Optimisation of protein and lipid extraction from low-value fish sources and investigation of high-value applications

The aim of this thesis was to investigate potential high value nutritional applications for protein and lipid raw material extracted from low-value pelagic marine fish, specifically blue whiting (Micromesistius poutassou), boarfish (Capros aper), and Atlantic herring (Clupea harengus), and to assess the effects of such products on the recipient’s gut microbiota. In Chapters 3 and 4 the proximate (biomass in terms of gross components, e.g. water, ash, protein and lipids) and gut microbial compositions of the three fish species were studied. Boarfish had the highest mineral (ash) content (6%), herring had the largest lipid fraction (11%) while protein level was similar in all three species (16 – 17%). Interestingly, the composition of the gut microbiome of the three fish were significantly different. The dominating phylum was Proteobacteria, followed by Tenericutes and Spirochaetes in all species, however, seven phyla were only recorded in one fish species and only two bacterial families (Vibrionaceae and Lactobacillaceae) were detected in the core microbiota of all three fish. Unique and novel viromes that separate distinctly in relation to fish species, and diverge from ocean and human viromes, were also found. Subsequent chapters in the thesis concentrate on blue whiting only. Chapter 5 and 6 focus on the protein fraction of the fish. Firstly, protein hydrolysates were produced using a range of enzymes. The resulting protein powders showed antioxidant properties and high solubility (> 80%) across a wide pH range in water. Moreover, their solubility improved further within a vitamin-tea beverage matrix (> 85%), proving them an ideal functional food for beverage fortification. In Chapter 6, two commercially-produced fish protein hydrolysate powders (1. powder of the soluble protein hydrolysate (SPH) fraction; 96% soluble protein, and, 2. powder of the partly-hydrolysed protein (PHP) fraction; 18% soluble protein) were used to supplement salmon aquafeeds high in plant protein. The growth performance of salmon fed the partly-hydrolysed protein hydrolysate powder equalled that of fish fed the positive control 35% fishmeal diet. Fish on diets supplemented with protein hydrolysates had significantly higher levels of many amino acids in their blood, including 27 – 48% more branched chain amino acids compared to fish fed the 35% fishmeal diet. Replacement of fishmeal with high levels of plant protein significantly altered the salmon gut microbiota. Alpha diversity was reduced and Spirochaetes as well as the families Moritellaceae, Psychromonadaceae, Helicobacteraceae and Bacteroidaceae were all found at significantly lower abundances compared to the control fishmeal diet. Supplementation with the SPH hydrolysate reduced the effects of high dietary plant protein on the gut microbiota. Finally, the last chapter of the thesis investigates the efficacy of fish oil dietary supplementation, on its own and in conjunction with the anti-depressant fluoxetine, as a treatment for depressive and anxiety-like symptoms associated with early life stress. The maternal separation model of early life stress was used in male Sprague-Dawley rats to test these treatments. Maternally separated rats showed depressive-like behaviours in the forced swim and open field tests. Depressive-like behaviour in the forced swim test was evaded by treatment with fluoxetine only. Anxiety-like behaviours in the open field test were not observed in any of the three treatment groups (fish oil, fluoxetine, fish oil + fluoxetine). Maternally-separated rats fed the fish oil, fluoxetine and fish oil + fluoxetine diets all had significantly lower plasma corticosterone levels and brain stem monoamine turnover was significantly changed compared to the untreated maternally separated rats. Fish oil significantly reduced caecal levels of butyrate while fluoxetine increased both acetate and propionate levels. Correlating changes were recorded in the SCFA-producing gut microbiota. Fish oil treatment significantly altered biological markers of stress and depression as well as the gut microbiota which translated to an evasion of anxiety-like behaviour. Taken together, the studies here within demonstrate that these three species of fish may provide a rich and valued source of ingredients for a variety of applications in animal, fish and human foodstuffs