Influence of spoilage and processing temperature on the quality of marine fish protein sources for salmonids

Press cake meals were prepared from previously frozen herring immediately following thawing and after storage for 8 or 12 days at 2-5 degrees C. Each of the raw sources of herring was subjected to two processing temperatures, 75 degrees C and 100 degrees C, during meal preparation. Also, protein hydrolysates were prepared using ocean perch when fresh or after storage at 2-5 degrees C for 4 or 8 days. Subsequently, each of the three hydrolysates was dried at 85 degrees C or 93 degrees C. In two separate experiments, each of the herring press cake meals and dried perch protein hydrolysates was blended with a reference diet in a 30:70 ratio (test protein source: reference diet). All diets contained 5 g kg(-1) chromic oxide as an indigestible marker. The reference diet and all test diets were provided to satiation to chinook salmon in salt water and rainbow trout in fresh water, with digestibility of organic matter, protein and energy measured by difference. Digestibility of protein was also measured by the pH-stat and dilute pepsin solubility in vitro techniques. The results indicated that variation in processing temperature to a maximum of 100 degrees C had little effect on digestibility of marine fish protein sources. By contrast, raw material storage for 8 days or more at 2-5 degrees C prior to processing was found to reduce organic matter digestibility and sometimes nitrogen digestibility in salmonids. In vitro measures of digestibility were of little help in predicting the nutritive value of the test protein sources. Cadaverine level in herring press cake meal was shown to be a good indicator of spoilage in the raw material.