Valorisation of fish waste biomass through recovery of nutritional lipids and biogas
Sammanfattning: Commercial fish catch in Eastern Africa is dominated by Nile Perch. Of the fish that is processed for human consumption, 30-40% is wasted. Currently, these wastes are not fully utilized; they are sold off at low price, converted to low valued products or left to decompose leading to environmental pollution and wastage of bioresource. This biomass has however a potential to generate considerable revenue and can be turned into a commercially viable business. It can be used in production of fish oils, bio-energy, proteins and organic fertilizers. Fish oils are a source of n-3 polyunsaturated fatty acids (PUFAs), in particular eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) associated with positive effect on human health. In this study, proteases were used to liberate oil from Nile perch (Lates niloticus) and salmon (Salmon salar) by-products. An oil yield of 11.2% and 15.7% of wet weight was obtained from warm water Nile perch and cold water salmon heads respectively, compared to 13.8% and 17.6% respectively, using solvents. Addition of water during the enzymatic hydrolysis decreased the oil yield. The DHA and EPA contents of oil extracted from Nile perch were 9 and 3 mol%, respectively. To further enrich DHA and EPA contents in Nile perch oil, use of lipases from Candida rugosa, Thermomyces lanuginosus and Pseudomanas cepacia were investigated. In the first case, the lipases were used to hydrolyse the natural oil. Non-regiospecific lipase from C. rugosa gave the best combined enrichment of EPA and DHA with EPA and DHA being enriched to 6 and 23 mol%, respectively. On the contrary, lipase from T. lanuginosus enriched DHA to 38 mol% but was ineffective in enriching EPA. Being a 1,3-specific lipase, the level of enrichment attained with T. lanuginous lipase was to a large extent influenced by the positional distribution of fatty acids within the triglyceride molecule. EPA was mainly in sn 1,3 positions while DHA was equally distributed in the 3 positions. To avoid complications associated with non-homogenous distribution of PUFAs in triglyceride molecules, free fatty acids (FFA) or fatty acid ethyl esters (FA-EE) derived from the natural oil were used as substrates in another study. In this case, lipase from T. lanuginosus was able to enrich both DHA and EPA. Evaluated lipases showed lowest specificity to EPA and DHA when present as ethyl esters and better recoveries of EPA and DHA were achieved when they were present as ethyl esters than when present as FFA or in glycerides. Both esterification of FFA and hydrolysis of FA-EE were more effective at enriching PUFAs than hydrolysis of the natural oil. In an attempt to add more value to fish oil, PUFA concentrate obtained from salmon heads by urea fractionation was used to lipophilize hydrophilic phenolic derivatives (vanillyl alcohol or rutin) which are natural antioxidants. Lipase from Candida antarctica was used to catalyse the esterification reaction. The synthesized lipophilic derivatives showed antioxidant activities with rutin esters showing more activity in the 2, 2-Diphenyl-1-Picrylhydrazyl (DPPH) radical assay than the vanillyl esters and on the contrary in the lipophilic medium, the vanillyl esters were found to be superior to the rutin esters. In bulk oil system, the antioxidant activities of rutin and vanillyl derivatives was lower than that of BHT and ?-tocopherol but in emulsion, they showed better activity than ?-tocopherol. The PUFA-phenolic molecules carry combined health beneficial properties associated with PUFAs and phenolics. In addition, the PUFAs are protected against oxidation by the phenolic moiety while PUFA makes the antioxidant more lipophilic which may enhance its function in lipid systems. To make maximum utilisation of the fish by-products, the insoluble fraction that remained after oil extraction was used for biogas production through anaerobic digestion. Methane yields before and after oil extractions were 828 and 742 m3 CH4/ton of volatile solids (VS) added, respectively. Despite the high methane yields, fish sludge/fish waste cannot be digested alone in a continuous anaerobic digester due to high content of proteins, lipids and light metals (sodium, potassium and calcium) that are inhibitory to methanogenesis. Co-digestion of the sludge with residues from crop cultivation was thus evaluated. Methane yields were 531 and 403 m3 CH4/ton of VS added when the ratio of Jerusalem artichoke residues: sludge was 1:1 or 3:1, respectively while that of JA alone was 283 m3 CH4/ton of VS. In conclusion, enzyme technology represents valuable tools that can be used in fish processing industries to convert fish waste into products with a higher market value. The use of proteases for the hydrolysis of the by-products results in maximum utilisation of the by-products since the intermediate hydrolysis products can be processed further for valorisation. Lipid fraction can be used for recovery of omega-3 fatty acids and biodiesel. The soluble protein fraction has several applications e.g. in food industries or in microbiological media and the sludge fraction can be used in anaerobic digestion for biogas production. Due to its high protein content, sludge fraction can also be used as animal feed or as biofertilizer due to high content of plant nutrients such as nitrogen, phosphorous and potassium.
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