Clair G. Wood Univers~tyof Maine at Orono Orono, 04473
The quantities of polysaccharide produced by marine algae quite likely exceed those of land and fresh water plants combined. It has been estimated that the algae fix 101' tons of carbon annually ( I ) . This not only is the source of carbon-based foods for all the animal population of the sea but also represents a vast potential resource that has remained relatively unexploited by man. The algae are simpler in structure than land plants, having no true r w t or leaf system. The larger species do have a rwt-like attachment called a holdfast and a stemlike portion, the stipe, which hroadens out into a leafy network termed the lamina (Fig. 1). In addition many of the seaweeds possess bladders, filled with air or having jellied material, to aid in floatation. The algae may range in size from unicellular to giants such as Macrocystis pyrifera which has been reported to he over 150 ft long with a weight exceeding 90 lhs (2). The algae are represented by four main classes, the Chlorophyceae (green), Cyanophyceae (blue-green), Rhodophyceae (red), and Phaeophyceae (brown). The first two classes encompass both fresh and salt water species, are generally unicellular, and of no economic importance. The red and brown algae contain the larger salt water forms commonly known as seaweeds. Historical Uses of Seaweed The earliest recorded use of seaweeds is to he found in the writings of Shen Nung (- 3000 B.C.), a Chinese pbysician, who told of their medicinal value. Later, during the time of Confucius (800-600 B.C.), the "Chinese Book of Poetry" praised housewives for cooking seaweed as food (3). The utilization of seaweed as a foodstuff still has significant economic importance in the far East. In the early Western world seaweed found usage as a fertilizer and a cattle and sheep fodder. The first commercial application was in the production of potash for glass making (4). Kelp was being dried and burnt for its ash in France and the United Kingdom a t the beginning of the 17th Century and, by 1692, Louis X N gave the Royal Company of Glassmakers the sole privilege of cutting kelp along the French coast. Roughly 20,000 tons of kelp were harvested annually in the United Kingdom until the industry was hrought to a near halt through the introduction of mineral potash by 1850. A revival of the kelp industry occurred with the discovery that iodine could be ohtained from its ash. The discovery of iodine is attributed to M. Courtois. a Parisian saltneter dealer. in 1813. Courtois leached kelp ash with wateiand then treated the mother-liauor with sulfuric acid. Boilins and condensation of the vapor gave crystalline iodine (5). A ton of dried kelp will produce between 30-40 lh of iodine. World production of iodine by this method reached its peak in 1917 when 240 tons of iodine were obtained from 4,000,000 tons of wet weed. The discoverv of Chilean iodine dewsits sent the kelp industry into its decond decline and the weed was not seriouslv harvested again until after W.W. n. English colonists to the new world hrought with them a -~~~~~~ ~
~~~
South Hadley, Mass., August 14-18,
Seaweed Extracts A unique ocean resource
Figure 1 . Three species of Phaeophyceae illustrating the basic physiology marine algae.
01 the
taste for blancmange, a pudding prepared from boiling the red seaweed Chondrus crispus or Irish Moss with milk. The colonists imported Irish Moss from Europe until 1835 when the mayor of Boston, J. F. C. Smith, discovered the plants growing off Scituate, Massachusetts and the first commercial harvesting of seaweed began in America (6). The United States, first large scale utilization of seaweed came about due to the need for large quantities of acetone during W.W. I. The Hercules Powder Company set up a plant in San Diego for the purpose of obtaining acetone from kelp with potash as a sideproduct (7). The industry processed 1200 tons of wet kelp daily, ohtaining 13 tons of 95% potassium chloride and over 1500 1 of acetone. At its peak the plant employed over 1000 workers; however, the pmcess was not economical and the plant closed a t the end of the war. Current Seaweed Use Silverthorne and Sorenson estimate the current market value for the world's annual seaweed harvest to exceed $350,000,000 (8). Nearly all of this revenue stems from the utilization of phycocolloids, the structural polysaccharides of the marine algae. Table 1 shows the major polysaccharides derived from the red and brown seaweeds. The algal polysaccharides are dissimilar from the land-based plant gums in that they have uronic acid or sulfate ester residues, although starch-like reserve polysaccharides are encountered. Cellulose, the main structural polysaccharide of the land plants is almost totally absent from marine algae. The major polysaccharide of the brown algae consists of a polymer of mannuronic and guluronic acid in ionic form commonly termed algin. The polysaccharides of the red seaweeds are more complicated, having a sulfate ester content ranging from 0% in the agarose fraction of agar to 36% in one form of carrageenan (9).Current helief is that all of the sulfated polysaccharides isolated from the red seaweeds are related by a common group of monosaccharides, with the degree of sulfonation and other variations dependent upon species and environmental conditions (10). The precise physiological function of the sulfate esters is unknown. One suggestion is that they are concerned with ionic balance and function as ion-exchange resins ( 1 1 ) . It has also been suggested that their Volurne51, Number 7. July 1974
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449
Table 1. Alaal - Polvsaccharides
Origins of the Major Algal Polysaccharides
I I Rhodophyceae I
Fucoids (rockweeds)
Kelps
I Fucus
Carrageens
I
I
Laminaria
Chondrua Gigartinin
Ascophyllum
\ ,
Table 2.
Extract Agar Carrageenan
Furcellaran Alginic Acid
1
/M'-YStis
Carrageenan
Algin
Agarophytes
I
Gmcilaria Gelidium
1
Agar
Furcellaran
I
Fureellaria
1
Furcellaran
Reoresentative Uses of Algal Extracts in the Food Industry
Dairy Products
Bakery Products
sherbets cheeses yogurt chocolate milk ice cream instant ~ u d d i n e s pie fillings cottage cheese imitation coffee creams milk puddings custards chocolate milk ice cream salad dressings dessert gels
icings and glazes jellied candies pie fillings bread breading and batter mixes
jellies icings mamalades pie fillings artificial cherries icings dry mixes
Meat, Fish & Poultry pickled meats canned tuna antibiotic ice canned meats and poultry antibiotic ice
meat pastes minced-meat pie fillings frozen fish sausage casings
Miscellaneous health foods preserves aspics frozen f ~ i t beer candies
fruit juice dietetic foods candies frozen fruit
extreme hygroscopic nature prevents desiccation of the weed a t low tide. All of the algal polysaccharides are indigestible and thus have no nutritive value in themselves; however, they are widely used in the food and food related industries for their properties as thickening, stabilizing, suspending, emulsifying, and gelling agents. Table 2 summarizes these uses of the phycocolloids of major economic importance. These and other uses of the seaweed extracts will he hriefly examined. Agar Undoubtedly the first algal polysaccharide to he extracted from the parent plant was agar, a cell wall coustituent of the red seaweeds. Extracted by boiling the weed in water, agar sets to a gel a t around 36°C and resists melting to 85°C (12). Legend has i t that agar was first discovered about 1658 when the Japanese Emperor was marooned a t an inn during a snow storm. The innkeeper prepared seaweed jelly for the Emperor's dinner and threw the excess outside on the snow. The next morning the jelly had turned into a dry papery translucent substance which the innkeeper found could be reconverted to its jellied form (13). The familiar use of agar as a culture media was first suggested in 1881 by Frau Fanny Hesse as an alternative to gelatin. When her husband, Dr. Walter Hesse, informed the famous microbiologist Robert Koch of his success with agar it rapidly replaced the use of gelatin and still remains the best media available (14). Selby states that some 400,000 lh of agar are used as culture media annually (121. The structure of agar has been under study for decades, primarily by Japanese chemists (15). However, as late as 1970, Percival states that the structure of agar was unresolved which points u p the difficulties in the structural 450
/ Journalof Chemical Education
elucidation of algal polysaccharides in general (16). Much of the difficulty lies in the extraction procedures where, due to the similarities in structure, a polysaccharide thought to he pure may in reality he a mixture. There is also the possibility of altering the structure of the polysaccharide during extraction or hydrolysis preceding analysis. Those polysaccharides containing sulfate ester are particularly susceptible to desulfonation and alteration of structure (17). Coupling this with the possibility of seasonal and species variations it is not surprising that the structures of most algal polysaccharides are not definitely known. Acetylation and fractionating procedures have shown the oresence of a t least two comoonents in the aear ex" tract (18). One of these, agarose, is a neutral polymer consisting of reneatine units of the disaccharide aearobiose shown in ~ i g u r e~ ~ ( 1 20). 9 , The other fraction, agaropectin, has not been defined beyond the fact that it contains agarose with varying amounts of sulfate ester. Recent studies have indicated it may well he a complex mixture (21, 22). Agar is somewhat more expensive than land based gums; however, it possesses properties which make it unique for certain usages. Agar forms one of the strongest
.
gels known, giving a tough strong gel a t concentrations as low as 0.5%. The gel itself forms well below the melting point as illustrated by a 1.5% solution which f o m s a gel a t 30°C that holds to a temperature of 85°C (23). This hysteresis effect finds many applications where autoclaving is necessary. Agar's ability to form strong gels which withstand higher temperatures has made i t particularly suitahle for the bakery-industry where it acts as a stahilizer and protective agent in meringues, pie fillings, icings, and glazes. Other major uses are as a stabilizer in sherbets, yogurt and other dairy products, as well as a protective gel for fragile canned and pickled meats. The non-nutritive character of aear ~~~~~~" has led to its use in bulkv health foods and laxatives. Lesser amounts go into the production of heveraees.,. net fwds. dental materials, and ~harmaceuticals. ~ilverthornehas estimated current world agar production a t 9,800 metric tons with a market value exceeding $46,000,000 (24). This makes agar the leading product obtained from the marine algae. ~
Carrageenan Carrageenan is the name given to a group of sulfated galactans extracted from the red seaweeds. The various carrageenans are differentiated from each other by their sulfate ester and 3,6-anhydrogalactose content (25). At least seven carrageenans have now been isolated, some of which may be precursers of the more common forms (26). Figure 3 shows the repeating units for kappa and lambda carrageenan, the most commonly encountered and first isolated of these polysaccharides. Kappa carrageenan forms strong gels with most cations, particularly potassium. The lambda form gives a free flowing solution with all cations even in cold water. This difference, which is partially due to the sulfate ester content, is exploited in the various uses to which carrageenan is put. Commercial carrageenan was originally the extract of Irish Moss and was comprised of 60% kappa and 40% lamhda carrageenan. The extracts of other members of this species normally have properties corresponding to one or another of these basic forms. Eighty percent of the carrageenan produced goes into the food industry. It is particularly valuable in the bakery and dairy industries due to its ability to gel in hoth milk and water systems. This property leads to carrageenan being added to such products as water dessert gels, pie fillings, syrups, puddings, and custards. It is also used to stabilize evawrated milk, infant formula, and other concentrates. he remainder of carrageenan production goes mainly to the pharmaceutical and cosmetic industries. carrageenan is second to agar in economic importance. In many cases i t is used as a less expensive alternative to
Figure 3. Top: A-Carrageenan illustrates the highly ionic nature of certain sulfated galactans. Bottom: K-Carrageenan with the 3.6-anhydro-group arising from desulfonation of a A-carrageenan-like precurser.
L
J"
Figure 4. A segment of alginic acid showing p-1.4-linked-D-mannuronic acid joined 10 a~-1,4-linked-L-guluronicacid.
agar when the properties desired are the same in hoth ~olvsaccharides. Carraneenan nroduction is estimated to be-about 9,000 metric tons-with a market value of $34,000,000 (28). Furcellaran Furcellaran or Danish agar is an extract of the red seaweed Furcellaria fastigiata. Third in commercial importance after agar and carrageenan among the sulfated galactans, furcellaran was developed during W.W. II as a substitute for agar. It is almost exclusively a product of Denmark. The structure of furcellaran is unknown hut it is thought to he similar to kappa carrageenan. Its properties are intermediate to those of agar and carrageenan, being soluble in hot water with the solution setting to a firm gel (29). As Table 2 indicates furcellaran is used in the same product types as carrageenan and for similar physical properties. Current p&uction runs around 1,200 metric tons annually with a market value 01 approximately 54,800,000 (301. Algin Alginic acid is the only extract of commercial importance presently obtained from the brown seaweeds. Most is extracted from the giant California kelp, Macrocystis pyrifera, although it is obtained in the Atlantic regions from Laminaria and Ascophyllum nodosum. Alginic acid consists of D-mannuronic and L-guluronic acid residues (31, 32). Figure 4 shows a repeating unit of alginic acid. Recent studies have shown that this alternating structure is occasionally interspersed with semicrystalline homopolymeric strands of pure guluronic or mannuronic acid (33). The sodium salt of alginic acid is sold under the name "algin" while other salts and esters are grouped under the term of alginate. Sodium alginate dissolves in water to form an extremely viscous solution; a 3% solution will barely pour. An algin solution can he made to gel by the addition of divalent ions, usually calcium, mineral acid, or a combination of hoth (34). Thus the gel strength can be varied by control of the p H and ionic concentration. In addition to the salts a propylene glycol ester of alginic acid is produced which is readily soluble, maintains high viscosity a t low concentration, and is not gelled by acid or divalent ions. Sodium alginate is used extensively in the dairy and bakery industries as a controlled gel and to improve texture, body, and smoothness of the product. Thus algin may be found in ice cream, dry mixes, pie fillings, and confectionaries. The propylene glycol ester is used as a stabilizer in French dressings, meat sauces, and frozen fruit. A major non-food use of algin is in the paper industry where alginic acid's hydrophobic nature is used to control ink penetration, thereby giving a brighter and more uniform surface to fine papers. Other uses are in shampoos, pressure sensitive tapes, lotions, and the prevention of boiler scale, to name but a few. Companies engaged in algin production are reluctant to release exact figures; however, estimates of algin production range from 9,000-12,800 metric tons annually with a maximum market value of $35,000,000 (34,35). Volume 5 1 , Number 7, July 1974
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451
Extracts with Potential Value
of gwd quality paper pulp having been produced from seaweeds bv the Kraft Process (48). Seaweed exnloitation -~ for the &pplementati~n of our offers ri& dwindline land resources. Certainlv the abundant supdies of seaweed along our shores deserve to be considerid as something other than a scenic nuisance. ~
Fucoidan Fucoidan is an extract of the hrown seaweeds and is comprised mainly of L-fucose or its ester sulfate (36). At least four different polysaccharides have been isolated from the original fucoidan extract and the term fucoidan is now used to refer t o any fucose-containing polysaccharide of the Phaeophyceae (37, 38). Fucoidan is a potential source of the rare sugar L-fucose and has been found to have anticoagulant activity similar to heparin (39). Funoran Funoran is a sulfated galactan produced on a limited scale in Japan. It is used in sizing, adhesives, and hairwaving preparations. Current production is about $1,000,000 annually (42). Laminarin Laminarin is a reserve carbohydrate consisting mainly of D-glucose and similar to the amylopectin of land plants. It also contains a small amount of the polyalcohol D-mannitol (40). Laminarin is a white, tasteless powder which has found limited use as a surgical dusting powder and i t has shown some tumor inhibitory effects (41). Future Potential Silverthome has estimated annual seaweed harvests to be around 1.700.000 metric tons (43). No true estimate of how much 'potentially harvestable seaweed exists has ever been made. One, now out of date, reference puts the figure a t 80,000,000 tons (44). The true potential harvest is more likely only a small fraction of this; yet it still represents a much larger value than is now being utilized. The United States produced some 20,000,000 lh of marine polysaccharide in 1970 with projections of 30,000,000 Ib by 1980 (42). Even with increased research into new uses for the phycocolloids, other uses for seaweeds must be found for the full potential of the oceans to be realized. Approximately 40,000 metric tons of seaweed are used annually as fertilizer and fodder (45). Only 10% of this utilization is in the United States and could well be increased with such products as packaged gardening materials. As mentioned previously seaweeds are an integral part of the Oriental diet, particularly in Japan where i t comprises some 25% of the diet. The Japanese consume some 65,000,000 l b annually, either dried (nori, kombu) or in sauces and salads (46). The Japanese are engaging in sophisticated ocean farming of seaweed; however, i t is unlikely that the Western diet will change sufficiently to allow for any similar activity in the United States. The Maritime Provinces of Canada produce some 725,000 lb of edible dulse (dried Rhodymenia palmata) annually but this industry is declining slightly. Increased research is needed to open other avenues in the utilization of seaweeds. One suagestion is the production of ammonia and oil from the dkstructive distillation of dried seaweed (47). A more promising one is the report
452
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Journal
of Chemical Education
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~
LRerature Cited (11 Pereival. E.. "Algal Polysaceharides and Thci. Biological Relationshipo." in Pmc. htern. Seaweed Symp.. 4th Bimitz, Ran-. 1961. The MannilIan Co., New Yolk. 1 W . p ~ .18JS. (2) Chapmsn, V. J., ''Seaweeds and Their Om." Mcthuen and Co.. Ltd.. London, 197o.p. 11. (3) Glicksman. M.. "Gum T c e h n o l ~ yin the Food Industry." Academic R-, he., 1969.~.W2. ,I, 1, 00.25-16. , ~w, 1 2 ., (5) Ref (2). P. 28. (61 Ref13J. p. 214. (7) Cmsrman. E. G.,SeienfificAmeric~~ 118.475i19181. (8) Silverthornc, W., and Sorenm. P. E.. "Marine Algae aa an Eeonomic Resour-," 7th Annual CodoronceofMarineTechoologicslSociety, 1971, p. 523. (9) Upham. S. D.. "The Structure of F M Seaweed Paly8acchsrides." Tech. Bull. S, Marine Collaidslnc.. Sprineflcld, N. J.. 1967. (10) hdorson. N. S.,Do1an.T. C. S.,endReea, D.A..Natue, 206,1060-62. (19651. (11) Epp1ey.R. W . , J Den Physiol.. 41.901 (19581. (12) Selby, H.. and Wynne, W. H.. in Whistler, R. L., (Editor). %dustrial Gums," 2ndEd.. Academic Ems, he.. 1973. pp. 29-48 1131 Ref(2J. 1111. 157-158. 114 h i m ) . i.:m (151 Mo1i.T.. inAduon. incorbohyd. Ckm., 8.322(1953). (16) Percival. E.. "The Carbohydrates, Chemistry and Biochemistry," Academic Pr-, hc.. NearYark 1970, ~ 5 5 5 . 117) Smith, F.. and ~ontgomery,R.. "The Chemistry of Plant Gums and Mueilaws." Van N ~ t r Reinhold ~ ~ d CO.. NevYark. 1959, pp. 403-4. (18) Araki,C., andArai.K., Bvil Chem. Soe. Jnp., 30.287(19571. (19) Araki, C.,J Chem. Soc Jop.. 58.1338 119371. ( m ) h a k i , c . , J. c h m Sac. Jap., 62.733 ii9"11. Chem. Absfr, 37.91 (19431. (21) Percivsl. E., and MeDowell, R. H.. "Chemistry and Eozymolw of Marine Aka1 Polysacchsrido%"AcademicRess.Inc., New York, i967.p. 133. (22) Duekworth.M., Corbohyd. R e g , 16.189(19701. 18.1(19711. 1231 Ref(31. p. 203. (24) Ref (8). p. 526. (25) Haw, A,, h r m , B., Smidsmd, 0.. and Pcrnaa, A. J., Aclo Chem. Scond.. 21.98 (19671. (28) Stanley. N. F., "The b r t i e s of Carragoenans as Relafed to Structure." Conference on Marine Sciences. Cham. Insifif, af Canads, Prince Edvardr laland, Aug. I€-18. 1970. (21) T o d c , A,, in "Industrial Gums." (Edilar: WhiaUer. R.L.). 2nd Ed., Academic P m , Inc.. IIT.oo.83-114. (281 Ref (8). p. 528. (29) Ref (3). pp. 2367. (30) Bjerxe-Peterxm. E.. Chrirtenaen, J., and Hemmingaen, P.. in "Industrial Gums? New Yolk, 1973. PP. (Editor: Whistler, R. L.), 2nd Ed.. Academic Press. h., 123-36; (31) Nelson, W. L.. and Cretchu, L. H.. J. Amer Chem. Soc.. 51. 1914(19291:52, 2130 (19301. (32) Fisher. F. G., and Dorfel. H..How.-Seykis Z Physiol. Chem.. 301. 224 (19551; 301.196(19551. (331 Ref 121). p. 110. (341 MeNeley. W. H., and Petcitt, D. J., in "Industrial Gums," (Editor: Whider. B. L.). 2nd Ed.. AeademicPress. Inc., New York. 1973.49-81. Ref (8). p. 5 2 l Percivsl. E. G. V.. and b a a . A. G.. J. Chem. Soe., 717 119501. Haug. A . Lamen. B..andPainfer.T.J..Acto Chom. Scond.. 24.3339(19101. Pereiva1.E.. Cmbohyd. Re& 7,272i19681. DO".~. L. w.. in "hdustrisl Gums." (Editor: Whistler, R. L.1. 2nd Ed.. Academic Press, he.. NcwYork. 1973.pp. 115-121. Peat. S., Whelan, W.I., a n d h r l e y , H. G . , J Chem S o t . 724i19581;729119581. B I S C ~w, A. P., and bar. E. T.. in"bdustrialGums," (Editor Whiatlcr. R. L.1. 2nd Ed., Aisdemie Press. In
(471 Reynolds, G. F., Chem inBritsin, 8,537119721. 1481 U s m , M.. Sci. Res. (DoeeoJ, 5.141 (19681: Cham. Abxfr, 70.793Mb (1969).
