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Abstract

This study was undertaken to assess the nutritional value of fish meals (Pacific herring, Clupea harengus pallasi) and protein hydrolyzates (ocean perch, Sebastes alutus) processed in different ways for chinook salmon (Oncorhynchus tshawytscha) in salt water and for rainbow trout (Oncorhynchus mykiss) in fresh water. Three levels of raw material freshness (fresh frozen, moderately spoiled and highly spoiled) and two processing temperatures (low and high) were employed in the present study. Spoilage of Pacific herring stored at 2-5°C, as determined by the levels of total volatile nitrogen(TVN) and trimethylamine(TMA), was slow for the initial 8 days but increased rapidly until day 15. The direct distillation method with MgO yielded significantly higher TVN values than the microdiffusion method with K₂CO₃ or KOH and the steam distillation method with NaOH. The TMA values also varied with the method employed. For the routine determination of TVN and TMA microdiffusion with K2CO3 is recommended. The levels of putrescine and cadaverine in herring also increased rapidly after 8 days of storage at 2-5°C. Histamine levels remained low (<5.0 ppm), even after 12 days of storage. Prefreezing of the herring prior to storage at chill temperatures may have delayed the formation of histamine. The levels of putrescine, cadaverine, histamine, TMA and TVN were lower in the press cake meals than in the raw material, which was possibly due to amine losses in the press liquor and thermal degradation of amines. Elevation of processing temperature from 75°C to 100°C had little effect on spoilage indicator levels but did increase the amount of press liquor released from the raw materials. Progressive spoilage of the raw materials also increased the yields of press liquor and concomitantly decreased the yields of fish meal. The levels of TVN, TMA, putrescine, histamine and especially cadaverine were considered to be useful in determining spoilage of the raw material but only the less volatile amines (putrescine, cadaverine and histamine) may prove useful in predicting the quality of fish meal. Spoilage of the raw material, regardless of the protein source, adversely affected apparent digestibility coefficients for organic matter, gross energy and digestible energy content in chinook salmon and rainbow trout. As spoilage of the raw material increased, digestibility values decreased. Protein digestibility values were least affected by the degree of spoilage. Since fish meals contain high levels of protein and lipid but almost no carbohydrate, the probable cause of reduced organic matter digestibility of spoiled protein sources was depression of lipid availability. Elevation of processing temperature did not adversely affect digestibility values in either species. Differences in digestibility values were noted between species for meals produced from highly spoiled raw material and freezedried material. Rainbow trout seem to be more sensitive than chinook salmon to spoilage of the raw material as digestibility values of the meals produced from spoiled raw material were more severly depressed in rainbow trout. Lipid oxidation may have accounted for these differences, as chinook salmon recently transferred to sea water may be susceptible to quality of dietary lipids. Ocean perch hydrolyzates were highly digestible in both chinook salmon and rainbow trout. Pepsin solubility was greater than 98% regardless of processing and spoilage conditions. The pHstat values (utilizing enzymes extracted from rainbow trout), increased as spoilage of the raw material progressed but were slightly reduced in the high temperature meals. The pepsin solubility values also increased as spoilage of the herring protein sources progressed. TVN and TMA showed little promise as methods for predicting the nutritive value of ocean perch hydrolyzates or herring fish meals, although monitoring TVN and TMA levels in the raw material is recommended. However, the relationship between levels of putrescine, cadaverine and histamine may prove useful in determining the quality of fish meal. In conclusion, advanced spoilage of raw material (> 8 days for herring and ocean perch stored at 2-5°C) before processing resulted in depressed digestibility values in both rainbow trout and chinook salmon.