Food Chemistry

4.2 Monosaccharides 263

Another industrially applied glucuronic acid syn-
thesis involves first oxidation then hydrolysis of
starch:

(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 polysaccha-
rides 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 ReactionsinthePresenceofAcidsandAlkalis...............

In the absence of amine components, monosac-
charides 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 reac-
tions) and secondary reactions of the fragments
(aldol additions) predominate.

4.2.4.3.1 Reactions in Strongly Acidic Media

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 isoma-

ltose and gentiobiose, are obtained from glucose:

(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 confor-

mation 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 as-

sumed that in the presence of strong acids an

oxonium cation is formed which, as an alkylat-

ing agent, reacts with the nucleophilic hydroxy

groups with the cleavage of H+(Formula 4.34).