Much confusion is undoubtedly created by the variety of terms and definitions used in describing the products of the seaweed industry. In some parts of the literature the terms are used rather loosely and, occasionally, erroneously. To a certain extent this situation is not surprising, as the subject is indeed somewhat complex chemically, and even specialized workers in this field have so far failed to reach common agreement upon certain classifications.
This note attempts briefly to classify, in as non-technical a manner as possible, some of the major terms encountered in the study of seaweed colloids. The descriptions, which in some instances are rather over-simplifed, are based principally upon the groupings and arrangements suggested by Chapman (1970).
The term phycocolloid is used to describe a colloid derived from seaweed; a colloid in turn is defined (Concise Oxford Dictionary) as a “non-crystalline substance with very large molecules; when dissolved the solution is viscous and sticky”. A phycocolloid is thus simply a seaweed gum.
The various gums of the red algae are closely related chemically and generally consist of at least two major polysaccharide compounds, one being responsible for their gelling powers, the other for their viscosity. A polysaccharide is a carbohydrate that can be decomposed by hydrolysis into two or more molecules of monosaccharides (which are, in effect, simple sugars). These compounds can be divided into three major groups; the agars, the carrageenans and the gelans. Reference is also frequently found to so-called “agaroids” but, as most of these yield carrageenans, Chapman recommends that this terms should be abandoned.
The compounds derived from the larger brown algae have certain properties similar to those obtained from red seaweeds and collectively these algal polysaccharides are known as phycocolloids.
The four major groups of phycocolloids and the seaweeds from which they are derived can thus be broadly classified as follows:
The agars - derived from the red seaweed genera Gelidium, Ahnfeltia, Pterocladia, Gelidiella and Acanthopeltis; the genus Gracilaria is also generally included in this group.
The carrageenans - derived from the red algae Chondrus crispus and Gigartina stellata; these two species represent the true carrageenans but generally Eucheuma, Phyllophora, Iridea1 and other species of Gigartina are also included.
The gelans - derived from a smaller group of red algae, represented by Furcellaria and Hypnea. The products Furcellaran and Hypnean are often included with the carrageenans and further research may eventually show that the term “gelan” is superfluous.
The algins - derived from the major brown seaweeds, for example, Macrocystis, Nereocystis, Sargassum, Fucus, Ascophyllum, Laminaria, Ecklonia, etc.
1 Recent work by Kim indicates that the genus Iridea can be reduced to synonym of Gigartina and that the term Iridea (or Iridaea) should be abondoned.