There are considerable advantages in pre-rigor filleting in terms of shelf life and reduced gaping/improved product texture and integrity and colour visualisation/translucency (Fig. 1). This extra robustness has a flow on effect in reducing the damage caused by downstream processing operations and consequently reduces downgrading. This significantly increases the yield of products most sensitive to downstream processing and handling operations.
The mechanism for reduced gaping of pre-rigor fillets is most likely derived from the lack of resistance of the skeleton against the muscular contraction of rigor processes. During the onset of rigor, the macroscopic structure of the muscle does not suffer any damage since the loose musculature, free to shrink, is not under any tension.
Figure. 1 Pre-rigor fillets showing their robustness and lack of gaping and translucency
Pinboning becomes an issue for raw pre-rigor fillets as pinbones are very difficult to remove just after pre-rigor filleting, which reduces pinboning speed and increases bone breakage. However, if fillets are stored (or transported) for 10-12hrs at 0-4 °C, productivity (pin boning speed) will be similar to a post-rigor process of 0.5 – 0.6 min/fillet. There may also be potential to electromassage fillets to speed rigor resolution (an assist in bloodspot removal) without causing flesh quality issues (once the flesh is off the bone).
Seasonal Variation in Flesh Texture
Atlantic salmon show seasonal trends in their flesh quality at different stages in their life history. Farmed salmon often reach sexual maturity in their first year at sea. Traditionally, at this time, market supply was interrupted due to maturity causing undesirable external, and flesh quality traits, i.e. pale colour, thin body wall, dark skin colouration, soft texture and gaping. Through manipulation of photoperiod and temperature (in freshwater and seawater), and the use of different stock types, fish farmers can now deliver market sized fish year round. However, there will always be some variations in flesh quality within these stock types throughout the year. Some of these seasonal trends are related to the time they have spent in the water as well as their growth history, maturity status, and water temperature. e.g. fast growing fish typically have lower muscle fibre and connective tissue density therefore an increased propensity to gape. Farmed salmon often experience a growth spurt after a period of slow growth, i.e. after winter or excessively high water temperatures that inhibit growth. This is where pre-rigor filleting is most effective in ameliorating gaping levels in these fish with a predisposition for flesh texture issues, hence providing greater consistency in quality caused by the seasonal ultastructural variations in fish muscle.
Freshness is, for most markets, the most important quality parameter. To supply the market with fresh processed fish it is therefore desirable for the processor to start production as early as possible after harvest. There is clearly significant potential for pre-rigor filleting, and the reduced gaping/improved texture provides more consistent year round quality for customers.