyzed in part by tyrosinase (T). DOPA, Dihydroxyphenylalanine.
Only part of melanin is shown.

products are often called melanoidins, indicating their vi- the reactions. Among sugars, pentoses are more reactive
sualsimilaritytothemelaninsofenzymaticbrowning.The than hexoses, and hexoses are more reactive than reducing
most common carbonyl compounds of foods involved in disaccharides. When free amino acids react with sugars,
the Maillard reaction are reducing sugars, and the most lysine appears to be the most active among them. In pep-
common amine compounds are amino acids. tides and proteins, the N-terminal amino acid is the most
The intermediate reactions and their relative velocities reactive, followed by a nonterminal lysine. Raising the
vary with the type of initial reactants and the conditions of temperature and/or the pH accelerates the Maillard reac-
tion. Intermediate water activity appears to maximize this
reaction.
Several pathways have been proposed for the forma-
tion of melanoidins through the interaction of carbonyl
and amine compounds. A simplified scheme is shown in
Fig. 14. This scheme involves first the condensation of a
carbonyl compound (an aldohexose in this scheme) with
an amine to a Schiff base via an intermediate product
(not shown). The Schiff base quickly cyclizes to an N-
substituted glycosylamine. Up to this step the process is
reversible because the glycosylamine can be hydrolyzed
back to the initial reactants. The N-substituted glycosy-
lamine is then rearranged to an N-substituted 1-amino-
1-deoxy-2-ketose, the Amadori compound, which is in
equilibrium with its enol form. If the initial carbonyl com-
pound is a 2-ketose (e.g., fructose), the corresponding N-
substituted 2-amino-2-deoxy-1-aldose is formed by the
FIGURE 13 Four phenolic compounds involved in enzymatic
browning. Heyns rearrangement, which is analogous to the Amadori