Texture, fat content and product yield of salmonids / Tekstur, fettinnhold og produktutbytte hos laksefisk
This thesis investigates factors that influence the texture, fat content, and yield of Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss Walbaum). Yield is defined in terms of weight loss during processing and liquid-holding ability. In addition, some aspects of experimental methods are discussed.
The breaking-strength of salmon flesh was determined during a one-year cycle and this revealed a negative correlation between hardness and growth rate. Likewise, sensory hardness was higher in smoked salmon whose rate of growth had been reduced by feed deprivation. Mechanical analyses of rainbow trout indicated hardening of the flesh towards the tail-end, but the tail softened substantially after freezing. Pre-freezing salmon in the gutted state caused softening of the flesh after smoking. In general the flesh of salmon and trout showed a positive correlation between hardness and muscle pH, and amount of salt-soluble proteins. Increasing fat content coincided with higher score for fatty-texture and occasionally with a softer texture. Sensory hardness correlated more closely with mechanical analyses carried out using cylindrical probes than results obtained with the Warner-Bratzler blade or sphere.
Fat content decreased during feed deprivation in Atlantic salmon, although this effect was negligible compared with variations related to season. The most significant change was an increase in fat content by 10-13%-units during the autumn. Measurements of rainbow trout revealed highest fat content in the belly flap and lowest in the tail. The fat content in the salmon fillets was more evenly distributed in the flesh compared with the rainbow trout where it to a higher degree was deposited as distinct visible fat deposits. The fat content of the salmon increased with increasing body weight, but the fish size only explained 27% of the variation in fat content for salmon larger than 2 kg. Fat content of the trout fillets was inversely correlated with protein content. Moreover, significant relationships were found between fat content and various physical and sensory properties.
Salmon with a slim body shape gave higher yield after evisceration and lower filleting yield, and increasing fat content was correlated with lower weight loss during salting and smoking. Liquid leakage from raw trout muscle increased towards the tail, where the fat content was lower and the water content higher. Similarly, water-holding capacity was positively correlated with the fat content of frozen stored raw salmon, while this correlation was negative for smoked salmon. Frozen storage resulted in higher liquid loss from both trout and salmon fillets. Freezing/thawing had its highest impact on water loss from raw salmon and fat loss from smoked salmon. Fat leakage of raw and smoked muscle correlated positively with fat content.
In conclusion, it its suggested that specific demands on product quality can be met, and the yield can be optimised, by planning slaughtering according to season or growth rate, and by using variations in quality attributes within and among individuals. One example is the grading of fillets according to intrinsic properties; another is the use of fillet sectioning.
