Food Chemistry 48 (1993) 331-335 Chemical and nutritional quality of raw, cooked and salted fish silages Oyedapo Fagbenro & Kim Jauncey Institute of Aquaculture, University of Stirling, Stirling FK9 4LA, Scotland, UK (Received 4 December 1992; revised version received and accepted 20 January 1993) Fermented fish silages were prepared from whole tilapia (Oreochromis niloticus), molasses and Luctobacillus pluntarum starter culture. The effects of cooking and addition of 5% salt on the proximate composition, various protein and lipid quality parameters were measured over a 30-day period of incubation at 30°C. The silages were stable during fermentation and storage, and no appreciable loss of nutrient contents was noted. The changes observed were increased degree of autolysis and ammonia production. Addition of salt or cooking of substrate before fermentation prevented continued protein hydrolysis by inhibiting the activity of endogenous autolyzing enzymes and decreased the formation of total volatile bases. The apparent digestibility of dry matter, of nitrogen and of the energy content of silage-based diets for tilapia was higher in both the cooked and salted fish silages than in the raw fish silage. INTRODUCTION Ensilage of fish and fish by-products has been used as an alternative to fish meal in animal feeds with reports that its nutritional value is comparable with that of fish meal (Lopez, 1990). Ensilage involves lowering the pH either by adding mineral and/or organic acids (acid-preserved silage) or by lactic acid fermentation (fermented silage). Fermented silage is preferred in developing countries as it precludes the purchase of expensive acids and avoids risk of handling acids which may be corrosive to silage-making equipment (James et al., 1977). Feeding experiments have generally indicated that fish silage has food replacement value when it is partly substituted for fish meal in rations for livestock and fish (Lall, 1991). However, standardized methods of fish silage preparation do not exist and the nutritional value depends on the fish species used as substrate, proper methods of preparation, and knowledge of its quality and properties during fermentation and storage. Such information is limited, particularly for silage pre- pared from warm-water fish species, which may form an important substrate for silage manufacture in tropi- cal developing countries where such technology is affordable and appropriate (Dhatemwa, 1989). This study was therefore conducted to determine the chemical changes that occur during the fermentation and storage of tilapia silage. Food Chemistry 0308-8146/93/$06.00 0 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain The nutrient quality of fish silage can be improved by limiting the extent to which proteins are hydrolysed to polypeptides and free amino acids. Termination of the ensiling process after 3-7 days resulted in improved weight gain, protein efficiency ratio (PER), biological value (BV) and net protein utilization (NPU) when these products were fed to mink (Skrede, 1981), calves (Offer & Husain, 1987), salmonids (Lall, 1991) and rats (Espe et al., 1992). Formaldehyde inhibits protein hydrolysis in fish silage (Haard et al., 1985; Husain & Offer, 1987) but may prove toxic to some animals. As an alternative, the present study was also conducted to assess whether inhibiting autolysis of fish silage by cooking before fermentation or the addition of 5% salt (sodium chloride, NaCl) to the silage mixture would give a stable product during storage; the digestibility of these silages for feeding fish (Oreochromis niloticus Trewavas) was also assessed. MATERIALS AND METHODS Raw materials Juvenile tilapias culled as a result of routine husbandry techniques were collected and kept deep-frozen (- 20°C) and thawed in running water at room temperature before use. Molasses (International Molasses Ltd, Grangemouth, Scotland, UK) was added as carbohy- drate substrate. A prefermented starter culture prepared with freeze-dried cultures of Luctobacillus plantarum (NCIMB 11974, NCIMB Ltd, Aberdeen, Scotland, 331