The application of natural hydrocolloids to foods and beverages - Natural Food Additives, Ingredients and Flavourings - page 190

meats, but by far the largest volume of agar continues to be used in Asia for
traditional dishes.
Newer developments have produced agars that dissolve at lower temperatures,
known as 'quick soluble agars', and increased production of the strip agar for its
natural, traditional status. There has also been increased interest in agar for its
health benefi ts as a dietary fi bre source, as it contains 95% soluble fi bre.

8.3.9 Alginate
Alginate is the most abundant polysaccharide extracted from brown seaweed. The
commercial product is extracted from a wide range of brown seaweed species,
including Laminaria, Ascophyllum, Lessonia, Durvillea and Macrocystis species,
and is harvested from many sites across the globe (South America, Australia,
South Africa and various northern hemisphere oceans). The different sources
produce alginates with slightly different structures, which affect the gelling
properties.
Alginate is present in seaweed as a mixed salt of sodium, potassium, calcium
and magnesium (Draget 2009). Extraction involves ion exchange followed by
fi ltration, precipitation (with alcohol, calcium chloride or acid) and recovery of
the alginic acid, or conversion into the appropriate salt - usually sodium alginate
(using sodium carbonate). This is a more intensive extraction process and the end
product is not identical to that found in the seaweed, making alginate the most
questionable natural hydrocolloid.
Alginate is a polymer of mannuronic acid (M) and guluronic acid (G). These
sugars are arranged in blocks of M and blocks of G, interspersed with regions of
alternating structure (MG blocks). Alginate has the benefi t of being cold water
soluble, and does not need to be heated and cooled to form a gel, so it is cold
setting unlike the other hydrocolloids. Instead, it requires the addition of
cations, most commonly calcium. Only the G blocks can react with calcium
to form a gel, so the higher G alginates give stronger gels. The gel is formed
using the egg box model, in a similar manner to LM pectin (Fig.8.2). Alginate
is very reactive to calcium, so the release of the calcium must be controlled, or
pre-gelling will occur, producing lumps of gel or 'fi sh eyes' in a liquid. There are
two methods of controlling calcium interactions: the diffusion method and the
internal setting method. The diffusion method is the simplest, and is used for
making thin strips or beads of alginate gel by dropping alginate solution into a
bath of calcium chloride solution. This is used to produce pimento fi llings for
olives and cocktail 'cherries', onion rings and beads - sometimes added to
beverages for novelty.
Internal setting is required for most alginate applications including bakery
fi llings, bakery custard, structured fruit and vegetables, structured meat and
aerated desserts. For the internal set, the calcium is in an inert form and
calcium ions are released in a controlled fashion within the alginate solution,
usually by reducing the pH. Common calcium salts are lactate, citrate and
phosphate. A sequestrant, normally sodium phosphate or citrate, is used to control

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