Seaweeds, have traditionally been used by the western food industry for their polysaccharide extract – alginate, carrageenan and agar. Carrageenans are naturally occurring linear sulphated polysaccharides which fill the voids within the cellulose structure of certain species of Red seaweed (Rhodophycae) of the Solieriaceae, Gigartinaceae, Furcellariaceae, Phyllophoraceae, Hypneaceae, Rhabdoniaceae and Rhodophyllidaceae families. Carrageenan-yielding algae are grown in the Philippines, Indonesia, Canada, the USA, France, Korea, Spain, Portugal, Morocco, Mexico, Chile, Denmark and Brazil. Euchema cottonii and E.spinosum are main species producing k and i types which grow around the Philipines and Indonesia and other island coasts in the Far East; Chondrus crispus, a small cold-water seaweed producing k and l types is the most familiar red seaweed widely distributed around the coasts of the North Atlantic; the large cold-water Gigartina species from which k and l types are produced can be found from the cold deep coastal waters off Chile and Peru and Furcellaria species are collected in the cold waters around Northern Europe and Asia. The cold-water seaweeds are harvested once a year, whereas the warm-water seaweeds grow on a 3-month cycle.

Carrageenan is generally considered a high-molecular-weight linear polysaccharide. Chemically, it comprises repeating galactose units and 3,6-anhydrogalactose (3,6-AG), both sulfated and non-sulfated, joined by alternating α(1-)-and β(1-4)-glycosidic linkages. Commercial carrageenans are available as stable sodium, potassium, and calcium salts or, most generally, as a mixture of these. The associated cations together with the conformation of the sugar units in the polymer chain determine the physical properties of the carrageenans. For example, k and i-carrageenans form gels in the presence of potassium or calcium ions. Carrageenans are used commercially as thickening, suspending, and gelling agents. Typical applications are as a thickener or ‘binder’ in toothpaste, a suspending agent for cocoa in chocolate milk, and a gelling agent for milk puddings, water-gel deserts, and air-freshener gels. Commercial food-grade carrageenans typically have the number average molecular weight, Mn values of ~200 x 103 to 400 x 103. At masses below 105 Da, the functionality of the carrageenans is largely lost.

The main carrageenan types, lambda, kappa and iota, can be prepared in pure form by selective extraction techniques. Mu and nu carrageenans are postulated as the precursor structures which, as a result of the internal rearrangement by alkali treatment, form kappa and iota carrageenans. The distinct carrageenan structures differ in 3,6-anhydrogalactose and ester sulfate content. Variations in these components influence hydration, gel strength and texture, and melting and setting temperatures, syneresis and synergism.

In 1844, the first successful extraction of carrageenan from Chondrus crispus was attributed to Schmidt, followed by a patentable process of extraction by alcohol precipitation. The production of carrageenan is a straightforward operation consisting of extraction, purification, and isolation. The seaweed should be cleaned to remove any sand before extraction proceeds. Alkaline conditions are necessary to modify the galactan structure, resulting in better gelling ability. Modifications as well as differences in extraction techniques exist. Three processes are most commonly used: alcohol precipitation, freeze-thawing and gel press.

What is Natural Carrageenans?

This form of carrageenan evolved into a food ingredient from a past that began as a way of treating raw Eucheuma seaweed in the Philippines prior to export. The carrageenan refiners in industrialized countries began to encounter environmental problems with their waste which is high in BOD and color. To counteract the growing cost of treating this waste there developed in the Philipines a pre-treatment process for the raw seaweed which yielded the item of commerce now called ‘alkali treated cottonii chips’. A powder made from these chips was developed in the early 1970s for use by manufacturers of canned pet food. The performance properties of the alkali modified flour approached those of refined carrageenan (which has been used for years as a binder, stabilizer, gelling agent), but at substantial cost savings to the pet food producers. However, the utilization of this kind of flour was limited in canned pet food only because the bacteria loadings in the flour were quite high as a result of sun or open bin drying was employed in the process. The finished products (canned pet food) are rendered sterile by retorting at high temperature.

By the early 1980s the quality of the alkali modified flour had been improved on color, odor, taste and bacterial reduction in which it can be incorporated as a food ingredients for human consumption. Since then, the names Philipine Natural Grade (PNG) carrageenan, and semi-refined carrageenan (SRC) were adopted by the producers to distinguish the product for human consumption from the alkali modified flour. The product must be officially labeled ‘carrageenan’ in some countries (e.g. the US and Canada) and ‘PES’ or ‘E407a’ in others (e.g. European Union and Japan).

The PNG process parallels the refined carrageenan process, except for the extraction step. In making refined carrageenan, the polysaccharide is extracted or dissolved from the seaweed to yield a carrageenan solution at a concentration of about two weight percent. PNG uses a reverse extraction process where impurities are extracted from the seaweed, and the polysaccharide is left in a gel state containing about 45 weight percent carrageenan and algal cellulose. PNG differs from refined carrageenan primarily in its relatively high algal cellulose content and lower salt content. Electron microscopy and atomic force microscopy have shed light on the organization of the cellulose and carrageenan in PNG. Electron microscopy has shown that the cell wall organization in untreated seaweed, where the carrageenan is known to reside, is at least partially carried over into the PNG product. It was found that this organization in PNG superior. Once the carrageenan in PNG has dissolved (heated to at least 85°C in an aqueous medium), and it then gels on cooling, the cellulose micro fibres play essentially no role in the tensile properties of the gel. The only property of interest to food processors that is imparted by the cellulose is to make the gels cloudy or turbid. Cellulose does appear to elevate slightly the viscosity of hot PNG solutions due to the relatively high excluded volume of the insoluble cellulose micro fibres.