First of all, what is rigor?
Rigor mortis is the process of muscle death
that follows the death of the fish. As the fish dies (hopefully for welfare and quality from instantaneous,
irrecoverable brain damage), the muscle cells continue
to respire, producing and consuming adenosine triphosphate (ATP), the primary
currency of cellular energy. When the ATP concentration falls to a critical
level there is insufficient ATP to dissociate all of the actin and myosin and
they begin to remain complexed as actomyosin; thus, the onset phase of rigor mortis
begins. These complexes continue to accumulate until the ATP concentration
reaches a critical minimum, at which time rigor is developed. Once rigor mortis
has developed, the muscle is not extensible (cannot “relax”) and becomes stiff.
Therefore, if we manage the harvest and slaughter process to increase the "pre-rigor window" we have the ability to fillet our fish before they enter rigor = pre-rigor filleting (PRF).
Why bother with PRF?
There are very noticeable 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 which reduces downgrading. This significantly increases overall yield.
How does PRF make the fillets so much better?
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 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
So why isn't everyone doing it?
There are more companies producing PRF's than ever before but it may require significant changes to harvest and processing infrastructure and methods which means significant costs. Payback time needs to be justified and top management needs to be convinced. In addition, the transport and sale of PRF is often seen as increasing the microbiological risk and reducing shelf life. And then there is the problem of removing pin-bones in most farmed fish species.
All of these problems have been shown to have solutions and there is no doubt that the increase in yield and production of consistent quality will ensure that more companies take on PRF and surmount the challenges listed above and reap the benefits to their bottom line.