Food Chemistry

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
  afterCuriepoint 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. TheMaillardreaction is
  important for the desired aroma formation
  accompanying cooking, baking, roasting or
  frying. It is equally significant for the gen-
  eration 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 sta-
  bilization of food against oxidative deteriora-
  tion.


• Losses of essential amino acids (lysine, argi-
  nine, cysteine, methionine).


• Compounds with potential mutagenic proper-
  ties.


• Compounds that can cause cross-linkage of
  proteins. Reactions of this type apparently also
  play a rolein vivo(diabetes).

( 4. 51 )

4.2.4.4.1 Initial Phase of the Maillard Reaction.

D-Glucose will be used as an example to illus-

trate the course of reactions occurring in the early

phase of theMaillardreaction. 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 theα-position (For-

mula 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 anAmadoricompound

(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.

Amadoricompounds 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, es-

pecially of patients suffering from Diabetes mel-

litus. As secondary amino acids,Amadoriand

Heynscompounds can be analytically detected by

amino acid analysis (cf. protein section).