2009_Book_FoodChemistry

270 4 Carbohydrates
Ammonia catalyzes the same reactions as alkali
and alkaline earths. Reactive intermediary products
can polymerize further into brown pigments
(“sucre couleur”) or form a number of imidazole,
pyrazine and pyridine derivatives.
4.2.4.3.3 Caramelization
Brown-colored products with a typical caramel
aroma are obtained by melting sugar or by
heating sugar syrup in the presence of acidic
and/or alkaline catalysts. The reactions involved
were covered in the previous two sections. The
process can be directed more towards aroma
formation or more towards brown pigment
accumulation. Heating of saccharose syrup in
a buffered solution enhances molecular fragmentation
and, thereby, formation of aroma
substances. Primarily dihydrofuran-ones, cyclopentenolones,
cyclohexenolones and pyrones
are formed (cf. 4.2.4.3.2). On the other hand,
heating glucose syrup with sulfuric acid in
the presence of ammonia provides intensively
colored polymers (“sucre couleur”). The stability
and solubility of these polymers are
enhanced by bisulfite anion addition to double
bonds:
(4.50)
4.2.4.4 Reactions with Amino Compounds
(Maillard Reaction)
In this section, the formation of N-glycosides
as well as the numerous consecutive reactions
classed under the term Maillard reaction or
nonenzymatic browning will be discussed.
N-Glycosides are widely distributed in nature
(nucleic acids, NAD, coenzyme A). They are
formed in food whenever reducing sugars occur
together with proteins, peptides, amino acids
or amines. They are obtained more readily at
a higher temperature, low water activity and on
longer storage.
On the sugar side, the reactants are mainly glucose,
fructose, maltose, lactose and, to a smaller
extent, reducing pentoses, e. g., ribose. On the
side of the amino component, amino acids with
a primary amino group are more important than
those with a secondary because their concentration
in foods is usually higher. Exceptions
are, e. g., malt and corn products which have
a high proline content. In the case of proteins, the
ε-amino groups of lysine react predominantly.
However, secondary products from reactions
with the guanidino group of arginine are also
known. In fact, imidazolin-ones and pyrimidines,
which are formed from arginine and reactive
α- andβ-dicarbonyl compounds obtained from
sugar degradation, have been detected.
The consecutive reactions of N-glycosides
partially correspond to those already outlined
for acid/base catalyzed conversions of monosaccharides.
However, starting with N-containing
intermediates, which with the nitrogen function
possess a catalyst within the molecule, these reactions
proceed at a high rate under substantially
milder conditions, which are present in many
foods.
(4.50)