Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
In the last two decades, the Maillard reaction has been the subject of seven international
symposia. 2–8 The literature dealing with all aspects of the Maillard reaction,
including under physiological conditions, has been covered in many excellent review
articles and books. 2–13 Therefore, only recent articles and important older papers are
cited here.
The significance of this reaction in foods stems from the fact that it alters
important food attributes such as color, flavor, taste, nutritional value, antioxidant
properties, and texture. These changes can be both desirable (e.g., flavor and color
development on roasting and baking) and undesirable (e.g., darkening of dehydrated
foods, formation of off-flavors, or development of toxic compounds). Despite ninety
years of effort, the Maillard reaction is not fully understood and our abilities to
control this reaction during food processing and storage are only very limited.
The intention of this chapter is to summarize recent findings on mechanistic
aspects of the Maillard reaction, with focus on degradation of the sugar moiety. With
some exceptions, the chapter does not cover the interactions and recombinations of
sugar fragmentation or amino acid degradation products.
18.2 FORMATION OF GLYCOSYLAMINES AND
REARRANGEMENT TO AMINO DEOXYSUGARS
The first step in the amino-carbonyl reaction between reducing sugars and amino
acids is the addition of a primary amino group of an amino acid to a carbonyl group
of a sugar, followed by the elimination of water. The resulting imine (Schiff base)
(18.1) undergoes cyclization to the corresponding N-glycosylamine (18.2) (Reaction
HO
HC
HC
CH
O
OH
HC OH
HC OH
CH 2 OH
Reaction 18.1
R NH2 HOCH NH R HC
N R
HC
OH
HO
CH
H O 2
HC
OH
HO
CH
HC OH
HC OH
HC OH
HC OH
CH OH 2
CH2OH CH 2 OH
18.1). Whereas glycosylamines derived from aromatic and heterocyclic amines show
a certain stability, those derived from amino acids are immediately converted into
further compounds. Aldosylamines undergo Amadori rearrangement, forming 1amino-1-deoxy-2-
ketoses, the so-called Amadori compounds (Reaction 18.2).
Ketosylamines rearrange to 2-amino-2-deoxy-aldoses via Heyns rearrangement
(Reaction 18.3). Both rearrangements start with protonization, causing opening of
the hemiacetal ring of glycosylamine and developing of the immonium ion. Loss of
a hydrogen proton leads via enaminol (18.3a,b) to aminodeoxyketose (18.4a) (from
aldosylamine) or aminodeoxyaldose (18.4b) (from ketosylamine). Amadori compounds
have been found in various heated and stored foods such as malt, soy sauce,
roasted cocoa, and dehydrated vegetables. 14–17
HO
OH
O
OH
18.1 18.2
NH R