There are several
carrageenans, differing in their chemical structure and properties, and
therefore in their uses. The carrageenans of commercial interest are called
iota, kappa and lambda.
Their uses are related
to their ability to form thick solution or gels, and they vary as follows.
Iota
|
Elastic gels formed with calcium
salts.
Clear gel with no bleeding of liquid (no synaeresis). Gel is freeze/thaw stable. |
Kappa
|
Strong, rigid gel, formed with
potassium salts.
Brittle gel forms with calcium salts. Slightly opaque gel, becomes clear with sugar addition. Some synaeresis. |
Lambda
|
No gel formation, forms high
viscosity solutions.
|
The carrageenan
composition in red seaweeds differs from one species to another.
Chondrus
crispus
|
mixture of kappa and lambda.
|
Kappaphycus
alvarezii
|
mainly kappa.
|
Eucheuma
denticulatum
|
mainly iota.
|
Gigartina
skottsbergii
|
mainly kappa, some lambda.
|
Sarcothalia
crispata
|
mixture of kappa and lambda.
|
There are two different
methods of producing carrageenan, based on different principles.
In the original method -
the only one used until the late 1970s-early 1980s - the carrageenan is
extracted from the seaweed into an aqueous solution, the seaweed residue is
removed by filtration and then the carrageenan is recovered from the solution,
eventually as a dry solid containing little else than carrageenan. This
recovery process is difficult and expensive relative to the costs of the second
method.
In the second method,
the carrageenan is never actually extracted from the seaweed. Rather the
principle is to wash everything out of the seaweed that will dissolve in alkali
and water, leaving the carrageenan and other insoluble matter behind. This
insoluble residue, consisting largely of carrageenan and cellulose, is then
dried and sold as semi-refined carrageean (SRC). Because the carrageenan does
not need to be recovered from solution, the process is much shorter and
cheaper.
Refined carrageenan is
the original carrageenan and until the late 1970s-early 1980s was simply called
carrageenan. It is now sometimes called filtered carrageenan. It was first made
from Chondrus crispus, but now the process is applied to all of
the above algae.
The seaweed is washed to
remove sand, salts and other foreign matter. It is then heated with water
containing an alkali, such as sodium hydroxide, for several hours, with the
time depending on the seaweeds being extracted and determined by prior small-scale
trials, or experience. Alkali is used because it causes a chemical change that
leads to increased gel strength in the final product. In chemical terms, it
removes some of the sulphate groups from the molecules and increases the
formation of 3,6-AG: the more of the latter, the better the gel strength. The
seaweed that does not dissolve is removed by centrifugation or a coarse
filtration, or a combination. The solution is then filtered again, in a
pressure filter using a filter aid that helps to prevent the filter cloth
becoming blocked by fine, gelatinous particles. At this stage, the solution
contains 1-2 percent carrageenan and this is usually concentrated to 2-3
percent by vacuum distillation and ultrafiltration.
FIGURE 44
Sun drying semi-refined carrageenan (alkali treated K. alvarezii).
Sun drying semi-refined carrageenan (alkali treated K. alvarezii).
FIGURE 45
Flow chart for the production of refined carrageenan (after Porse, 1998).
Flow chart for the production of refined carrageenan (after Porse, 1998).
The processor now has a
clear solution of carrageenan and there are two methods for recovering it as a
solid, both rather similar to those described previously for agar production.
An alcohol-precipitation method can be used for any of the carrageenans. A gel
method can be used for kappa-carrageenan only, and the gel can be dehydrated
either by squeezing or by subjecting it to a freeze-thaw process.
In the alcohol method,
isopropanol is added until all the carrageenan is precipitated as a fibrous
coagulum that is then separated using a centrifuge or screen (a fine sieve).
The coagulum is pressed to remove solvent and washed with more alcohol to
dehydrate it further. It is then dried and milled to an appropriate particle
size, 80 mesh or finer. For the process to be economic the alcohol must be
recovered, both from the liquids and the dryer, and recycled.
The gel method relies on
the ability of kappa carrageenan to form a gel with potassium salts. The gel
may be formed in various ways. For the freeze-thaw process it is convenient to
form it as spaghetti-like pieces by forcing the carrageenan solution through
fine holes into a potassium chloride solution. The fine "spaghetti"
is collected and washed with more potassium chloride to remove more water,
pressed to remove surplus liquid and then frozen. When allowed to thaw,
separation of water occurs by synaeresis, the pieces are washed with more
potassium chloride, chopped up and dried in a hot air dryer. Inevitably the
product contains some potassium chloride. The alternative to freeze-thaw is to
force water out of the gel by applying pressure to it, using similar equipment
to that used for agar (Figure 10). After squeezing for several hours the sheets
of gel are chopped, dried in a hot air dryer and milled to an appropriate
particle size. Many agar processors are now using their equipment and similar
techniques to produce kappa carrageenan as well.
Figure 45 summarizes the
above processes.
Semi-refined carrageenan
(SRC) was the name given to the product first produced by the second method of
processing noted in Section 7.1. This is the method in which the carrageenan is
never actually extracted from the seaweed.
In the production of
SRC, Kappaphycus alvarezii, contained in a metal basket, is heated
in an alkaline solution of potassium hydroxide for about two hours. The
hydroxide part of the reagent penetrates the seaweed and reduces the amount of
sulphate in the carrageenan, increases the 3,6-AG so the gel strength of the
carrageenan in the seaweed is improved. The potassium part of the reagent
combines with the carrageenan in the seaweed to produce a gel and this prevents
the carrageenan from dissolving in the hot solution. However, any soluble
protein, carbohydrate and salts do dissolve and are removed when the solution
is drained away from the seaweed. The residue, which still looks like seaweed,
is washed several times to remove the alkali and anything else that will
dissolve in the water. The alkali-treated seaweed is now laid out to dry; in
hot climates, like the Philippines, usually on a large concrete slab (Figure
44). After about two days it is chopped and fed into a mill for grinding to the
powder that is sold as SRC or seaweed flour.
The above process is
summarized in Figure 46 (seaweed flour branch).
However, the seaweed
flour is coloured, often has a high bacterial count and is not suitable for
human consumption. Nevertheless it immediately found a large market in canned
pet food because it is a good gelling agent and was so much cheaper than
refined carrageenan. The temperatures used in the canning process destroy any
bacteria so the high bacterial count in the SRC is not a problem. Sometimes the
dried product is just chopped into pieces, not milled, and sold as a raw
material to refined carrageenan processors. It is called alkali treated
cottonii (ATC) or alkali treated cottonii chips (ATCC), or even simply cottonii
chips. If this treatment is done in the country of origin of the seaweed, such
as the Philippines or Indonesia, this means processors in Europe and United
States of America have cheaper transport costs per tonne of carrageenan,
compared with shipping dried seaweed. They have also left behind some waste
products, which reduces their waste treatment costs.
FIGURE 46
Flow chart for the production of seaweed flour and PES/PNG carrageenans (after Bixler, 1996).
Flow chart for the production of seaweed flour and PES/PNG carrageenans (after Bixler, 1996).
Kappaphycus alvarezii is used in this process because it
contains mainly kappa carrageenan and this is the carrageenan that forms a gel
with potassium salts. Iota-containing seaweeds can also be processed by his
method, although the markets for iota carrageenan are significantly less than
those for kappa. Lambda carrageenans do not form gels with potassium and would
therefore dissolve and be lost during the alkali treatment.
The simplicity of the
process means the product is considerably cheaper than refined carrageenan.
There is no alcohol
involved that must be recovered, no distillation equipment to purify alcohol,
no equipment for making gels, no refrigeration to freeze the gels, nor any
expensive devices to squeeze the water from the gel.
Producers in the
Philippines developed a higher quality product, suitable for human consumption,
by modifying the process just described for SRC.
After the alkali
treatment and water washing, the product is chopped and treated with bleach to
remove the colour (chopping improves penetration by the bleach, and bleach also
helps to reduce the bacterial count). After washing to remove any bleach, the
product is dried in a closed dryer. In this type of dryer, indirectly heated
hot air passes up through a bed of the unground pieces or chips that are being
carried through the dryer on a chain-type belt. This closed system dryer is
usually sufficient to keep the bacterial count low enough to make a human-food
grade product. If bacteria reduction is required, the dried chips can be milled
and then washed with alcohol (ethanol) followed by vacuum evaporation to
recover the alcohol. A simpler process is to treat the milled powder with
superheated steam.
The above process is
summarized in Figure 46 (PES/PNG carrageenan branch).
The product was
originally called Philippine natural grade carrageenan (PNG).
Attempts to market this
product as food grade in the United States of America and Europe resulted in
strong opposition from the producers of refined carrageenan who did not wish to
lose market share to this cheaper product. Eventually in the United States of
America, the Food and Drug Administration declared it suitable for use in human
food and to be labelled as "carrageenan", the same status as that of
the refined product.
In Europe, both refined
and PNG are permitted in human food, but carry different labels:
- refined carrageenan is labelled
"carrageenan" and E-407; while
- Philippine natural grade is labelled "processed Eucheuma seaweed" or "PES", and E-407a.
So PNG and PES are the
same grade of carrageenan.
The main difference
between refined carrageenan and PNG is that PNG contains the cellulose that was
in the original seaweed while in refined carrageenan this has been removed by
filtration during the processing. Refined carrageenan will therefore give a
clear solution, while PNG gives a cloudy solution. Where clarity of a user's
product is of no consequence, PNG is suitable.
For
further details
Detailed information on
any methods of carrageenan extraction are not easy to find. As Stanley (1987)
said, they are closely guarded as trade secrets by the several manufacturers.
Some information can be found in Stanley (1987), Stanley (1990) and Therkelsen
(1993).
