252 Organic waste recycling: technology and management
5.4.1 Algae as food and feed
Protein content of algae is about 50%, which is higher than soya bean, while
other vitamins and minerals are present in desirable proportions (Tables 5.10
and 5.11 show the chemical compositions of various algal species and soya
bean). However Waslien (1975) reported that there is an imbalance in the amino
acid composition of the algal protein, notably in the absence of sulfur-containing
amino acids, methionine and cystine, making the nutritional value of algae to be
considerably less than conventional foods such as eggs and milk. Another
problem with the algae is their high nucleic acid content which is about 4
percent or more (see Table 5.11, the values of RNA + DNA). These nucleic
acids, when being ingested in large quantity, can result in unacceptably high
level of uric acid (an illness) in human blood. In addition, except the Spirulina
sp., most of the waste-grown algae have thick cell walls, which are not well
digestible by human and non-ruminant animals (such as poultry). Therefore
these cell walls have to be ruptured such as by heat or acid treatment or some
mechanical means such as ball milling and crushing.
Table 5.10 Chemical composition of different algae compared with soya (% dry matter)
(Becker 1981)
Component Scenedesmus Spirulina Chlorella Soya
Crude protein 50-55 55-65 40-55 35-40
Lipids 8-12 2-6 10-15 15-20
Carbohydrates 10-15 10-15 10-15 20-35
Crude fibre 5-12 1-4 5-10 3-5
Ash 8-12 5-12 5-10 4-5
Moisture 5-10 5-10 5-10 7-10Table 5.11 Chemical composition of algae (g/100 g dry matter) (Becker 1981)
Component Spirulina Scenedesmus
Total nitrogen 11.0 8.3
Non-protein nitrogen 1.5 1.05
Protein-nitrogen 9.5 7.25
Available lysine (g/100 g
protein)2.96 3.66Ribonucleic acid (RNA) 2.90 4.4
Deoxy-ribonucleic acid (DNA) 1.00 1.6
Calcium 0.75 0.85
Phosphorus 1.42 1.9The thin-walled algae, such as Spirulina, thus appear to be a suitable algae
species that should be cultured for use as human food supplement. However, as