Chemical and Functional Properties of Food Saccharides

© 2004 by CRC Press LLC
Alginates are widely used as thickeners and stabilizers as well as for their gelforming
properties and are found in ice creams, dairy products such as milk shakes,
soups, and sauces. They are also an important ingredient in beer manufacture to
stabilize dissolved solids in commercial beer and prevent sedimentation. Alginate
gels are popular in the dessert and bakery sectors for pie fillings and also for
restructuring meat. Alginates can also be used to form films that can be used as
edible packaging. An important market for alginates uses chemically modified alginate,
mainly propylene glycol alginate PGA. PGA has greater acid solubility than
native alginate and can therefore be used in a range of applications where native
alginate would start to precipitate out.
15.6.2 AGARS
Agars are obtained from red seaweeds (Rhodophyta) mainly from the genus Gelidium,
of which some 15 species are now used to produce agar from the cell wall
mucilage. The main center of production has historically been Japan, and this bias
continues to this day. Production involves aqueous extraction of seaweed in hot
water, with the extract frozen and thawed to precipitate polysaccharides and remove
water-soluble contaminants. Agarose is a purified form of agar with typically shorter
chains and a lower molecular weight.
Agars are formed from linear chains of both L- and D-galactose residues linked
alternately α(1-3) and β(1-4) and can be sulfated at the 2, 4, or 6 positions. Some
of the L-galactose residues are converted to 3,6-anhydrogalactose by the formation
of a bridging bond across the ring between the C-3 and C-6. Formation of the 3,6anhydrogalactose
changes the conformation of the sugar ring and favors the formation
of a helix by the polysaccharide chain. 95 Gelling can occur by formation of
junctions between the helical regions of two agar chains that intertwine to form a
double helix. Agar gels are thermoreversible and show significant hysteresis, a large
difference between the melting (80–90°C) and gelling temperatures (35–45°C),
which reflects the slow rate of double-helix formation and the strength of the
association once it is formed. 96
Agar gels are formed at low concentration and are hard and brittle. Generally,
they are too hard for many food uses and are therefore more commonly used in
nonfood applications. Food uses include confectionery and sweets, icings, and
restructured meat pie fillings.
15.6.3 CARRAGEENANS
Carrageenans are a family of sulfated polymers with a linear chain of D-galactose
linked alternately α(1-3) and β(1-4). The sulfate groups are present at the 2, 4, or
6 positions. An important feature of carrageenan structure is that some of the
galactose residues are converted to 3,6-anhydrogalactose by the formation of a
bridging bond across the ring between the C-3 and C-6. This transformation can
occur after chain synthesis catalyzed by an enzyme in the algae or it can be promoted
during extraction by the use of alkaline conditions which result in the loss of some
sulfate groups from galactose residues. Formation of 3,6-anhydrogalactose changes