Carrageenan and The Structure
Carrageenan are commercially important hydrophilic colloids (water-soluble gums) which occur as matrix material in numerous species of red seaweeds (Rhodophyta) wherein they serve a structural function analogous to that of cellulose in land plants. Chemically they are highly sulfated galactans. Due to their half-ester sulfate moieties they are strongly anionic polymers. In this respect they differ from agars and alginates, the other two classes of commercially exploited seaweed hydrocolloids. Agars, though also galactans, have little half-ester sulfate and may be considered to be nonionic for most practical purposes. Alginates, though anionic, are polymers of mannuronic and guluronic acids and as such owe their ionic character to carboxyl rather than sulfate groups. In this respect alginates are more akin to pectins, found in land plants, than to the other seaweed hydrocolloids.
Red Seaweed (Rodophyta)
Carrageenans have the common feature of being linear polysaccharides with a repeating structure of alternating 1,3-linked b -D-galactophyranosyl arid 1,4-linked a -D-galactophyranosyl units. The 3-linked units occur as the 2- and 4-sulfate, or unsulfated, while the 4-linked units occur as the 2-sulfate, the 2,6-disulfate, the 3,6-anhydride, and the 3,6-anhydride-2-sulfate. Sulfation at C3 apparently never occurs. Pyruvate has been reported present in the carrageenans from some Gigartina species; these carrageenans have been termed "pi-earrageenan" . Methoxyl groups occur in sulfated polysaccharides from the Grateloupiaceae family. There is some question, though, as to whether these have the alternating structure characteristic of carrageenans.
There are Three Main Commercial Classes of Carrageenan
1. Kappa: strong, rigid gels. Gels with potassium ions, reacts with dairy proteins from Eucheuma cottonii.
2. Iota: soft gels. Gels with calcium ions. Produced mainly from Eucheuma spinosum
3. Lambda: Does not gel, used to thicken dairy products. The most common source is Gigartina.
Extraction Processes
Specific details of extraction processes are closely guarded as trade secrets by the several manufacturers of carrageenans, but broadly these follow a similar pattern. Weed, usually dried and baled, is received at the processing location from the harvesting location. The shipment may be sampled and the sample subjected to a test extraction to evaluate the quality of the extractive. Other need quality factors such as contents of moisture, sand and salt, and non-carrageenophytes are evaluated at this stage. Obtaining a representative sample from a weed shipment is not a trivial exercise, as weed quality may vary widely not only from one shipment to the next but also within a shipment, due to factors over which the processor may have but limited control. Sampling protocols used generally represent what is feasible rather than what a statistician might regard as adequate.
Prior to plant-scale extraction the weed may be washed to remove adhering salts, sand, stones, and marine organisms. Washed, or unwashed, weed, usually as a blend selected to achieve the desired properties in the extractive, is then digested with hot water under alkaline conditions to exhaustively extract the carrageenan. The alkali, usually calcium or sodium hydroxide, performs two functions: firstly it promotes swelling and maceration of the weed to aid in bringing the carrageenan into solution, while, secondly, when employed at sufficiently high concentrations, it effects cleavage of 6-sulfate groups from the carrageenan to generate 3,6-anhydro-D-galactose residues in the polysaccharide chain. These function to enhance the water gel strength and milk reactivity of the carrageenan. Maceration is promoted by agitation of the resultant paste. Conversion of 6-sulfated moieties to the 3,6-anhydride continues during resting of the paste at temperatures near 100°C.
When the desired degree of conversion has been achieved the solution of carrageenan is separated from weed solids by filtration, or by centrifugation followed by filtration. Concentration of the filtrate by evaporation, and adjustment of pH, are done prior to the recovery of the carrageenan from solution.
The foregoing processing operations inevitably involve some degradation of the polysaccharide, due to the rigors (e.g., heat, alkalinity) of processing. Although carrageenans are reasonably stable under the conditions of alkalinity encountered in processing a drop in pH can occur from the consumption of alkali for the neutralization of sulfuric acid formed by cleavage of half-ester sulfate groups. Saccharinic acids may also be formed through alkali-catalyzed "peeling" reactions.
Several methods have been used to recover the carrageenan from solution. Direct drying of the concentrated filtrate on steam-heated rolls has been used extensively. Products of much higher quality are obtained by precipitation of the carrageenan from solution by 2-propanol or other alcohols. An interesting historical note is that perhaps the earliest process described for recovering carrageenan from Irish moss employed alcohol precipitation.
Precipitation is followed by further alcohol washes to dehydrate the coagulum. Vibrating screens or basket centrifuges may be used to separate the coagulum from the alcohol following precipitation and each wash. Following the final wash the coagulum is dried under conditions permitting recovery of the residual alcohol.
The fibrous carrageenan from the dryer is ground and sifted to specified particle sizes which may range from 80 mesh to 270 mesh. This basic product, segregated into batches, is sampled and tested for compositional and functional properties (e.g., moisture, viscosity, gel strength).
Another process presently in use for the recovery of carrageenan from solution was originally developed for furcellaran production but is also employed for kappa-carrageenan. This cakes advantage of several properties common to furcellaran and kappa-carrageenan. First, solutions of these polysaccharides form gels in the presence of potassium ions. Second, these gels exude water by syneresis on standing, the more so when squeezed in a press. Third, much water separates from the gel when it is frozen and then allowed to thaw. The latter phenomenon is the same as that used for the production of agar.
In the case of Furcellaria, the weed may be treated in the cold with an alkaline solution for one or more weeks. This alkaline treatment removes colouring matter and some proteins and makes the gum more easily extractable. Some alkaline elimination of 6-sulfate may also occur during this treatment. Extraction follows a procedure generally similar to that described above for the alcohol process. Following concentration by evaporation the filtrate is extruded through spinnerets into a cold 1-1.5% solution of potassium chloride. The resulting gelled threads are further dewatered by subsequent potassium chloride washes followed by pressing. The gel is then frozen, thawed, chopped, again washed with fresh KCl solution, and air-dried.
A limitation of the freeze-thaw process as applied to carrageenans is that it is applicable only to furcellaran and kappa-carrageenan, which are the only types whose gels with potassium ions exhibit marked syneresis. Moreover the requirement that potassium be present precludes making products wherein sodium is the major counterion.
The "gel-press" process, used by some minor producers of carrageenans and agar, likewise relies on pressure to dewater the gel, but omits the freeze-thaw cycle.
The "gel-press" process, used by some minor producers of carrageenans and agar, likewise relies on pressure to dewater the gel, but omits the freeze-thaw cycle.
The economics of extraction processes are strongly affected by the cost of the energy required to bring the carrageenan into solution and subsequently to recover it in dry form. This includes the heat necessary to digest and cook the seaweed, to concentrate the filtrate in evaporators, to dry the coagulum, and, in the case of alcohol precipitation, to recover the spent alcohol by distillation. Steam, generated by oil-fired boilers, is the usual source of process heat. Given the volatility of oil prices in the present-day market it will be appreciated that the cost of energy from this source has changed drastically in the past and can be expected to do so in the future. The impact of energy cost can of course be less in any locality where local low-cost fuels can be exploited. As an example, one major manufacturer of carrageenans uses locally-mined peat as a source of process heat.
High energy costs can be countered by employing cogeneration to supply the not inconsiderable electrical power requirements for plant operation. Alternatives to evaporation (e.g., ultrafiltration) have been investigated. These capital-intensive measures are not presently economical, but may be expected to become so in the likely event of rising energy costs.
An adequate supply of cheap, good quality, fresh water is an obvious prerequisite for the economical operation of an extraction process. In at least one instance a carrageenan factory was forced to relocate to another area when it was found chat its expanding demands exceeded the capacity of the local water supply.
Filtration must be employed if refined, completely water-soluble, products are to be produced. Owing to the viscosity of the extract and the swollen, gelatinous nature of the residual solids pressure filters are a necessity for efficient throughput and a filter aid must be added to the feed to prevent clogging ("blinding") of the filter medium and to afford a porous filter cake that will drain well and can be washed in the filter press to recover retained carrageenan. The filter aids most commonly employed are calcined diatomaceous earth and expanded volcanic glass. Since large amounts of aid are required the choice may depend on the location of the carrageenan processing plant with respect to the source of supply of aid.
Diatomaceous filter aids are available in a range of grades of retentiveness. A retentive aid may be used for a secondary "polish" filtration of the effluent from the primary filtration. This may be preceded by treatment of the primary filtrate with activated carbon to decolorize it. However this practice is now uncommon as the expense of these added steps usually cannot be recouped as added value of the product. Moreover the tendency of carbon to peptize and pass through even the most retentive filter can result in the product's having an undesirable grayish color.
It has been estimated that for a carrageenan extraction plant to be economical it should have a capacity of at least 450-750 metric Cons of product annually. This would require processing 1 400-2 300 MT of dried seaweed. To allow for variations in the harvest, available seaweed sources should exceed these tonnages by 40-50%. Initial investment for procurement, production, and marketing has been estimated to be $ 4 500 000 - $ 6 750 000. While these may be taken as ballpark estimates, obviously many factors other than scale of production determine whether a given operation will prove profitable. It has been recommended that any move toward production should be preceded by a three- to four-year pilot program of harvesting and selling dried seaweeds to other producers. This will serve to develop information required for starting an extraction plant.
A new process, and product, is semi-refined carrageenan. This process, which is distinguished by its low energy input, uses Eucheuma cottonii as a raw material. Semi-processed E. cottonii is prepared by a method which superficially resembles that for French fried potatoes. A basket of seaweed fronds is immersed and cooked in hot aqueous potassium hydroxide and then soaked in fresh water to extract most of the residual alkali. The product is dried and ground to produce a flour having many of the properties of conventional extracted carrageenans.
The economic advantage lies in not extracting the carrageenan from the seaweed but rather performing the reaction which maximizes gel strength upon the polymer within the plant structure. By doing this the ratio of process water to product is minimized, thereby reducing the cost of isolating the dry product.
Top (Left-Right): Carrageenan, Dry Seaweed, Wet Seaweed; Bottom (Left-Right): Ice Cream, Tooth Paste, Cosmetics Cream (Examples: products that contain carrageenan)
Uses
Carrageenan is used in food production as a thickening agent, an emulsifying agent and a stabilizing agent. Carrageenan is a vegetarian and vegan alternative to gelatin. Carrageenans are large, highly flexible molecules which curl forming helical structures. This gives them the ability to form a variety of different gels at room temperature. They are widely used in the food and other industries as thickening and stabilizing agents. A particular advantage is that they are pseudoplastic—they thin under shear stress and recover their viscosity once the stress is removed. This means that they are easy to pump but stiffen again afterwards.
Products
Basically there are three types of carrageenan of commercial importance. These are kappa, iota, and lambda-carrageenan. Furcellaran may be considered to be an extreme kappa type. These commercial extractives approximate to the limit polysaccharides, their criteria being functionality rather than strict chemical characterization.
Batches of the basic carrageenans are tested for their functional properties and then blended to produce standardized products. Diluents, usually sucrose or glucose, may be added for standardization. Food grade salts, such as potassium chloride or citrate, and other gums, particularly locust bean gum, may be incorporated in the blend to achieve desired functional properties. In all, more than two hundred different carrageenan and furcellaran blends, tailored to meet specific applications, are presently offered by the several manufacturers, as well as blending houses, under their various trade names.
Future Prospects
After nearly fifty years of development the carrageenan industry can now be said to have come of age and to be a mature industry. As previously mentioned its close ties to the food processing industry, likewise in its maturity, presages that future growth should be steady, if unspectacular. New products and applications can be expected, but these may be slow in coming. An insight into the time scale may be gained from the observation that the last "new" application for carrageenan of any great commercial significance was air freshener gels in the early 1970s. Opportunities exist wherever the functionality of carrageenans can confer advantages not possessed by cheaper competitive gums.
A favorable factor has been the stabilization of seaweed supplies and prices due to the advent of Eucheuma farming. Barring political upheavals in the harvesting regions the industry can remain assured of adequate supplies of good quality weed at reasonable prices. Future progress now appears to lie in the areas of achieving cost reduction in processing and developing more versatile and better quality-controlled products.
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