2009_Book_FoodChemistry

4.2 Monosaccharides 271
These reactions result in:
• Brown pigments, known as melanoidins,
which contain variable amounts of nitrogen
and have varying molecular weights and
solubilities in water. Little is known about
the structure of these compounds. Studies
have been conducted on fragments obtained
after Curie point pyrolysis or after oxidation
with ozone or sodium periodate. Browning
is desired in baking and roasting, but not in
foods which have a typical weak or other
color of their own (condensed milk, white
dried soups, tomato soup).
• Volatile compounds which are often potent
aroma substances. The Maillard reaction is
important for the desired aroma formation
accompanying cooking, baking, roasting or
frying. It is equally significant for the generation
of off-flavors in food during storage,
especially in the dehydrated state, or on heat
treatment for the purpose of pasteurization,
sterilization and roasting.
• Flavoring matter, especially bitter substances,
which are partially desired (coffee) but can
also cause an off-taste, e. g., in grilled meat or
fish (roasting bitter substances).
• Compounds with highly reductive properties
(reductones) which can contribute to the stabilization
of food against oxidative deterioration.
• Losses of essential amino acids (lysine, arginine,
cysteine, methionine).
• Compounds with potential mutagenic properties.
• Compounds that can cause cross-linkage of
proteins. Reactions of this type apparently also
play a role in vivo (diabetes).
4.2.4.4.1 Initial Phase of the Maillard Reaction
D-Glucose will be used as an example to illustrate
the course of reactions occurring in the early
phase of the Maillard reaction. The open-chain
structures will be used for simplification although
the hemiacetal forms are predominantly present
in solution.
Nucleophilic compounds like amino acids or
amines easily add to the carbonyl function
of reducing carbohydrates with the formation
of imines (Schiff bases). As a result of the
hydroxyl group present in the α-position (Formula
4.50), the imines can rearrange via the
1,2-eneaminols corresponding to the 1,2-enediol
(cf. Formula 4.36). This rearrangement leads to
an aminoketose called an Amadori compound
(1-amino-1-deoxyketose) after its discoverer.
If fructose reacts in a corresponding way with
an amine or an amino acid (Formula 4.51),
an aminoaldose, called a Heyns compound
(2-amino-2-deoxyaldose), is formed. Since the
addition of the amine to fructose or the addition
of the H-atom to the intermediate aminoenol can
proceed from two sides, an enantiomeric pair is
obtained in each case.
Amadori compounds with different amino acid
residues have been detected in many heated and
stored foods, e. g., in dried fruit and vegetables,
milk products, cocoa beans or soy sauce. Amadori
compounds are also found in the blood serum, especially
of patients suffering from Diabetes mellitus.
As secondary amino acids, Amadori and
Heyns compounds can be analytically detected by
amino acid analysis (cf. protein section).
(4.51)