Evaluation of Fish Viscera Silage as a Feed Resource

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Abstract
The chemical composition of 5 different fish viscera silages [for animal feeding] was determined. The silages were produced at industrial-scale processing plants in the north of Norway. The method included preservation with .apprx. 0.75% formic acid and 0.75% propionic acid, autolysis, heating to 95.degree. C and removal of fat by centrifugation. Two of the silages were concentrated by vacuum drying. The method resulted in microbiologically acceptable and stable products. Propionic acid was to a certain extent lost during vacuum drying, but concentrated silages revealed good storage stability. Proximate analysis revealed considerable variation between silages, especially with regard to fat contents which varied between 3.1-15.2% of the dry matter. The residual fat fraction was of poor quality as judged by free fatty acids and peroxide values. Tryptophan was partly lost during processing and storage of the silage. Comparison with fresh viscera indicated a 50% loss of tryptophan in fish viscera silage. Other amino acids appeared not to suffer notable damage during processing. Autolysis causes a degradation of the protein by the inherent enzymes of the viscera. The silage could be separated by centrifugation into a soluble phase and an insoluble sludge. The latter represented, an average 12.7% of total dry matter, but contained .apprx. 1/2 of the total fat and most of the true protein. The soluble phase contained almost all the ammonia and more ash compared with the insoluble sludge. Free amino acids represented most of the amino acid N in the soluble phase. Amino acid analysis revealed a higher concentration of the aromatic amino acids, phenylalanine, tryptophan and in particular tyrosine, in the insoluble sludge than in the soluble phase.

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