Raw material characteristics and treatment - effects on yield and quality of cold-smoked Atlantic salmon / Råmaterialets betydning for utbytte og kvalitet på kald-røykt Atlantisk laks
The aim of this thesis was to study the effects of fat content, fatty acid profile, freshness and freezing of the raw material on yield and selected quality characteristics (i.a colour, texture, salt content, sensory attributes) of cold-smoked Atlantic salmon fillets.
The effects of fat content, gutted weight and condition factor on the process yield and quality of cold-smoked farmed Atlantic salmon (Salmo salar) fillets was investigated in Paper I, using 120 salmon varying in fat content from 140 to 230 g kg-1. Filleting loss was in this study not found to be related to any of the measured parameters, whereas about 8% of the variation in trimming loss was related to the fat content and condition factor of the fish. Loss during smoking was influenced by the gutted weight and dry matter content of the fish, and these factors explained 52% of the variation in the smoking loss of brine-injected fillets. Gutted weight explained approximately 13% of the variation in total loss during the processing of cold-smoked salmon fillets, with larger fish giving a higher process yield. However, the total process yield was not affected by raw material fat content. The measured raw material characteristics did not, in this study, effect sensory evaluated quality of the smoked fillets.
Possible effects of fatty acid profile were investigated in the study described in Paper II. In a feeding experiment lasting for 138 days, groups of salmon were fed fishmeal-based diets with the following as oil source: 100% fish oil (FO) or 50% fish oil and 50% rapeseed oil (50RO) or 30% fish oil and 70% rapeseed oil (70RO), respectively. As expected from earlier studies, the fatty acid compositions of the fillets reflected those of the diets, with increasing proportions of C18:1n-9, C18:2n-6 and C18:3n-3 in the muscle with increased inclusion of rapeseed oil (RO) in the diet. The amounts of eicosapentaenoic acid (20:5n-3) (EPA) and docosahexaenoic acid (22:6n-3) (DHA) were significantly lower when RO was included in the diets. The fatty acid profile did not change during smoking. Inclusion of rapeseed oil in the diet caused a minor increased water loss from smoked fillets compared to a 100% fish oil diet. Compared to the effects of ice storage and smoke processing, the dietary treatments tested had only minor effects on product quality and storage stability.
In Paper III and IV the main focus was on the effects of fillet freshness as influenced by the process of pre-rigor filleting. When investigating pre-rigor filleting of salmon it was found that the incidence and severity of gaping in raw fillets were significantly reduced with pre-rigor filleting as compared to post-rigor filleting. The differences became even more pronounced as the fillets were stored. Fillet yield and water binding capacity was not affected by pre- or post-rigor filleting, but there was a positive effect of pre-rigor filleting on visual colour of the raw fillets. Thus, pre-rigor fillets of salmon are of potential interest as raw material for smoking. However, brining, by injection or dry salting, and smoking of pre-rigor fillets at different stages postmortem, showed that early processing greatly influenced both salt uptake and salt homogeneity of cold-smoked fillets. Compared to traditionally post-rigor filleting and smoking, salt content was significantly lower in smoked salmon filleted pre-rigor. The process yield of dry salted fillets was not affected by filleting pre- or post-rigor, thus, finding suitable salting procedures for pre-rigor fillets should be encouraged.
Effects of freezing the raw material as gutted fish or fillets before smoking was the main topic investigated in Paper V. Short-term freezing before smoking resulted in increased product yield and water content, but softer texture and increased K-value in smoked fillets. Freezing reduced the content of astaxanthin but increased the lightness and the colour intensity of the flesh. Freezing the fish as fillets increased gaping compared to freezing the gutted fish. Freezing only after smoking led to fewer changes in the measured quality traits than freezing before smoking, while re-freezing the finished products has little additional effect on quality.
