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In this project, we investigate how and to what extent the state of rigor mortis affects frozen raw materials.

Last update

Read in Norwegian

Start

15. Jan 2023

End

15. Jan 2025

Funded by

FHF – Norwegian Seafood Research Fund

Background

It is well-known in the industry that fish that is frozen in a pre-rigor state can present challenges in further storage and processing; however, there is no clear consensus on how significant this effect is. 

In the frozen fish industry, it is often claimed that the rigor state is released after a few weeks in cold storage, meaning it does not represent a problem in practice. 

In the 60s, FAO published a guideline indicating that the rigor energy is released during 8 weeks of cold storage, which parts of the industry use as a guideline for fish that are frozen in a pre-rigor state. 

Historically, much of the fish that has been frozen on board have been handled in a way (e.g. stress during the catch and crowding during transportation to the plate freezer) that makes it reasonable to assume that relatively few of the fish are actually in a pre-rigor state at the moment when they are frozen. 

However, there is much to suggest that there will soon be increased opportunities for freezing raw materials in a pre-rigor state as a result of new practices such as reduced hauls, improved on-board handling, and an increased share of the live catch being delivered onshore. 

With an increased focus on the production of high-quality frozen raw materials comes the need for fundamental knowledge about how the state of rigor affects raw materials that are being frozen. 

A useful start will be to document this process thoroughly through trials with cod and haddock carried out under controlled conditions. These trials should document the state of rigor’s effect on raw materials being frozen and stored frozen.

Goal

Document how rigor status influences frozen raw materials from cod and haddock. The different stages of rigor are here defined as the difference between “pre-rigor”, “in-rigor” and “post-rigor”.

The effect of these different stages on frozen raw materials will be examined to achieve the main objective of this project. 

Sub goals

  • Subgoal 1: Sample, describe, and then freeze the raw material to be used in the various freezing and cold storage experiments described in sub-goals 2–4. 
  • Subgoal 2: Document the effect of the state of rigor on thawing loss, subsequent drip loss, and sensory quality in connection with cold storage. This will be studied using the following parameters: cold storage over time, different storage temperatures (-20°C vs -35°C), thawing methods (fast vs slow), shrinkage by thawing, and differences between frozen whole fish, fillets, and loin pieces. 
  • Subgoal 3: Document the effect of the state of rigor on the development of yellow/brown color during cold storage. This will be investigated at different storage temperatures (-20°C vs -35°C). 
  • Subgoal 4: Conduct studies to better understand the mechanisms that affect quality to reduce yield loss. 
  • Subgoal 5: Coordinate experiments for relevant industry stakeholders based on the insights gained through sub-goals 1–3, and then disseminate the research to the industry. 

What we are doing

This is a research project led by Nofima. The research activity will mainly be coordinated and carried out by Nofima, but the aim is that the stakeholders in an internal company group consider the results so relevant that industrial tests will also be carried out as part of the project. 

It is imperative to ensure good quality raw materials and a correct freezing process to gain reliable results from the research activities of this project. We will ensure this by assessing and describing the properties of the raw material before freezing and cold storage. 

We intend to measure the k-factor (length, weight), liver weight, and blood values (lactate) for all fish included in the experiment to document variations and distribute samples so that the variations are distributed as equally as possible among the various groups. 

The rigor process is monitored by conducting texture measurements on whole fish using a penetrometer. All the experiments aim to compare how different conditions affect the thawing loss, drip loss, and sensory quality for three different rigor states before freezing. 

Both thawing loss and subsequent drip loss are good representations of the damage ice crystals can cause during freezing, cold storage, and thawing. In these experiments, all samples that are frozen will be packaged to be able to monitor the drip loss. 

A loin and fillet index will be used as a method for evaluating sensory quality, with a focus on fillet gaping, meat surface, and consistency. 

Previous experiments by Nofima indicate that light conditions may have an impact on the development of yellow discoloring on the surface of the fish muscle. This will be investigated further. 

In addition, it will be investigated whether the presence of non-frozen water has any effect on the yellowing reaction. Previous observations show a clear yellow discoloring that is easily captured in conventional photographs. In addition, instrumental color measurements (using a Minolta Chroma Meter with the L* a* and b* scale) will be taken to record the progress of any yellowing (yellow index) in contrast to whiteness (white index). 

Thawing losses will also be measured to record if there is any connection between drip loss and yellowing. 

Nofima has recently built up expertise in studying fish muscle structure through the use of MRI (magnetic resonance imaging) techniques. MRI makes it possible to identify damage and structural changes throughout the entire fish muscle without modifying the sample in any way. 

Nofima has access to high-resolution MRI equipment at the University Hospital but also has suitable MRI/NMR equipment at its lab. Nofima has, among other things, published studies where freezing damage to white fish has been examined using various MRI techniques and texture analyses. 

In this project, we aim to use MRI technology to gain a better understanding of what happens to samples with significant drip loss after thawing. These insights will be vital for finding practical ways to solve this challenge. 

Facts about rigor 

  • Rigor (mortis) is the Latin term for the phenomenon of stiffness in death, when the muscles contract and become stiff. 
  • It is a gradual process, but we usually refer to the organism being “in-rigor” when the process has progressed so far that there is a noticeable hardness in the musculature. 
  • For fish such as cod, this can occur relatively quickly after death (less than 2 hours), but it can also take up to 24 hours before the fish appears stiff. 
  • Raw materials in the period before a pronounced stiffness is evident are often referred to as pre-rigor raw materials. 
  • The length of the rigor process varies a great deal and is influenced by several factors such as temperature, stress level, and fitness.  
  • The fish exit a state of rigor when the stiffness ceases, which usually occurs within 1–3 days, after which it is referred to as being in post-rigor.