Leung's Encyclopedia of Common Natural Ingredients Used in Food, Drugs, and Cosmetics

ALGIN

Source: Brown algae, commonly including
members of the following genera:Macro-
cystis,Laminaria, andAscophyllum.

Common/vernacular names:Algin, salts of
alginicacid(alginates),andparticularlysodium
alginate.

GENERAL DESCRIPTION

Algin is a collective term for the hydrophilic
colloidal substance isolated from certain
brown algae (class Phaeophyceae). The most
commonly used algae include members of the
following genera: Macrocystis,Laminaria,
andAscophyllum.
A major source of algin in the United States
isMacrocystis pyrifera(L.) C. A. Agardh. or
“giant kelp” that grows along the West Coast
of North America. Other sources includeAs-
cophyllum nodosum(L.) LeJolis andLami-
naria digitata (L.) Edmonson and related
species that are used by countries bordering
theAtlantic Ocean.Laminariaspeciesare also
used by Japanese producers.
The process for algin manufacture basi-
cally involves a prewash of the seaweed
whereby undesirable salts are leached out
and removed, followed by extraction with a
dilute alkaline solution that solubilizes the
alginic acid present in the seaweed. The
resulting thick and viscous mass is clarified
and the algin is obtained as free alginic acid
on treatment with mineral acids. The alginic
acid can then be converted to sodium algi-
nate. Sodium alginate is the major form of
algin currently in use.1,2
The major producing countries include the
UnitedStates,UK, Norway, France, and Japan.
Alginic acid and its calcium salt are insol-
uble in water, but its ammonium, sodium,
potassium, and magnesium salts as well as its
propylene glycol ester are readily soluble in
cold and hot water in which they form viscous
solutions. The viscosity of algin solutions
depends on various factors including concen-
tration, pH, degree of polymerization (DP),

temperature, and presence of polyvalent metal

ions. Viscosity increases with DP; it decreases

with increase in temperature but will regain its

original value on cooling to its initial temper-

ature, provided the solutions are not held

above 50C for long periods. Between pH

4 and 10 the viscosity of algin solutions is

generally stable.

Algin solutions form gels with calcium

ions due to the formation of insoluble calci-

um alginate. These gels are not thermally

reversible but may be liquefied by calcium

sequestrants.

Propylene glycol alginate is more acid-

tolerant than the other alginates. Its solutions

are stable below pH 4 (down to pH 2.6).1,2

CHEMICAL COMPOSITION

Alginic acid is a linear polymer consisting of

(1,4)-linked residues ofb-D-mannopyranosy-

luronic acid anda-L-gulopyranosyluronic ac-

id. TheseD-mannuronic acid andL-guluronic

acid residues are arranged in the polymer

chain in blocks. Blocks of mannuronic acid

are separated from those of guluronic acid by

blocks made up of random or alternating units

of mannuronic and guluronic acids.2–4The

homogeneous blocks (those composed of

either acid residues alone) are less readily

hydrolyzed than the interconnecting hetero-

geneous blocks.^5 Alginates from different

sources vary in their proportions of blocks of

mannuronic and guluronic acid residues; for

some alginate samples, values of mannuronic

acid to guluronic acid ratios range from 0.3 to

2.3.6–8Thesevalues can be readily determined

by infrared spectroscopy.^6

Molecular weights of alginates range from

10,000 to 1,870,000 depending on algal

sources and methods of analysis.8–11

PHARMACOLOGY AND BIOLOGICAL

ACTIVITIES

Sodium alginate has the ability to reduce

strontium absorption, and the sodium algi-

nate with the highest proportion of guluronic

16 Algin