No more dry cod
In his doctoral dissertation, researcher Dagbjørn Skipnes at Nofima studied how to give cod the optimal water-binding and texture. The solution proved to be far more complex than what was previously believed.
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“Until now, the conclusion has been that the higher the temperature, the greater the loss of water,” Skipsnes explains. “But this turns out to be incorrect. My research shows that in the interval from 28°C to around 50°C, the water-binding capacity decreases, before increasing in pace with the temperature.”
“Dagbjørn’s research is an important contribution to understanding what transpires during industrial heat treatment of fish and other types of food,” adds his supervisor Morten Sivertsvik, head of research at Nofima. “Now the industry can for example optimize juiciness and taste by using the correct process conditions without compromising on food safety and shelf life. This is a prime example of industrial gastronomy in practice.”
Cod offers many challenges
Lean fish such as cod is more fragile than the more fatty fish types, making heat treatment more challenging. Cod is often perceived as dry if water loss during cooking is substantial or if the water-binding capacity has been reduced. In addition, the binding protein collagen changes already at lower temperatures (below 40°C), causing the blocks of muscle to separate from each other.
“I studied the quality of cod loin that had been heat treated in a vacuum pack, in order to discover the optimal combination of heat treatment duration and temperature,” Skipnes says. “It turns out that the interval between 64 and 72°C provides high quality cod. This interval proves favorable for killing dangerous bacteria, of which Listeria monocytogenes is the most heat resistant, and for achieving a good texture and water-binding capacity in the fish muscle. No single temperature is optimal, however, because factors such as the size of the fish pieces also determine what is optimal.”
In collaboration with Prekubator, Dagbjørn Skipnes has developed a prototype cup that facilitates new types of tests and analyses. The cup enables him to heat food quickly and homogeneously and also to measure water loss during cooking, water-binding capacity, and texture without removing the fish slice from the body. Contrary to what was previously believed, it turns out that the relationship between water loss and water-binding capacity, as well as the duration and temperature of heat treatment, is non-linear.
Far more complex than previously assumed
Previous tests have used approximately 60°C as their starting point. As a result, the many variations in water-binding capacity went undetected, as did the connections between water loss during cooking, water-binding capacity, protein changes, texture, and muscular structure. The possibility of being able to heat food quickly and homogeneously and to achieve more complete measurements has also provided new insights, especially in regard to how the proteins in the cod muscle are denatured (changed).
“The complex connections may be largely explained by the fact that the denaturation of the different types of protein occurs stepwise and gradually overlaps during heating – and at different temperatures,” Skipnes explains. “So far we have identified five steps in the denaturation of proteins, but the effects on the fish muscle vary depending on which protein group that is being denatured.”
A model to optimize heat treatment processes
The results from the work can be applied directly when optimizing the heat treatment of cod. The many experiments have mapped out which combinations of duration and temperature that provide high water-binding capacity and low water loss during cooking, while simultaneously maintaining food security. A method of calculation has been devised for optimizing the desired quality, and the calculations can be controlled by observing the decomposition of particular enzymes, called acid phosphatases, that exist naturally in cod fillet and that can indicate whether Listeria bacteria have been neutralized.
Tests and analyses are currently being conducted to map out salmon in a similar fashion, so far producing a similar pattern.
“The logical next step,” Skipnes says, “is to continue the work that has been done by integrating the results with a numerical model, which the food industry can then use to optimize most heat treatment processes that are adapted to vacuum-packed cod loins.”
The research is financed by the Research Council of Norway.