Carrageenan

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.

7.1 Carrageenan production methods

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.

7.1.1 Refined carrageenan and filtered carrageenan

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).

FIGURE 45
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.

7.1.2 Semi-refined carrageenan and seaweed flour

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).

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.

7.1.3 Philippine natural grade (PNG) and processed Eucheuma seaweed (PES)

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).