Cold-smoking of Atlantic salmon (Salmo salar) - Influence of processing conditions and raw material characteristics on quality and yield / Kaldrøyking av Atlantisk laks (Salmo salar) - betydning av prosessbetingelser og råstoff for kvalitet og utbytte
This thesis investigates the effects of different salt-curing and cold-smoking procedures on quality parameters and processing yield of cold-smoked Atlantic salmon (Salmo salar), and is based on six papers. The quality parameters investigated were texture, muscle gaping, colour, carotenoid concentration, liquid holding capacity and salt and phenol content. The extraction of protein-bound astaxanthin by brine was also investigated because it may have a close relationship with the visually assessed colour.
The injection-salting technique is increasingly gaining popularity due to its time saving potential and the higher processing yields obtained when compared to conventional salt-curing techniques. However, the effects of its use on quality parameters of salmon products are poorly documented. Therefore, the effects of brine injection pressure, needle velocity, needle density and salt injection mode on muscle gaping and processing yield were investigated (Paper I). Muscle gaping was negatively affected by increased injection pressure but not by increased needle density. Processing yield increased significantly by increasing injection pressure and increased needle density. This is explained by the higher amount of brine injected at high pressure and increased needle density. The injection system was checked for stability, and the amount of brine injected was stable and typically 8%±0.8% (w/w). Improving processing yield may be facilitated by increased needle density without compromising muscle gaping.
A major finding in our investigations was that choice of processing strategy (e.g. salt-curing method) and fat content and carotenoid composition of the raw material have significant effects on quality traits of cold-smoked Atlantic salmon (Paper II). The injection-salting and dry salting procedures represented a tough and gentle processing treatment, respectively. The utilization of the injection-salting technology caused the smoked products to have significantly different quality traits and increased smoking yields (16%) compared to smoked products subjected to dry salting. Thus, injection-salted fillets were substantially lighter and had a more yellowish hue, had a less firm texture, exhibited increased muscle gaping and higher liquid holding capacity compared to the dry salted fillets. Fillets with a high fat content were less firm than fillets with a low fat content. Canthaxanthin affected redness negatively and yellowness positively. Nevertheless, the relatively small effects of processing procedure indicate that salmon represent a fairly robust material for cold-smoking.
Temperature during cold-smoking is a fundamental parameter that may influence product quality. Two groups of Atlantic salmon either fed a diet with fish oil as sole oil supplement or soybean oil as sole oil supplement were smoked at six different temperatures in the range 21.5-29.9°C (Paper III). Temperature significantly affected texture, colorimetric parameters, salt and phenol contents, total liquid loss and processing yield. However, the regression between quality parameters and smoking temperature were low or moderate (R²=0.02-0.48). A negative relationship was found between processing yield and smoking temperature, whereas the relationship between firmness, elasticity, yellowness, total phenol content and total liquid loss were positive. Dietary oil supplements only affected redness, colour saturation, total liquid loss and liquid loss during storage significantly. Of importance is that soybean oil appeared to have a negative effect on astaxanthin concentration in the muscle and therefore colouration. Although siginficant, the effects of temperature on quality were generally low and again indicate that salmon is a fairly robust raw material for cold-smoking.
Cold-smoke processing caused an overall loss of 13% of the astaxanthin (Paper IV). The salting step appeared to be quantitatively more important than the cold-smoking step. Significantly more astaxanthin was lost when the fillets were smoked at 20°C compared to at 30°C. Conversely, the changes in colorimetric parameters were higher at 30°C than at 20°C. The effect of dry salting on colorimetric parameters was higher than the effect of injection-salting. The variation in colour parameters appeared to be higher in dry salted fillets than in injection-salted fillets and in fillets smoked at 30°C compared to 20°C. Thus, processing conditions may influence the colorimetric parameter stability and astaxanthin loss, and emphasize the need for thorough quality control during smoke-processing of salmon.
Both temperature and duration during dry- and brine salting have significant effects on colorimetric parameters (L*, a*, b*, Hab, C*, ΔE) of cold-smoked salmon fillets (Paper V). Significantly higher changes in colorimetric parameters were found at the highest salting temperature. By increasing salting time the ΔL*-, Δb*, ΔC*- and ΔE-values decreased and the ΔHab-value increased in the smoked fillets. The Δa*-value was not significantly different at the lowest and highest salting temperature. Although there was a relatively high correlation (R²=0.64) between a* and total carotenoid concentration in smoked fillets, the correlation between a* in the raw fillets and smoked fillets was rather low (R²=0.19). The coefficient of variation for Δa*, ΔHab, and ΔC*-values were significantly higher in the cold smoked product than in the raw material. This shows that smoke-processing itself contributes significantly to the variation in product quality.
Considerable amounts of protein-bound astaxanthin (up to 21%) were extracted from minced muscle of Atlantic salmon by water and brine (Paper VI). The highest amount of astaxanthin was extracted in the pH range 6-7 and for brine concentrations in the range 1-4M. This indicates that extraction of astaxanthin from the fillet surface during processing is a plausible contributor to carotenoid losses. Thus, care should be taken during de-salting of the fillets with water to avoid quality deterioration.
In conclusion, the experiments presented in this thesis have shown that important quality parameters and product yield are significantly affected by processing procedure and process parameters such as temperature and duration. This shows the importance of careful quality control during manufacture of cold-smoked Atlantic salmon. Furhermore, raw material characteristics have a significant impact on end quality, but in general salmon was found to be a rather robust raw material for cold-smoke processing. The processing protocol should be developed carefully because processing parameters may have a significant effect on the variation in product quality.
