NF Monographs: Carrageenan

Carrageenan

Carrageenan.
Carrageenan

[9000-07-1].

» Carrageenan is the hydrocolloid obtained by extraction with water or aqueous alkali, from some members of the class Rhodophyceae (red seaweeds). Carrageenan consists chiefly of potassium, sodium, calcium, magnesium, and ammonium sulfate esters of galactose and 3,6-anhydrogalactose copolymers. These hexoses are alternately linked

-1,3 and

-1,4 in the polymer. The prevalent copolymers in the hydrocolloid are designated kappa-, iota-, and lambda-carrageenan. Kappa-carrageenan is mostly the alternating polymer of D-galactose-4-sulfate and 3,6-anhydro-D-galactose. Iota-carrageenan is similar, except that the 3,6-anhydrogalactose is sulfated at carbon 2. Between kappa-carrageenan and iota-carrageenan there is a continuum of intermediate compositions differing in degree of sulfation at carbon 2. In lambda-carrageenan, the alternating monomeric units are mostly D-galactose-2-sulfate (1,3-linked) and D-galactose-2,6-disulfate (1,4-linked). The ester sulfate content for Carrageenan ranges from 18 percent to 40 percent. In addition, it contains inorganic salts that originate from the seaweed and from the process of recovery from the extract.

Carrageenan is recovered by alcohol precipitation, by drum drying, or by freezing. The alcohols used during recovery and purification are restricted to methanol, alcohol, and isopropyl alcohol. Carrageenan that is recovered by drum-roll drying may contain mono- and di-glycerides or up to 5 percent of polysorbate 80 used as roll-stripping agents.

Packaging and storage— Preserve in tight containers, preferably in a cool place.

Solubility in water— Not more than 30 mL of water is required to dissolve 1 g at a temperature of 80

Identification—

A: A solution (1 in 50) prepared by heating a uniform dispersion in a hot water bath to 80

(Solution A) becomes more viscous upon cooling and may form a gel.

B: To 10 mL of Solution A, while still hot, add 4 drops of potassium chloride solution (1 in 10), mix, and cool. A short-textured (“brittle”) gel indicates a carrageenan of a predominantly kappa type; a compliant (“elastic”) gel indicates a predominantly iota type. If the solution does not gel, the carrageenan is of a predominantly lambda type.

C: Dilute a portion of Solution A with about 4 parts of water, and add 2 to 3 drops of methylene blue TS: a blue, stringy precipitate is formed (also positive for furcellaran, a similar colloid).

D: Obtain IR absorption spectra on the gelling and non-gelling fractions of the specimen by the following procedure. Disperse 2 g in 200 mL of potassium chloride solution (1 in 40), and stir for 1 hour. Allow to stand for 18 hours, stir again for 1 hour, and transfer to a centrifuge tube. (If the transfer cannot be made because the dispersion is too viscous, dilute with up to 200 mL of the potassium chloride solution.) Centrifuge at approximately 1000 g for 15 minutes.

Remove the clear supernatant, resuspend the residue in 200 mL of potassium chloride solution (1 in 40), and centrifuge again. Coagulate the combined supernatants by adding 2 volumes of dilute alcohol (9 in 10). (Retain the sediment for use subsequently as directed.) Recover the coagulum, and wash with 250 mL of the dilute alcohol. Press the excess liquid from the coagulum, and dry it at 60

for 2 hours: the material so obtained is the nongelling fraction (lambda carrageenan).

Disperse the sediment retained from the foregoing procedure in 250 mL of cold water, heat at 90

for 10 minutes, and cool to 60

. Coagulate the mixture, then recover, wash, and dry the coagulum as described above: the material so obtained is the gelling fraction (kappa- and iota-carrageenan).

Prepare a solution (1 in 500) of each fraction, cast films 5 µm thick (when dry) on a suitable nonsticking surface, and obtain the IR absorption spectrum of each film. Carrageenan has strong, broad absorption bands, typical of all polysaccharides, in the 1000 to 1100 cm

1 region. Absorption maxima are 1065 cm

1 and 1020 cm

1 for gelling and nongelling types, respectively. Other characteristic absorption bands and their intensities relative to the absorbance at 1050 cm

1 are as shown in the accompanying table.

Wave Number cm1	Molecular Assignment	Absorbance Relative to 1050 cm1
		Kappa	Iota	Lambda
1220 to 1260	Ester sulfate	0.7 to 1.2	1.2 to 1.6	1.4 to 2.0
928 to 933	3,6-anhydrogalactose	0.3 to 0.6	0.2 to 0.4	0 to 0.2
840 to 850	Galactose-4-sulfate	0.3 to 0.5	0.2 to 0.4	—
825 to 830	Galactose-2-sulfate	—	—	0.2 to 0.4
810 to 820	Galactose-6-sulfate	—	—	0.1 to 0.3
800 to 805	3,6-anhydrogalactose-2-sulfate	0 to 0.2	0.2 to 0.4	—

Viscosity

911

— Transfer 7.5 g to a tared, tall-form, 600-mL beaker, add 450 mL of water, and disperse with agitation for about 15 minutes. Add water to bring the weight to 500 g, and heat in a water bath, with continuous agitation, until a temperature of 80

is reached. Add water to adjust for loss by evaporation, cool to between 76

and 77

, and place in a constant-temperature bath maintained at 75

. Provide a suitable rotational viscosimeter with a spindle 1.88 cm in diameter and 6.51 cm high, using an immersion depth of 8.10 cm (No. 1 spindle). Allow the spindle to rotate in the solution at 30 rpm for 6 revolutions, then observe the scale reading. Convert the scale reading to centipoises by multiplying by the constant for the spindle and speed employed. The viscosity at 75

is not less than 5 centipoises.

Microbial limits

— The total bacterial count does not exceed 200 cfu per g, and the tests for Salmonella species and Escherichia coli are negative.

Loss on drying

731

— Dry it at a pressure not exceeding 10 mm of mercury at 70

for 18 hours, cool in a desiccator, and weigh: it loses not more than 12.5% of its weight.

Acid-insoluble matter— Transfer about 2 g, accurately weighed, to a 250-mL beaker containing 150 mL of water and 1.5 mL of sulfuric acid. Cover with a watch glass, and heat on a steam bath for 6 hours, rubbing down the wall of the beaker frequently with a rubber-tipped stirring rod, and replacing any water lost by evaporation. Transfer about 500 mg of a suitable filter aid, accurately weighed, to the beaker, and filter through a tared filtering crucible provided with a 2.4-cm glass fiber filter. Wash the residue several times with hot water, dry at 105

for 3 hours, cool in a desiccator, and weigh. The difference between the total weight and the sum of the weights of the filter aid, crucible, and glass fiber filter is the weight of the acid-insoluble matter. It is not more than 2.0% of the weight of Carrageenan taken.

Total ash

561

: not more than 35.0%.

Arsenic

211

: 3 ppm.

Lead

251

: 0.001%.

Heavy metals, Method II

231

: 0.004%.