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Seaweed

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Seaweed is a generic term for a range of coloured algae which grow in shallow seas and are classified primarily by their colour - brown, red and green. These have been harvested, and sometimes grown agriculturally, for thousands of years for a range of different purposes. One of the most well used species of seaweed is Kelp.

Use as a fertiliser[edit]

In some coastal communities, seaweed has historically been an important crop and source of income. In Scotland[1], the alkaline, nutrient rich seaweed contrasted with the acidic soils, so small farmers (known locally as crofters) would spread seaweed directly onto their land and see a dramatic yield improvement.

In the 17 century, the seaweed industry in the North of Scotland became a large growth industry as it was such an important source of potassium but was decimated a century later with the discovery of cheaper sources of potassium.

Today gardeners can buy a range of dried and extracts of seaweed which are acknowledged to be beneficial for plant growth[2] and is allowable for use in organic agriculture by some standards [3]

Use as food[edit]

Some cultural groups have diets which include seaweed. These include the eating of Laver in Wales, Kelp and other species in Scotland[4] and Scandinavian countries. These tend to be historical parts of the diet, however in Asia seaweed is still a major part of many meals. Japanese cuisine, in particular, is well known to contain many different kinds of seaweed to the extent that some scientists suggest there are unique enzymes produced by gut microbes in Japanese people that assist with seaweed digestion[5]. Some have suggested that this part of the diet contributes to the longevity of life expectancy in Japanese people. This seems to be largely assertion rather than proven fact, however it is known that the seaweed provides essential nutrients that some people do not get elsewhere in their diet.

Use for alginate production[edit]

Alginates are chemicals that have applications in printing, to thicken food and in medical/pharmaceutical applications. They can be extracted in particular from brown species of seaweed.

The FAO guide[6].suggests two extraction methods for alginates from seaweed:

The first is to add acid, which causes alginic acid to form; this does not dissolve in water and the solid alginic acid is separated from the water. The alginic acid separates as a soft gel and some of the water must be removed from this. After this has been done, alcohol is added to the alginic acid, followed by sodium carbonate which converts the alginic acid into sodium alginate. The sodium alginate does not dissolve in the mixture of alcohol and water, so it can be separated from the mixture, dried and milled to an appropriate particle size that depends on its particular application.

The second way of recovering the sodium alginate from the initial extraction solution is to add a calcium salt. This causes calcium alginate to form with a fibrous texture; it does not dissolve in water and can be separated from it. The separated calcium alginate is suspended in water and acid is added to convert it into alginic acid. This fibrous alginic acid is easily separated, placed in a planetary type mixer with alcohol, and sodium carbonate is gradually added to the paste until all the alginic acid is converted to sodium alginate. The paste of sodium alginate is sometimes extruded into pellets that are then dried and milled.

Alginates have also been recently investigated as a low impact source of biofuel[7]

Use for agar production[edit]

Agars are gels extracted from types of red seaweed (primarily Gelidium spp and Gracilaria spp) and are used in food (either strips or powdered) and as a growth substance for nursery plants, but it is perhaps best known as a growth substrate in microbiological laboratories.[8]

At the most basic level, agars are produced by boiling specific species of seaweed in water until the it disintegrates into a gel, and then removing unwanted impurities. The water is then removed and the agar is dried and milled, ready to be reconstituted as a gel.

In food contexts, agars can be used as alternatives to animal derived gelatines and are very popular in Japan - where they are called kanten.

Use for carrageenan production[edit]

Carrageenans are gels extracted from a small number of red seaweed species, in particular Chondrus crispus and Mastocarpus stellatus and is mostly used in the dairy industry, where it is added in small amounts to prevent separation and improve texture. It is also used in toothpastes, in petfoods air freshener gels and as a preservative of meats[9].

There is some controversy regarding the use of carrageenan, particularly in organic foods, where it has been linked to gastrointestinal disease

  1. http://www.hebrideanseaweed.co.uk/history.html
  2. Royal Horticultural Society factsheet
  3. Soil Association (2009)
  4. BBC: Should we eat more seaweed
  5. Science mag news article "Japanese guts are made for sushi' from 2010
  6. FAO 2003 A guide to the Seaweed Industry Chapter 5
  7. Berkeley Energy Review 2012 - unlikely source of biofuel
  8. FAO 2003 A guide to the Seaweed Industry Chapter 3
  9. FAO 2003 A guide to the Seaweed Industry Chapter 7