Bioresource Technology 46 (1993) 207-211 CHEMICAL AND NUTRITIONAL QUALITY OF STORED FERMENTED FISH (TILAPIA) SILAGE Oyedapo Fagbenro & Kim Jauncey Institute of Aquaculture, University of Stirling, Stirling, UK, FK9 4LA (Received 10 February 1993; revised version received 27 March 1993; accepted 1 April 1993) Abstract Fermented fish silage was produced from minced tilapia (Oreochromis niloticus), 15% molasses and 5% Lactobacillus plantarum culture. Initially, batches of the fish silages were fermented at 5, 20, 30 or 35°C for 30 days. The pH declined rapidly at 20, 30 and 35°C but at 5°C, pH decline was slower. Protein solubilization (autolysis) in the fish silages was temperature-dependent as the non-protein nitrogen content (NPN) increased rapidly at higher temperatures. In a second experiment, fish silage was fermented at 30°C for 7 days after which autolysis was halted, then stored at 30°C for 180 days and analysed at intervals. The NPN attained a maximum of 51"4% of total nitrogen content after 180 days. Proximate composition of tilapia silages varied slightly during storage and there was a slight loss of tryptophan during fermentation and storage. The stored wet tilapia silages were blended with soybean meal (1:1, w/w) and dried. The nutritional value of co-dried tilapia silage as a dietary protein supplement for ca(fish (Clarias gariepinus) was studied in digestibility experiments. Although the protein quality of wet tilapia silage was reduced during storage, there were no differences (P > 0"05) in the apparent protein digestibility coeffi- cients of diets containing co-dried tilapia silages. It is suggested that autolysis in the stored tilapia silages had little effect on protein digestibility in the ca~ish. Keywords: Fermentation, temperature, nutrient quality, fish silage, protein digestibility. INTRODUCTION Efforts are primarily directed towards the preserva- tion of fish for human food. However, seasonal varia- tions in catches, transportation cost, lack of ice or storage facilities, inadequate processing facilities, unsold, undersized or low-value fishes, etc., have resulted in considerable amounts of fish being wasted, Bioresource Technology 0960-8524/93/S06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain 207 discarded at sea or left unutilized. These underutilized fish are usually converted to fish meal and used as a protein supplement in pet food, animal- and fish-feeds. Ensiled fish and fish by-products have been used as alternatives to fish meal in animal feeds, particularly in situations where fish meal production or preservation by deep freezing is not available or uneconomical (Brown & Sumner, 1985; Dhatemwa, 1989). Fish silage is prepared either by acid preservation (acid silage) or by anaerobic microbial fermentation (fermented silage). The latter is particularly preferred in developing countries because it is relatively cheaper to produce and it involves simple, artisanal technology which is adaptable at cottage/village level (Dhatemwa, 1989). Fermented fish silage has been included as a dietary-protein ingredient for poultry and pigs (Tibbetts et al., 1981; Brown & Sumner, 1985; Hassan & Heath, 1986) but the evaluation of its suitability in aquaculture diets is limited to a few formal studies (Djajasewaka & Djajadiredja, 1980; Wee et al., 1986; Edwards et al., 1987). The various sources, combinations and proportions of the different raw materials (fish, carbohydrate, inoculum, salt, water) coupled with the various condi- tions (pH, incubation temperature, duration) of fer- mentation and storage make the standardization of quality for lactic acid fermentation impossible (Hassan & Heath, 1987). Therefore, pH and non-protein nitrogen (NPN) content are generally accepted and used as indicators of fish silage quality (Batist a et al., 1989; Hardy & Masumoto, 1990). Information is limited on the nutritional quality and value of fer- mented fish silage prepared using warmwater fish, which constitute an important source of raw materials for fish silage manufacture, particularly in the tropics (Disney et aL, 1978). The fact that only small quantities of fish wastes may be available, or their supply irregular or geographically dispersed constitutes a limitation to the use of fish silage. The practical solution to this is to prepare and store the fish silage as the substrates become available. The significant changes that occur during storage of fish silage are autolysis of the tissues and release of ammonia (Espe et al., 1989). A decrease in nutritional value with increasing degree of fish silage autolysis has