Salt is an essential component of salted fish and clipfish. However, how does the quality of the salt affect the yield or the product quality?
Several details has been discovered about this during the SALT research project, funded by FHF – Norwegian Seafood Research Fund.
“The composition of the salt influences the quality of the salted fish product,” says Project Manager and Senior Scientist Grete Lorentzen at Nofima.
The Norwegian salted fish and clipfish industry use around 180,000 tonns of salt annually, which corresponds to a value of approx. NOK 120 million. What is good salt? Does it make a difference if we use new or used salt? What happens to the properties of salt if it is purified after use. Do we need more salt or a longer time during the process if we use purified salt instead of new salt? Moreover, how does different types of salt influence parameters like yield, colour, taste and appearance of the product?
“The overall aim of this project has been to create an overview showing the status of knowledge about salt used in the production of salted fish and clipfish. This overview has now been finalized,” says Grete Lorentzen.
Product quality affected
The Norwegian Quality Regulations relating to Fish and Fish Products stipulate that salt to be used for salting fish industry, should have a clean appearance and not contain coloured particles or foreign crystals. The regulation also require absence of off-odour and that it should have a fresh, and pure taste.
“Salt which is used in the saltfish industry contains about 97-98% salt, while the remaining part includes calcium, magnesium, sulphates, copper, and iron,” says Grete Lorentzen.
The composition of salt changes after use. The levels of calcium, magnesium, copper and sulphates decreases, while the water and protein content increases. This affects the product quality.
The scientist explains:
“Elevated calcium and magnesium levels in the salt can cause the surface of the fish to become more solid, which can reduce the rate of salt uptake , drying, and de-salting.”
However, the condition of the raw material, i.e. frozen- thawed vs. fresh fish, the temperature during the salting process, the salting time, the storage time and physical pressure are all parameters that have a greater impact on quality variations of the product than variations of the salt quality,” says Grete Lorentzen.
However, there is no reason to throw away used salt. Used salt can be purified and reused.
“Some companies have recently introduced mechanical purification of used salt, with positive results. It has been reported that there are no differences in salt uptake, drying times or yield when comparing new with purified salt. Fully saturated fish is achieved after the time regardless of new or purified salt,” says Grete Lorentzen.
Summary of trials
The Norwegian salt fish industry applies one part of salt to one part of fish. Several trials have been conducted to study potential changes of the salt at this salt fish ratio.
Here is a brief summary of these trials:
• Less calcium, magnesium, copper and sulphates: calcium and magnesium are important for whiteness, firmness and taste. Too much calcium and magnesium result in a hard, compact surface on the fish. This involves a longer salting, drying, and de-salting time.
• Lower pH: lower pH levels in the salt alters the solubility of calcium sulphate (CaSO4). This can affect the whiteness of the fish.
• Increased water content: more water in the salt results in a longer salting, drying and desalting time. The fish will have lower weight yield, increased water content and a lower salt content.
• Increased protein content: remnants of fish and blood proteins causes a greyish colour of the salt. In case of a temperature above 8 C in combination with the presence of these proteins, microbial growth of extreme halophiles (Halophilic Archaea) can occur. The growth of these halophiles results in a red to pinkish colour of the salt. Moreover, presence of blood results in higher levels of iron, and iron oxidises fat which promotes a rancid taste.
• Changes in distribution between the particle size: the size of the salt grains and the volume between the different sievedfractions in the salt are changed after use. The ratio between the different sieved fractions are important for salting times and salt uptake. Too many large salt crystals results in longer salting times due to less contact between the fish and the salt, while too many small salt crystals makes the fish sticked together.