Lichens as food

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Dr. Hansteen, who was the chief lecturer in the Agricultural School at Aas, Norway in 1911, prophesized that lichen was to become the great popular food of the masses, because of its cheapness and nutritive properties (Swartz 1911). This didn't happen, but lichens have frequently been used as food by people. They have often been used as famine food, but there are also many peoples who have used lichens for food on a more regular basis. Lichens are sometimes even been used as a delicacy (like Umbilicaria esculenta in Japan) or a dessert (like Cetraria islandica in Scandinavia).

There are two problems that people have generally encountered when eating lichens. The first problem is the secondary lichen compound often found in lichens. Most lichens contain a variety of secondary compounds. These compounds are generally unique to lichens and because of this are referred to as 'lichen compounds'. Lichen compounds are usually acids and thus have an acrid flavor. Only two lichen compounds have been found to be poisonous, vulpinic acid and pinastric acid, and these compounds would have to be ingested in significant amounts to be fatal for humans. But many other lichen compounds are herbivore deterrents, and can be very bad tasting, a digestive irritant, and would could probably even be toxic if eaten in large quantities for extended periods of time.

The second problem with eating lichens is that the complex carbohydrates in lichens are not easily broken down in the human digestive tract. Lichens contain a variety of polysaccharides. They usually contain lichenin (soluble in hot water) and/or isolichenin (soluble in cold water, turns iodine purple), and can often also contain other lichen polysaccharides such as evernin and usnin (Swartz 1911). Lichens can also often contain small quantities of polysaccharides often found in other plants, such as cellulose and inulin (Perez-Llano 1944). Lichen carbohydrates were fairly well studied over a century ago, after Külz suggested in 1874 that they could be eaten as substitute carbohydrates by diabetics (Swartz 1911). They did not discover a cure for diabetes, but they did discover that these lichen polysaccharides were not digestible by humans, dogs, or rabbits (Swartz, 1911). However, if lichenin and isolichenin are hydrolyzed, they yield glucose and other readily digestible simple sugars.

People have traditionally used various preparation methods to make lichens edible by removing the lichen secondary compounds and hydrolyzing the lichen polysaccharides. Table 1 summarizes these various techniques. The most frequently used preparation technique is boiling or steaming. This has been used by groups of people from North America, Europe, and India. Boiling would help to hydrolyze the lichen polysaccharides into digestible forms. It would also help to remove many lichen compounds. It is often recorded that people would boil the lichen and discard the water, which indicates that the boiling water was being used to remove the lichen compounds.

The lichen was also often soaked or rinsed with water. This could have removed some lichen compounds as well, but they are generally not very soluble in pure water. Both the Iroquois and northern Europeans are recorded to have soaked the lichens in ash water. Wood ash is alkaline, and so it would have been a lot more effective in removing the acidic lichen compounds. Alkali could also help to hydrolyze lichen polysaccharides.

The addition of dilute acid, or acidic things like onion, is common when cooking lichen. Acids could possible have helped to hydrolyze lichen polysaccharides, or they might make some lichen compounds more water soluble.

The value of lichens as a food stuff is probably usually just as a source of carbohydrates. The nutrient composition of lichens varies widely between different species of lichens but they are generally high in carbohydrates and low in most other nutrients.

Lichens may also provide some other nutrients. Lal and Ranganatha Rao (1956) found calcium and iron levels to higher in lichens than cereals and more comparable to green leafy materials. The calcium to phosphorus ratio they found was from 2 to 14, showed that lichens could serve as a good source of calcium. Peltigera canina has been found to be relatively high in protein and essential amino acids. Various studies have shown lichens to contain some vitamins, but results have not been consistent.

Table 2 summarizes the findings of different studies on the nutritional value of lichens. The various findings have not been consistent. This variation probably partly arises from variation in nutrient composition between and within species. Some of the variation is also likely experimental error as some of the studies are quite old.

Lichens can also accumulate toxins from their environment. Cetraria islandica and Cladina spp. have been found to contain particularly high levels of lead, cadmium, and mercury. Parmelia saxatilis and Xanthoria parietina have been found to absorb enough beryllium from their environment to harmful to animals (Perez-Llano 1944). In some areas Parmelia molliuscula can contain toxic levels of selenium salt (Perez-Llano 1944). And the natural radionuclides Po-210 and Pb-210 both accumulate in lichens, as well as Cs-137 and Sr-90 from nuclear test explosions (Airaksinen et al. 1986).