Industrial thawing of fish : to improve quality, yield and capacity

Abstract

Melting of frozen water in food products is denoted thawing. The phase change requires energy, and takes place at a constant temperature for pure water. For mixtures of water, fat, protein and ashes (i.e. foodstuffs) this phase change will take place at a gliding temperature. Thawing is physically the opposite process to that of freezing. The heat flow is reversed and instead of extracting heat from the product, heat is directed into it. Although opposite processes, thawing is more difficult to carry out with respect to predictability and controllability. This is due mainly to three aspects:

1. Increased heat flow resistance as the thawing proceeds.

2. Reduced temperature difference (ΔT) between product and media.

3. More difficult to monitor the process and product end temperature accurat

The food processing industry depends on a continuously and safe supply of raw material, in order to utilise process equipment better, improve production planning and to create stable and secure working environment for the employees. The fish industry is very important for Norway, and its export value (NOK 30,6 billion in 20011) is the second highest after oil and higher than gas. The fact that supply of fresh raw material (i.e. fish) often is dependent on seasonal variations, weather conditions, quotes and regulations (governmental and international), has been a great intensive to use frozen raw material in the fish process industry.

A survey amongst 155 fish processing plants throughout Norway showed that 75 % of them used thawing in their production and further 23% claimed that they would do so in the future. 93 % of the companies that thawed did so in an uncontrolled manner. The challenges in industrial thawing are many:

• Block frozen products changes size during thawing and are generally more difficult to handle

• The product texture are temperature dependent

• Size variations – both single fish and batches

• Use of both fresh and frozen raw material

• Company culture

Thawing by heat transfer through the surface has been studied for three different products; Salmon, Cod and Mackerel. A slightly different approach has been used for these products depending on the raw material availability, industrial needs and relevance.