Biochemical structure of protein (reactive SH content, content ratio of SH/SÐS and concentration of D-Asp as % of total (D L)-Asp) indicating digestibility of dietary protein was changed under different processing conditions. Based on ®sh crude enzyme extract, in vitro digestibility of different ®sh materials processed under different conditions correlated positively with reactive SH content and content ratio of SH/SÐS and negatively with D-Asp concentration. In vitro digestion of different experimental feeds, based on Atlantic salmon crude enzyme extracts, was studied in association with growth trials in order to investigate its value as a criterion for industrial strategy in predicting feed quality. Crude enzymes were extracted from the pyloric caeca before feeding. Signi®cant differences in in vitro digestibility between the experimental feeds were observed whereby there would be differences in feed conversion ef®ciency within 3 months of feeding. There were associations between the in vitro digestibility and other parameters for dietary quality, such as mink digestibility and the biochemical structure parameters of the dietary protein due to different processing conditions. Crude enzyme extracts from rainbow trout and European seabass were also used for in vitro digestibility study of different experimental feeds by standardising trypsin activity to that of Atlantic salmon crude enzyme extract. The results indicated that different ®sh species have different digestion ability to the same feed types, and the effective time for feed utilisation and growth is dependent on ®sh sensitivity and the extent of difference in digestibility between the feeds consumed as observed in the Atlantic salmon trials. For the species investigated, sensitivity ranking of the enzymes to feed quality under the condition studied was Atlantic salmon > rainbow trout > European seabass. The results indicated that in vitro digestibility study of experimental feeds using pyloric caecal crude enzyme extract from a speci®c species at an age of interest could be a practical, quick and reliable method for testing feed quality in growth trials. By standardising the crude enzyme extract with regards to trypsin activity, the in vitro digestibility values could be comparable not only within the same species but also between different species.

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The effect of processing conditions on protein digestibility and fluorodinitrobenzene (FDNB)-reactive (available) lysine in the production of fish meal and extruded fish feed has been studied under pilot and commercial conditions using mink as model animals. Fish meal produced under pilot-plant conditions at processing temperatures below 70-80 • C (FM1) had protein digestibility of 929 (grams of protein digested per 1000 g protein consumed) compared with 905 when processed at temperatures above 100 • C (FM2). A low-temperature-processed commercial fish meal (CFM1) had protein digestibility of 940 compared with 888 for a standard commercial fish meal (CFM2). Pilot-produced extruded fish feed had protein digestibility of 913 when based on FM1 as the main protein source (95% of total protein) compared with 892 when based on FM2. Commercial extruded fish feed had protein digestibility of 912 when based on CFM1 compared with 871 when based on CFM2. Varying extrusion conditions at the pilot scale, ie temperatures from 100 to 126 • C and moisture contents from 21 to 12%, did not affect protein digestibility. Similarly, under commercial conditions, variation in temperature from 89 to 110 • C and moisture from 24.5 to 19.5% did not affect FDNB-reactive lysine and protein digestibility. The FDNBreactive lysine content and protein digestibility of the extruded feed were less than the values calculated from the ingredient mixture before extrusion. Thus, despite different extrusion conditions not giving different FDNB-reactive lysine and protein digestibility, the total process, ie extrusion, drying and oil coating, caused a reduction.

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The aim of this study was to investigate the impact of two types of fish raw material on fishmeal (FM) physicochemical properties, specific mechanical energy, starch gelatinization and physical feed quality. Eleven feed mixes based on six independent FMs from herring (FMH) and five from sand eel (FMSE) were extruded according to standardized conditions. Type of fish raw material influenced FM physicochemical properties and had significant impact on starch gelatinization and pellet hardness. Multivariate models were established based on partial least squares regression (R2 = 0.637–0.999). Improved degree of starch gelatinization was associated with increased degree of protein hydrolysis (P = 0.011) and by replacement of FMH with FMSE in the feed mix (P = 0.012). A large span in pellet hardness was observed (4.9–94.1 N). Improved hardness was associated with decreased level of non‐soluble protein (P < 0.001) and by replacement of FMSE with FMH (P < 0.001). For FMSE, improved hardness was associated with decreased level of non‐soluble protein (P < 0.001) and increased degree of protein hydrolysis (P = 0.002), compressed bulk density (P = 0.039) and dust fraction (P = 0.001). The underlying physical and chemical mechanisms are discussed. The study documents differences in technical quality of FM produced from herring and sand eel with significant impact on fish feed extrusion and pellet quality.

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