Freshwater and marine microalgae are in many respects interesting new raw material sources. In comparison to conventional energy crops, microalgae produce about five to ten times more biomass per unit of time, and algal biomass can also be harvested continuously throughout the year. In addition to sunlight and CO₂, algae only require nutrients such as phosphorus and nitrogen for growth. These nutrients can easily be recovered from wastewater streams. In contrast to land plants, microalgae consume low quantities of water and do not have to be grown on agricultural land. Scientists from the fields of process engineering, food technology, agriculture, animal nutrition and nutritional medicine are studying the potential of using this renewable and environmentally friendly nutrient and feed resource.

Depending on the species and cultivation conditions used, microalgae produce proteins, fatty acids or carbohydrates (such as starch). Vitamins, carotenoids (which can be used as dyes and antioxidants), phytosterols (which can be used as cholesterol-lowering agents) and fatty acids such as omega-3 fatty acids are of particular interest to the pharmaceutical, food and feed industries. Omega-3 fatty acids are essential for humans and have to be ingested with food; omega-3 deficiency is associated with an increased risk of cardiac infarction and stroke. In addition, omega-3 fatty acid products are used as anti-inflammatory agents for medically treating rheumatoid arthritis and multiple sclerosis.

The objective of the research network “Microalgae – integrated use for food and feed” in the Baden-Württemberg Bioeconomy Research Programme, is to use the various nutrient fractions as completely as possible in interconnected and cascaded use to develop sustainable processes for the bioeconomy. The project is being carried out at the Institute of Interfacial Process Engineering and Plasma Technology IGVP at the University of Stuttgart, with which the Fraunhofer IGB’s is closely connected through various teaching activities as well as joint operations, and is funded by the Baden-Württemberg Stiftung and the Baden-Württemberg Ministry of Science, Research and the Arts.

"The composition of algal biomass is very diverse and contains substances that are different from those of land plants, consequently it is not easy to adapt current extraction methods to algae. New extraction methods had to be developed. It is important to process algal biomass selectively in order to efficiently extract a broad range of high-quality algal ingredients. We are using a strategy based on cascaded processing of the algae,” explains Dr.-Ing. Ursula Schließmann, a process engineer who has been head of the Department of Environmental Biotechnology and Bioprocess Engineering (UTB) at the Fraunhofer IGB since 2011.

"The desired end products determine the processing technique used. This includes the selective extraction of different ingredient classes with defined degrees of purity, yields, etc. from the algal biomass produced.”

Microalgae are unicellular microorganisms that occur in nature in well over a million species that differ in size, cell wall structure and biomass composition. The IGB researchers have managed to find the needle in the haystack, i.e. three microalgal strains that are characterised by fast growth, simple cultivation conditions and high biomass production per time unit. The team uses mechanical and enzymatic methods to break up the cells gently in order to retain the functionality of the ingredients extracted.

The liquid algal suspension needs to be separated into its solid and liquid components and subsequently dried so that valuable lipophilic ingredients such as fatty acids and carotenoids can be accessed. "The drying step consumes a great deal of energy," says Dr. Schmid-Staiger, explaining the drawbacks of methods involving high temperatures. Dr. Schmid-Staiger relies on the extraction of wet biomass using supercritical fluids to achieve a positive energy balance. This involves extracting the desired ingredients from the biomass using so-called supercritical fluids such as CO₂ at moderate temperatures and under high pressure. Gases such as CO₂ become liquid above a certain pressure and behave like solvents. When the pressure is reduced again, CO₂ returns into its gaseous form and is then easy to separate from the extract.

"The advantage of this extraction method is that both the recovered fraction and the remaining biomass contain no harmful solvents, in complete contrast to extraction methods involving organic solvents. The extracted product can be processed further without any additional cleaning steps and then marketed as a dietary supplement, for example," explains Schmid-Staiger. In collaboration with the University of Hohenheim (Nutritional Sciences) and the Max Rubner-Institut in Karlsruhe, the process parameters were optimised in a number of collaborative projects. The objective was to obtain fractions with stable functional properties.

In a further step, the researchers were able to extract carotenoids as well as triacylglycerides using solvents (such as ethanol). Triacylglycerides are currently of interest as platform chemicals and, above all, fuel - either by way of the oil itself or as biodiesel after the transesterification of the triacylglycerides.

It was possible to selectively extract carbohydrates under growth-limiting conditions in the absence of nitrogen and phosphate. Starch, for example, can be used as a substrate for ethanol production. A two-stage process was established at the IGB in order to achieve this. The process was subsequently transferred outdoors and further developed to pilot production on the kilogramme scale. Another fraction, i.e. soluble proteins, is separated from the residual biomass by filtration prior to the CO₂ or ethanol extraction step, so as to make the proteins available to University of Hohenheim project partners involved in food production. The remaining biomass residues (proteins, carotenoids and cell wall polymers) can be used for animal feed production at the Institute of Animal Nutrition, University of Hohenheim.

Can climate-damaging carbon dioxide be removed from the atmosphere? It is a nice idea in these times of climate change when carbon dioxide is known to have widespread and harmful effects. It is an issue that really got the minds of scientists at the Fraunhofer IGB going. The researchers are developing processes in which microalgae use the CO₂ from combustion and industrial processes for photosynthesis. They rely on a coupled process in which all cycles are closed: to start with, valuable substances are extracted and then the residual biomass is fermented to biogas. After generating electricity and heat from the biogas in a block heat and power station, CO₂, a by-product of the process, is returned to the cycle process and used to produce algal biomass. This is an important step towards a positive energy balance.

A prospective approach involves introducing CO₂ from the atmosphere or from combustion and industrial processes, and this is also being pursued by Schmid-Staiger’s team. "However, gas treatment is still relatively expensive; a major challenge is presented by the nitrogen oxides present in the gas." Cooperation and investment partners and project funding through the industry and the Baden-Württemberg Ministry of Science, Research and the Arts could give this promising approach the necessary impetus.

Another approach to sustainable biomass production is a process that uses biogas production wastewater. This contains inorganic nutrients such as phosphate and ammonium, which algae need for growth. The Fraunhofer IGB succeeded in controlling the composition of algal biomass and optimising the cultivation and treatment processes in technical systems, including the use of suitable process measuring and control technology devices.

"In the pilot projects we have been able to show the feasibility of the cascaded, targeted extraction of valuable materials. Possible follow-up projects could be used to further develop the processes with an eye on greater profitability," says Schließmann, expressing his hopes for the future. After all, the many advantages of cultivating algae and using them for producing basic materials with high value creation potential are still held back by high investment and operating costs. "Based on our extensive experience, we offer investors and interested parties from industry, finance and government the transfer of the findings into further multi-faceted projects at any time." The use and further development of sustainable processes from microalgae cultivation has the potential to reduce the pressure on currently available raw materials and thus make an important contribution to the bioeconomy.