Development of fermented fish silage derived from pelagic sardine and apple pomace by-product for inclusion in diets for the European seabass (Dicentrarchus labrax)
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The ensilage of fish and fisheries derived by-products as alternative ingredients to fish meal in diets for aquaculture may be an effective strategy to reduce feed and production costs of farmed marine fish. In this context, the production of fish silage by means of the fermentation of whole sardine (Sardina pilchardus) using Lactobacillus plantarum starter culture together with apple pomace co-product as the principle carbohydrate source was developed. Apple pomace is an important co-product from the cider industry in the UK and was ideally suited for this purpose. The minimum of total sugars necessary for a successful fermentation was found to be 5% (w/w) which was equivalent to a ratio of 0.15: 1 apple pomace to minced sardine. The optimum conditions for the fermentation process during ensilage of sardines in apple pomace and molasses were evaluated. Successful fermentation was obtained at 25 and 35°C with an inoculum size of more than 10² cfu gˉ¹ The effect of autolysis and storage on chemical composition of whole fish and fish silage was also investigated. Fish silage was compared before and after fermentation and acidification as well as in the fresh state and after 7, 14, 21 and 30 days of storage at different temperatures for the fermented fish silage product. Specific changes in the proximate composition (i.e. moisture, protein, lipid and ash) of raw fish after the ensiling process were obtained. Reduction in the bound protein amino acids and consequent elevation in "free" amino acids occurring during the ensilage process was also noticed. The assessment of fermented fish silage storage conditions showed minor effects on the proximate composition and amino acid profile of the finished silage material. Fish silage samples tested after storage at all temperatures gave a sufficiently low pH to maintain a successful fermentation condition. Storage time and temperature dependency effect showed an increase in non-protein nitrogen (NPN) values in both fermented fish silages. An elevation in total volatile bases (TVB-N) values occurred in apple pomace and molasses fish silage as storage time increased. Increases of Free Fatty Acid (FFA) content and associated high values of Thiobarbituric Acid Reactive Substances (TBARS) in stored apple pomace and molasses fish silages could indicate a problem with rancidity. A comprehensive evaluation of the nutritional effect of the partial replacement of fish meal by fish silage products in the diet of European sea bass (Dicentrarchus labrax) was conducted under laboratory conditions. Feeding sea bass with fish silage based diets generated similar or lower growth performance, feed utilization, protein efficiency and retention compared to the fish meal based diet. Extended investigations showed no evidence of any morphological, histopathological or haematological changes in seabass. Incorporation of a polychaete worm meal in the apple pomace fish silage diets appeared to increase the palatability of diets and improved the microvilli morphology of the posterior intestine of seabass. In separate experiments, the apparent digestibility coefficient of dry matter (ADCdry matter) of the experimental diets was relatively low. However the apparent digestibility coefficients for protein (ADCprotein) and energy (ADCenergy) were deemed acceptable for all silages tested. The overall findings of this study indicate that fish silage could be generated by biological fermentation using by-catch, over-catch and processing waste fish with carbohydrates obtained from industrial waste streams such as, apple pomace from cider production. This silage product was an effective replacement of expensive fish meal in aquaculture feed with potential for future applications in aquafeeds.
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