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An Elusive Green Dream?
BY LEONTIEN BRAAKMAN
Algae take light, water and carbon dioxide and turn them into healthy ingredients for food. Once scientists thought algae could feed the world. But the technology of mass production is not simple.
1 January 2005
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You don’t need to know a lot about cows to realise that their genetic code is different to that of horses.
Scientists became interested in algae as a possible food source in the 1960s. You could feed the whole world in this way, idealists thought, and they set to work. Hardly surprising, since algae are tiny plants that can grow quickly using few S resources. They contain important nutrients to boot. For instance, algae contain up to 70 per cent protein. They make healthy ingredients such as vitamins, minerals, antioxidants and unsaturated fatty acids atlittle cost. The fact that algae might be able to clean exhaust gases andwaste water – after all, they use ‘water pollutants’ and carbon dioxidefor their metabolism – and turn them into valuable raw materialsseemed promising. Micro-algae are unicellular plants. They grow injust 12 to 24 hours and are not particularly demanding. They need only need light, carbon dioxide, water, and a minimal quantity of nutrients such as phosphates, nitrates and sulphates.
The first open-air culture ponds were in Mexico, the United States (including Hawaii) and Australia. Many still exist. They are large shallow lakes, about 30cm deep, covering several hectares. They are not very efficient: the yield is only 0.5g to 1g of algae per litre of water. That’s due to poor penetration of light, sedimentation of algae and barely controllable climatic conditions.
The method is indiscriminate and only suitable for some algae: strains that can be cultivated selectively or thrive under competition from other algae and bacteria. But, because of the large volumes and the low ‘reactor’ costs, these flooded .elds are still the major source for commercial micro-algae and micro-algal products.
The ideal of ‘feeding the world’ was abandoned long ago. But the health-promoting substances present in algae make the organism highly interesting for commercial cultivation. And the market for algae and algal ingredients is growing steadily. Most of the algal biomass used as food supplements in western countries or as ingredients for cosmetics.
Algae produce some substances that are difficult to make by chemical means and therefore expensive, such as the red pigment astaxanthin, a carotenoid.
In the US, this is produced on a large scale by the algae, Haemotococcus pluvialis. Astaxanthin is also a strong antioxidant and is therefore believed to contribute to the prevention of cardiovascular diseases, skin diseases and age-related eye defects. Such valuable ingredients supplied by algae are the subject of intensive research. Essential fatty acids are another example. Some algae are rich in omega-3 fatty acids and long-chain poly-unsaturated fatty acids (PUFAs), such as EPA (eicosapentaenoic acid).
Types of algae
Microalgae are microscopic – unicellular – plants.
The autotrophic strains from this heterogeneous group of organisms use light as a source of energy
to convert carbon dioxide and water into biomass. They use chlorophyll for this assimilation. Mixotrophic strains can use both light and organic substances such as glucose as their energy source for growth. Heterotrophic strains do not need light to reproduce, but derive all their energy from the organic substances. Macroalgae are multi-cellular, such as the group of seaweeds.
Why algae?
Algae versus fish
Ingredient suppliers also have discovered algae as a useful source. For example, Nutrinova produces a highly concentrated, vegetarian form of DHA (docosahexaenoic acid). The company markets this omega-3 fatty acid under the Nutrinova DHA brand name. While DHA-oils are commonly extracted from fatty cold water fish, DHA is extracted from microalgae via a patented fermentation process. Nutrinova’s DHA is said to have many advantages over typical fish oil. It contains a much higher concentration of DHA and a lower amount of undesirable fatty acids.
Infants and embryos need these ingredients to build the brain and the retina. But they’re also interesting to adults, because of they are believed to help prevent cancer and cardiovascular diseases. Whole algae Spirulina and Chlorella are commercially available as powder or pills in organic food shops. Antioxidants, omega-3 fatty acids and vitamins (B and E) are produced as functional ingredients. The algae Dunaliella salina is also a recognised source of natural colorants (betacarotene).
At the moment, most of the market still consists of whole algae, produced in culture ponds outside Europe. For food supplements it is necessary to isolate the components from the organisms, extract them and process them into pure substances.
This is only possible using more advanced technologies, such as membrane .ltration, centrifuging, fiocculation or (chemical) extraction. That is why the cultivation methods should be advanced. Economic production needs high yields.
Open lakes have limitations for growing algae. So with rising demand, the focus shifted to .nding alternatives. A technologically more advanced method of making algae grow more ef.ciently was developed: the photobioreactor.
This reactor is a translucent, closed vessel in which the temperature and composition of the substrate—the medium in which the algae grow—are controlled, and the intensity of the incoming light is regulated accurately.
The first laboratory-scale photobioreactors were glass cylinders with water in which air enriched with carbon dioxide is injected from the bottom. Artificial light (from .uorescent or discharge lamps) produces the energy required for the photosynthesis so that the necessary organic substances can be produced.
The bubble column has a drawback. The light does not penetrate allthe way into the tube. The algae absorb and reflect it. The diameter of the column is often about 20 cm, but the illuminated zone is only 1 cm deep. Efficiency can be poor: just 5 to 10 per cent.Other systems involve tube or plate reactors. They can absorb a lot of light. You can scale up such reactors by adding more tubes and plates, or making the tubes and plates bigger.
A horizontal tube, for instance, can be made as long as 100 metres. But there’s a problem. Oxygen is produced by the process. It can only escape through the end of the tube. It accumulates. Once the oxygen content exceeds 42 per cent, it starts to hamper the growth of algae. Nevertheless, American and German firms use tube reactors for largescale cultivation of algae for cosmetics and organic food.
Plate reactors are different. They’re like very narrow aquariums. Two glass walls contain water and biomass. Carbon dioxide gas is injected at the bottom. Excess gas can escape at the top. The reactor is illuminated from the side.
The light has a shorter distance to travel than in circular reactors. At the same time, the illuminated area per unit of reactor volume is much larger. As a result, algae are exposed to light for longer. Shorter light paths mean algae are less likely to remain in each other’s shadow. Increased turbulence can also increase exposure.
Wageningen University and Research Centre (WUR) in the Netherlands has made a plate reactor with efficiency of 15 per cent in the Photosynthetic Active Region—visible light with wavelengths of 400-700 nm. Calculations suggest that in practice an efficiency of 20 to 25 per cent could be obtained. (That’s below the theoretical maximum of 33 per cent, as some energy is lost as heat.)
The ‘turnover time’ of algae governs the ef.cient use of light. The organisms need 10 to 100 milliseconds to convert the captured light into energy for their metabolism. We can optimise this by varying the distance between the plates, the light intensity and turbulence (due to gas injection) to match the algae’s metabolic cycle.
SMALL IS BEAUTIFUL
Small reactors have advantages. Algae can ‘capture’ more light and more often, and thus will thrive better. They’re less likely to block each other in shadow. Scaling up is proving problematic.
In the laboratory, research reactors use arti.cial light. But that needs a lot of energy and is expensive. Commercial production must use sunlight if algae production is to live up to its environmental promise. Wageningen University has developed a ‘Green Solar Collector’. It consists of a closed system of several interconnected plate reactors. Researchers used a strain of freshwater algae Monodus subterraneus, which is renowned for its sensitivity, to develop the reactor. Other strains should be easier to produce.
The university has applied for a patent, and plans a spin-off companyto commercialise the technology. It’s now analysing the market for the strains of algae and substances with the greatest demand. The Wageningen researchers believe they can produce algae much more cheaply than the current cost of around €20 (US$27) a kilogram. They believe there’s enough sunlight, even on cloudy European days. But not all algae need light. Autotrophic strains do. Heterotrophic algae use carbon instead, often in the form of glucose, for their metabolism. An American firm, Martek Biosciences has used genetic modi.cation to convert algae that need light to grow, to get their energy instead from sugar. It’s done in an enclosed fermentor, like yeast or bacteria. The company introduced a gene encoding a glucose transporter into the algae’s genome. This sunlight-to-sugar growth change is called ‘trophic conversion’. (Organisms that use light for energy are called ‘phototrophs’; organisms that grow in the dark using sugars are called ‘heterotrophs’.) But the strain of algae has become a genetically modi.ed organism (GMO) as a result: consumers may not like this and reject products made with it.
For now, the vast majority of commercially available algae and ingredients from algae still come from open culture ponds. A few closed systems are used for producing high-value substances. But with promising cheaper technologies for producing algae emerging from the laboratories, we’re likely to see more applications for this interesting ingredient.
As a scientist of the Fraunhofer Institute in Germany suggests that, for example, ‘perhaps in the future we will eat soft green algae bread as part of our daily diet.’ Products made from seaweed—a type of algae— are common in East Asian food. Algae producers must be hoping for the day when a sifter containing dried algae is standard equipment in every kitchen around the world.
OUR THANKS TO ROUKE BOSMA AT WAGENINGEN UNIVERSITY RESEARCH CENTRE FOR HELP IN PRODUCING THIS ARTICLE. AND THANKS ALSO TO OUR SISTER MAGAZINE, FOOD ENGINEERING & INGREDIENTS, IN WHICH IT FIRST APPEARED.
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