Algae contain fibers whose nutritional value is comparable to that of vegetable fibers.
IN PARTS OF THE FAR EAST algae have been used as vegetables in soups and salads since ancient times. In France they serve mainly as a source of iodine and fertilizer and as gelatinizing or texturing additives. Although eleven species of algae were recently accepted as vegetables by the French health authorities, their chemical composition and metabolism are poorly understood. Analysis of the fibers they contain nonetheless has illuminated the sources of their nutritional value.
The modern vogue for fibers began in the early 1970s when the British physician Denis Burkitt discovered a correlation between certain digestive, metabolic, and cardiovascular diseases and low levels of consumption of foods rich in fibers. Fibers are macromolecules that resist digestion by human enzymes. For the most part they make up the cell walls of plants—cellulose, for example, and various other molecules composed of chains of monosaccharides, or elementary sugars. Their chemical complexity dampened the enthusiasm initially aroused by Burkitt’s finding, but researchers have now developed sufficiently powerful analytical tools to carry on his work.
Fibers and Digestion
At the Institut National de la Recherche Agronomique station in Nantes, Marc Lahaye and his colleagues have used such tools to study the dietary fibers of marine macroalgae. It used to be customary to classify fibers according to their degree of solubility in response to various enzymatic treatments. Water-soluble fibers—certain pectins (fruit polysaccharides that cause jellies to gel), algal polysaccharides, and certain kinds of hemicellulose—were distinguished from insoluble fibers such as cellulose, other kinds of hemicellulose, and lignin. The soluble fibers, many of which have interesting rheological properties, were thought to reduce the blood concentration of cholesterol and to act on the metabolism of glucids and lipids. Insoluble fibers, on the other hand, seemed to accelerate bowel movements.
Researchers have confirmed the main features of these properties in recent years, and the Nantes team refined the classification of the dietary fibers in algae by modifying a technique of molecular separation known as the gravimetric method, which was used to precipitate macromolecules in various environments, such as water and alcohol, after the elimination of starch and proteins by enzymatic treatment. In 1991 Lahaye and his coworkers used it to determine the quantity of polysaccharides solubilized in environments that approximately reproduce those of the digestive tract.
They went on to apply this method to algae and found that the total concentration of dietary fibers in wakame, for example, can be as great as 75%, as opposed to only 60% in Brussels sprouts, the root vegetable that is richest in fiber. To discover how such fibers behave in the digestive tract, the Nantes chemists first studied Laminaria digitata, several thousand tons of which are processed every year in order to produce the gelling agents known as alginates (in 1992 alone some 65,000 tons were recovered and treated by the French seaweed fertilizer industry). Its polysaccharides—glucose polymers known as laminarins—are essentially soluble in very acid environments, whereas alginates dissolve in a neutral environment. The insoluble fibers of Laminaria species are principally constituted by cellulose.
On the other hand, the fibers of dulse (Palmaria palmata), a common red seaweed consumed by Europeans since the eighteenth century, seem to be continuously solubilized in the successive sections of the digestive system. In the case of sea lettuce and sea hair—two of the eleven species of algae that can now be sold as vegetables or condiments in France—the soluble fibers are xylorhamnoglycuronane sulfates (made from sugars and composed, in particular, of xylose and rhamnose), and the insoluble fibers are principally glucans (glucose polymers). Their solubilization more nearly resembles that of dulse.
Algal Thickening Agents
Improved understanding of the nature and assimilation of these foods has stimulated interest in developing markets for underused algal fibers and for the large quantities of residues that are now extracted from alginates every year. The extraction procedure begins with repeated washings in an acidified water solution, which eliminates the laminarins and fucans and transforms the alginate into alginic acid. Dietary fibers are solubilized in these baths. Then the alga is tossed in a warm basic environment (often with sodium carbonate), and the insoluble part is separated out with the aid of a flocculating agent and air currents. The resulting flocs, or tuftlike masses, constitute a second coproduct consisting mainly of cellulose.
Fibers from vegetable sources—beets, cereals, and fruits—are now incorporated in breakfast products or used as an ingredient in various prepared foods. Algal fibers can serve the same purposes. Looking to the future, research in the chemistry of algal fibers holds out the prospect that processing methods can be developed similar to those used in the milling of wheat and other grains, which depend in large part on an understanding of their elaborate structure. But whereas the chemistry of starch is already well developed, the listing of the elementary sugars that make up algal macromolecules has only recently been completed, and the exact order of their sequence is in many places still uncertain, as is the type of chemical bonds linking the sugars. Now that chemists have decoded the alphabet of algal polysaccharides, it remains for them to learn how to form words and put them together.