SOURCES AND USES OF COMMERCIAL ALGAE
Very likely the use of Arthrospira as food in Chad dates back to the same period, or even earlier, to the Kanem Empire (9th century AD), indicating that two very different and very distant populations discovered independently the food properties of Arthrospira. Human consumption of this cyanobacterium in Chad was reported for the first time in 1940 by the French phycologist Dangeard in the little known Journal of the Linnean Society of Bordeaux. He wrote about an unusual food called dihe´ and eaten by the Kanembu of Chad, and concluded that it was a pure´e of the filamentous cyanobacterium Arthrospira platensis. However, at that time, his report failed to capture the attention it deserved because of the war. In 1966 the botanist Leonard, member of the 1964–1965 Belgian Trans-Saharan Expedition, confirmed the observations of Dangeard. He reported finding a greenish, edible substance being sold as dry cakes in the market of Fort-Lamy (today N’Djamena, the capital of Chad). His investigation revealed that these greenish cakes, called dihe´, were a common component of the diet of the Kanembu populations of Chad and Niger, and that they were almost entirely composed of Arthrospira, blooming naturally in the salinesoda lakes of the region. Like tecuitlatl, dihe´ was commonly eaten as a thick, pungent sauce made of chilli peppers, and spices, poured over millet, the staple of the region. In 1976, Delpeuch and his collaborators of ORSTOM (Office de la Recherche Scientifique et Technique Outre-mer, Paris, France) carried out a study on the nutritional and economic importance of dihe´ for the populations of the Prefectures of Kanem and Lac in Chad. The use of Arthrospira by Kanembu was mentioned again in 1991 in a canadian survey of food consumption and nutritional adequacy in wadi zones of Chad, which suggested that dihe´ makes a substantial contribution to vitamin A intake.
Arthrospira is still harvested and consumed by the Kanembu who live around Lake Kossorom, a soda lake at the irregular northeast fringe of Lake Chad, in the Prefecture of Lac. Arthrospira is harvested from Lake Kossorom throughout the year, with a minimum yield in December and January, and a maximum in the period between June and September during the rainy season. The bloom is present as a thick blue-green mat floating onto the surface of the lake only few hours a day, early in the morning. When the sun is high, the temperature of the water rises, and the bloom disperses, therefore the harvesting begins at sunrise and it is over in about 2 h.
The algal suspension is then carefully poured into the holes, and the surface of the biomass is smoothed with the palm of the hand. Within a few minutes, almost all of the extracellular water will have seeped out of the biomass, and the dihe´ is then cut into 8–10 cm squares, 1–1.5 cm thick, and removed from the holes as soon as it is firm enough for the squares to be handled without breaking (Figure 7.2). The drying is completed on mats in the sun. Dihe´ is traded in the local markets, and in the markets of the main towns of Chad, from where it can also be taken across the borders of Chad to Nigeria, Cameroon, and other countries. It is estimated that about 40 tons of dihe´ are harvested from this lake Kossorom every year, corresponding to a local trading value of more than US$100,000, which represents an important contribution to the economy of one of the poorest nations in the world.
Dihe´ is mainly used to prepare la souce, a kind of vegetable broth served with corn, millet, or sorghum meal, which occasionally can have fish or meat as additional ingredients. Well-dried dihe´ is crumbled in a bowl either by hand or with a mortar and pestle; cold water is then added to disperse the lumps, and the suspension is strained through a fine sieve to remove such solid impurities as sticks, grass, and leaves. The suspension is poured away from most of the sand that settles to the bottom of the bowl. The cleaned dihe´ is cooked for 1–1.5 h, which further disperses the lumps, yielding a blue-green broth that still contains small amounts of plant debris and sand. This broth is transferred into a bowl, left to settle for 5–10 min to allow sedimentation of any residual sand, strained very carefully once more, and then poured over saute´ed onions. Salt, chili peppers, bouillon cubes, and gombo (Hibiscus esculentus) are added, and la souce is then simmered and occasionally stirred until cooked.
A minor utilization of dihe´ is as remedy applied onto wounds to speed up the healing process, or as a poultice to soothe the pain and reduce the swelling of mumps.
Arthrospira biomass has a high content of protein, about 55–60% of the dry matter, with respect to other foods such as milk, eggs, etc.; the proteins are low in lysine and sulfured aminoacids such as methionine and cystein, but their amount is much higher than in other vegetables, including legumina. Phycobiliproteins represent a major portion of proteins, and among them phycocyanin can reach 7–13% of the dry matter; carbohydrates reach 10–20% of the dry weight, and consist mainly of reserve products, while lipids account for 9–14% of the dry weight. The mineral fraction represents 6–9% of the dry biomass, rich in K, P, Na, Ca, Mn, and Fe. Group A, B, and C vitamins are also present, with an average β-carotene content of 1.5 mg g-1 of Arthrospira, corresponding to 0.25 µg of vitamin A. Considering a recommended dietary allowance (RDA) of vitamin A of about 800 µg, and taking into account a natural 20–30% decrease in β-carotene level due to dihe´ storing conditions, we can say that a daily consumption of 5 g of dihe´ would provide about 100% of RDA.
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