2009_Book_FoodChemistry

4.2 Monosaccharides 263
Another industrially applied glucuronic acid synthesis
involves first oxidation then hydrolysis of
starch:
The reverse of glycoside hydrolysis (cf. 4.2.4.5)
i.e. re-formation of glycosides, occurs in dilute
mineral acids. All the possible disaccharides and
higher oligosaccharides, but preferentially isomaltose
and gentiobiose, are obtained from glucose:
(4.31)
Depending on their configuration, the uronic
acids can form lactone rings in pyranose or
furanose forms.
A number of uronic acids occur fairly abundantly
in nature. Some are constituents of polysaccharides
of importance in food processing, such as
gel-forming and thickening agents, e. g. pectin
(D-galacturonic acid) and sea weed-derived
alginic acid (D-mannuronic acid, L-guluronic
acid).
4.2.4.3 Reactions in the Presence of Acids
and Alkalis
In the absence of amine components, monosaccharides
are relatively stable in the pH range 3–7.
Beyond these pH limits, more or less extensive
conversions occur, depending on the conditions.
Enolizations and subsequent elimination of water
with retention of the C-chain predominate in an
acidic medium. In a basic medium, enolizations
with subsequent fragmentation (retro-aldol reactions)
and secondary reactions of the fragments
(aldol additions) predominate.
4.2.4.3.1 Reactions in Strongly Acidic Media
(4.32)
Such reversion-type reactions are observed, e. g.,
in the acidic hydrolysis of starch.
In addition to the formation of intermolecular
glycosides, intramolecular glycosidic bonds can
be readily established when the sugar conformation
is suitable. β-Idopyranose, which occurs
in the 1 C 4 -conformation, is readily changed to
1,6-anhydroidopyranose, while the same reaction
with β-D-glucopyranose ( 4 C 1 -conformation)
occurs only under more drastic conditions,
e. g., during pyrolysis of glucose, starch or
cellulose. Heating glucose syrup above 100 ◦ C
can form 1,6-anhydrogluco-pyranose, but only in
traces:
(4.33)
In the formation of reversion products, it is assumed
that in the presence of strong acids an
oxonium cation is formed which, as an alkylating
agent, reacts with the nucleophilic hydroxy
groups with the cleavage of H + (Formula 4.34).