Quality effect of freeze-chilling in cod and rainbow trout

‘Meal elements’ is the name of a concept in which elements of a meal, e.g. portions of pre-fried meat, sauces, fish or pre-processed vegetables are prepared industrially. The meal elements are distributed to professional satellite kitchens for instance in hospitals and canteens, where the kitchen staff combines the different meal components to a complete meal.

Freeze-chilling is a process consisting of freezing and frozen storage followed by thawing and chilled storage and could be an ideal technique to combine with the concept of meal elements. Freeze-chilling would enable manufacturers to produce large quantities of frozen meal elements to be released into the chill chain according to market demands.

This procedure would allow the products to thaw during transport, and by arrival the thawed meal elements would be ready for use or chill storage. We have studied the influence of freeze-chilling on the quality of raw fish portions as an example of a meal element. The thawing of frozen products during transport was mimicked by placing cardboard boxes with frozen, vacuum packaged portions of fish in a chilling facility and allowing them to thaw slowly.

To mimic possible subsequent chill storage at the satellite kitchens the quality was also followed during a storage period. The quality changes were evaluated on the basis of the results from descriptive sensory analysis and analysis of different chemical parameters. The high quality shelf life was determined from these results.

As the quality changes are known to differ among fish species, the present study included the popular species cod (Gadus Morhua) and rainbow trout (Oncorhynchus Mykiss). Principal component analysis of the sensory results clearly showed that after frozen storage at -30 °C for 1 month and subsequent chill storage at +2 °C, trout had a shelf life of approximately 10 days as a high quality product which was perceived indistinctly from the freshly thawed samples by the sensory panel.

The corresponding high quality shelf life for cod was 6 days. In conclusion, the consistent quality during storage and the high quality shelf life is practically applicable and cod and rainbow trout seem potential candidates for freeze-chilled meal elements.