Modeling, Estimation and Control of Freezing and Thawing Processes: Theory and Applications

Fisheries and aquaculture are very important sectors in the Norwegian industry. Fish and its by-products are considered wholesome foodstuff and are popular especially due to their nutritional value. Therefore, consumer safety is a very important topic and methods for prolonging shelf-life play a vital role in the fishing and aquaculture industries. These methods are mostly based on cooling and freezing techniques, as bacteriological and enzymatic activities are generally reduced at low temperatures. However, freezing and cooling are quite energy- and time-consuming. Freezers are often designed as batch-operations, especially on board fishing vessels, and thus act as bottlenecks in the production chain. Therefore, an optimal operation should be imposed in order to minimize the time the good is spending in the freezer. If a good is in the freezer longer than necessary, this operation can be denoted as overfreezing. Hence, the aim is to monitor and operate the freezing processes in an optimal way with respect to quality, but also with regards to energy use and environmental impact. This can be achieved by a better understanding of the factors that influence the quality of fish and seafood during freezing and thawing. Furthermore, comprehension of the actual processes that happen during phase change are of importance; this includes the derivation of a mathematical model to describe the temperature evolution in the food. This model can also be used to monitor and operate the reversed freezing process, namely thawing. Considering not only freezing but also the thawing process as an additional and often inevitable production step makes sense due to the fact that it is difficult to process frozen fish. The thesis is structured into five parts, which are divided thematically: Introduction and Preliminaries, Mathematical Modeling, Freezing and Thawing Applications, Experiments and Closing Remarks. Part I gives an introduction to the context of this thesis. The importance of the fishing industry for the Norwegian economy is highlighted. Furthermore, preliminaries about topics related to freezing and thawing of foodstuff in general, and fish and fish products in particular, are presented. These preliminaries include methods of industrial applicable and more theoretical nature. Furthermore, a Chapter is dedicated to the concise introduction to quality aspects of fish, in a general way, but also with respect to freezing and thawing applications and processes. In Part II