Carragenan
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ABSTRACT
Industrially important sulphated galactans, known as carrageenans are extracted from a number of red seaweeds. Theses polysaccharides have a linear structure of alternating 3-linked-¦Ð-D-galactopyranose (G-units) and 4-linked-¦Ð-D-galactopyranose units (D-units). For food industrial application purposes, carrageenan is classified into three types as kappa (¦Ðš-), iota (¦Ð™-) and lambda (¦Ð›-) carrageenan according to the number (one-two and three) of sulfate groups per repeat unit of disaccharide, respectively. Under proper conditions, ¦Ðš-carrageenan and ¦Ð™-carrageenan in aqueous solution undergo a thermoreversible sol-gel transition, while no gelation takes place in ¦Ð›-carrageenan having more electrolyte groups. In general terms, ¦Ðš-carrageenan gels are hard, strong and brittle, whereas ¦Ð™-carrageenan forms soft and weak gels. Carrageenans are used as thickeners, stabilizers and gelling agents. Therefore, they are widely used in food formulations, the dairy sector representing the largest part and, in meat applications.
INTRODUCTION
Carrageenans are a family of sulfated polysaccharides extracted from red algae. Despite their widespread application, especially in the food industry as thickening and stabilizing agents, the mechanism of gelation has been a matter of discussion over the past years and further investigation is still needed for a full comprehension of the process.
The objective of this paper is to introduce the reader to some generalities of carrageenans and the latest findings regarding their mechanism of gelation.
HISTORY
Carrageenan is the generic name for a family of gel-forming, viscosifying, sulfate polysaccharides that are obtained commercially by alkali extraction of certain species of red seaweeds (Rhodophyceae). The main species responsible for most of todays carrageenan production belong to the following genera: Gigartina (Argentina/Chile, France, Morocco), Chondrus (France, North Atlantic), Iridaea (Chile), and Eucheuma (Philippines/Indonesia) (Van de Velde et al, 2002).
For several hundred years, carrageenan has been used as a thickening and stabilizing agent in food, in Europe and the Far East. In Europe the use of carrageenan started more than 600 years ago in Ireland. In the village of Carraghen on the south Irish coast, flans were made by cooking the so-called Irish moss (red seaweed species Chondrus crispus) in milk. The name carrageenin, the old name for carrageenan, was first used in 1862 for the extract from C. crispus and was dedicated to this village. Commercial production began in the 1930s in the U.S. During that time, the trading shifted from dried seaweed meal to refined carrageenan. After the Second World War, a general increase in the standard of living forced an increase in carrageenan production. Fractionation of crude carrageenan extracts started in the early 1950s, resulting in the characterization of the different carrageenan types. A Greek prefix was introduced to identify the different carrageenans. In the same period, the molecular structure of carrageenans was determined. Today, the industrial manufacture of carrageenan is no longer limited to extraction from Irish moss, and numerous red seaweed species are used. Traditionally, these seaweeds have been harvested from naturally occurring populations. Seaweed farming to increase the production started almost 200 years ago in Japan. Today, nearly a dozen seaweeds are cultivated commercially, lowering the pressure on naturally occurring populations (Van de Velde et al, 2002). During the past few years, the total carrageenan market has shown a growth rate of 3% per year, reaching estimated worldwide sales of 310 million US$ in the year 2000. At the end of the 20th century, a few large corporations that account for over 80% of the supply dominate the carrageenan market, including: FMC Corporation (USA), CP Kelco (USA), Degussa (Germany), Danisco (Denmark), Ceamsa (Spain), and Quest International (The Netherlands) (Van De Velde & De Ruiter, 2002).
CHEMICAL STRUCTURE
Carrageenan is a high molecular mass material with a high degree of polydispersity. The molecular mass distribution varies from sample to sample, depending upon the sample history, e.g., age of the harvested seaweed, season of harvesting, way of extracting, and duration of heat treatment. Commercial food-grade carrageenans have an average molecular mass ranging from 400 to 600 kDa with a minimum of 100 kDa (Van De Velde & De Ruiter, 2002).
Carrageenan is a complex mixture of at least five distinct polymers designated by greek letters: kappa (¦Ðš-), iota (¦Ð™-), mu (¦Ðœ-), lambda (¦Ð›-) and nu (¦Ð-). In general, carrageenans are distinguished by the amount of the 3,6-anhydro group, sulfation and position of the sulfate group (Janaswamy & Chandrasekaran, 2002). For instance, ¦Ðš- carrageenans, ¦Ð™-carrageenans and ¦Ð›- carrageenans contain one, two and three sulfates group per unit of disaccharide, respectively (Yuguchi, Y. 2001).
Carrageenans are linear polymers of about 25,000 galactose derivatives with regular but imprecise structures, dependent on the source and extraction conditions (Falshaw et al., 2001). The general structure is shown in Fig. 1.
Figure 1. Carrageenan consists of alternating 3-linked-¦Ð-D-galactopyranose (G-units) and 4-linked-¦Ð-D-galactopyranose units (D-units).
Of all these sulfated polymers, ¦Ðš, ¦Ð™ and ¦Ð›-carrageenans are the most important in regard to food applications. Idealized structures are given below but it should be noted that native carrageenans often contain combinations of these idealized units, with variations in structure occurring not only between different species of seaweed but also within the different life-stages of a single species. The structural differences between various carrageenan types also affect their properties and, therefore their uses (Falshaw et al., 2001).
¦Ðš-carrageenan (kappa-carrageenan)
¦Ðš-carrageenan is produced by alkaline elimination from ¦Ðœ-carrageenan, isolated mostly from the tropical seaweed Kappaphycus alvarezii (also known as Eucheuma cottonii). It produces the strongest gel within the family; the gels are rigid and elastic in the presence of potassium,