Recent years has seen industry hunt to find new, alternate and sustainable fish feed ingredients, primarily in anticipation of an increasing world population and predicted insufficient supply of fishmeal – a critical raw material component of aquaculture feed.
A huge amount of effort in this direction has focused on microalgae.
For use in aquaculture, microalgae needs to be non-toxic and possess the essential nutrients, wrote Hamed Safafar in his PhD study on the subjected, conducted at the National Food Institute at DTU.
Commercial production of microalgae through photosynthesis, though, is costly, so Safafar said he set about finding a method to make it an affordable ingredient within fish feed.
As part of his PhD study, he evaluated different microalgae species, eventually identifying two types that he said have a high content of protein and/or omega 3 fatty acids as well as antioxidants, and are suitable for growing under Danish conditions.
The nutritional composition of microalgae depends on the species, environmental conditions and growth medium composition, he added.
Overview
The two microalgae species identified, Nannochloropsis salina and Chlorella pyrenoidosa, are grown in industrial process water that is a low-price growth medium rich in lots of nutrients while being free from toxic compounds, he said.
Safafar also developed methods to harvest, dry and store the algae biomass that ensure that valuable substances in the biomass deteriorate as little as possible.
The drying process he established is said to be environmentally friendly in that it uses around 30% less energy than spray drying techniques currently in use. In the new process, the microalgae are dried in a stream of air in a specially designed drying chamber.
That drying process is said to be far gentler on pigments and omega-3 fatty acids in the biomass than spray drying.
He expects to be able to modify the drying method so it can be used on other algae species and, eventually, also on other types of biomass. The National Food Institute said it has a patent pending for the technology.
Even minor changes in growth conditions can lead to the growth of unwanted organisms in the algal biomass, rendering it unusable for fish feed, so Safafar said it is necessary to select species that are less sensitive but also to have systems in place to quickly identify contaminating organisms in the biomass during the cultivation process.
Thus, he also developed a method to trace such organisms in the biomass easier and quicker than existing methods by analyzing the composition of fatty acids and pigments.
The species Chlorella pyrenoidosa contains pigments such as lutein and other carotenoids, which can be used as natural colors in food products – he said the extraction and sale of pigment as a separate product would make the production of microalgae for fish feed more profitable.
Moreover, storage of the algae biomass at optimum conditions minimize the deterioration of valuable compounds, he added.
Charlotte Jacobsen, professor at the National Food Institute and Safafar's PhD supervisor, told FeedNavigator the research project also involved input from other academic researchers at DTU, the IFAU, and industry players like Ecolipids, BioMar, and LiqTech.
“LiqTech contributed studies on the applicability of their microfiltration technology for the harvesting of the biomass. BioMar contributed through the production of fish feed containing algae biomass,” she said.
In terms of whether the protein levels in the microalgae identified compared to those of fishmeal, Jacobsen said: “The best microalgae contained around 85% of the protein level found in fishmeal. It is not possible to make a direct one for one replacement, thougth, at this point in time because the algae contains more carbohydrate than fishmeal and the protein is not as digestible. We are looking into this aspect.”
Study in-depth
A pre-gasified industrial process water with high concentration of ammonia and free from toxic compounds, representing effluent from a local biogas plant was used as a low price growth medium in the project, according to the PhD study.
Safafar said screening of various microalgae species was carried out to find microalgae capable of growing on industrial process water and producing a biomass containing high levels of protein, long-chain polyunsaturated fatty acids (LC PUFA), and bioactive compounds such as natural antioxidants.
He evaluated the effects of growth media composition, concentration and cultivation time on the nutritional composition of the biomass, variations in proteins, lipid, fatty acid composition, amino acids, tocopherols, and pigments.
Species studied
The studied species included Nannochloropsis salina, Nannochloropsis limnetica, Chlorella sorokiniana, Chlorella vulgaris, Chlorella pyrenoidosa, Desmodesmus sp. and Arthrospira platensis.
“The microalgae Chlorella pyrenoidosa grew well on the industrial process water, efficiently valorized the compounds in the growth medium (ammonia and phosphorous) and produced reasonable amounts of the biomass (6.1 g/L).
“The resulting biomass included very high levels of protein (65.2±1.30% DW) as well as promising amino acid and carotenoid compositions. Chlorella pyrenoidosa was selected as a source of proteins and amino acids while lacking LC PUFAs.”
The microalgae Nannochloropsis salina which was grown on a mixture of standard growth medium and industrial process water produced a biomass containing high eicosapentaenoic acid (C20:5 n-3, as 44.2% ± 2.30% of total fatty acids), representing a rich source of LC PUFA, said the researcher.
He said data from laboratory scale experiments were translated to large scale and both species were successfully cultivated in flat panel photobioreactor systems.
Chromatographic methods were developed and employed for characterizing algal biomass at both pre- and post-harvest stages and were based on the analysis of fatty acids (gas-liquid chromatography) and pigments (high-performance liquid chromatography).
“These methods represented rapid, routine and reliable control measures to verify the variations in the purity of the biomass the microalgae biomass during cultivation, and its quality during the processing and storage,” said the author.
Energy consumption
He said he established a new downstream process, which included crossflow microfiltration by SiC ceramic membranes, heat treatment for inactivation of enzymes, up concentration by bowl centrifuge and, finally, drying by a swirl flash dryer.
The process aimed at reducing the energy consumption and minimizing deterioration of value-added bioactive compounds such a carotenoids, and LC PUFA.
“Energy consumption per kg of the product was evaluated as 2.2 KWh, which was estimated as 28% lower than known current processing technologies which are being applied to microalgae.”
The swirl flash dryer was specifically designed to handle microalgae paste like feeds, said the author.
As the final part of the study, he investigated impact of the storage time (0-56 days), storage temperature (5 degrees C, 20 degrees and 40 degrees ) and the packaging conditions, under vacuum or ambient pressure, on a high LC PUFA biomass from Nannochloropsis salina.
“The storage time and temperature strongly influenced the oxidation reactions, which resulted in deterioration of bioactive compounds such as carotenoids, tocopherols and LC PUFA.”
The study, he said, showed the oxidation reactions, which led to the creation of primary and secondary products, occurred mainly during the first days of storage.
“These findings reveal new opportunities and open new doors toward research, processing and quality control in the microalgae industry,” concluded the author.