The food processing industry is facing challenges when it comes to the thawing of fish and fish products. These challenges include most importantly the quality of the product to be thawed, the profitability of the operation and to some minor degree the environmental impact of the whole operation. Without doubt, thawing (and freezing) has a large impact on the structure and other quality factors of fish. In addition, both serve as bottlenecks in the production chain with direct effects on the profitability and hence also on the environment, for example, if the thawing process takes longer than necessary, energy is wasted and the quality of the product might suffer. In this concise review, several methods for (industrial) thawing of fish are presented and discussed, mainly with respect to their technical specifications and functional principles. These methods can be divided into two main principal types, namely the ones that provide heat to the frozen fish only via its boundary layers, and the ones that generate heat also in the frozen fish's inner spatial domain. Both types come with advantages and disadvantages; however, the latter types are generally not (yet) suitable for industrial large‐scale operation. The theory, functional principles and advantages/disadvantages will be highlighted in this work. In addition, an outlook on future developments as well as proposals for further research will be provided. Practical applications Thawing of fish is a very important step in the production chain from catch to salable product. Freezing serves both as shelf‐life extension for large catches and as a possibility to dampen out seasonal fluctuations of different species to make them available all year round. However, a frozen headed and gutted fish cannot be processed further (e.g., filleted), it has to be thawed to perform further processing of the product.

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Abstract-In this paper an approach for optimal boundary control of a parabolic partial differential equation (PDE) is presented. The parabolic PDE is the heat equation for thermal conduction. A technical application for this is the freezing of fish in a vertical plate freezer. As it is a dominant phenomenon in the process of freezing, the latent heat of fusion is included in the model. The aim of the optimization is to freeze the interior of a fish block below −18 o C in a predefined time horizon with an energy consumption that is as low as possible assuming that this corresponds to high freezing temperatures.

In this work the stability properties of a nonlinear partial differential equation (PDE) with state-dependent parameters is investigated. Among other things, the PDE describes freezing of foodstuff, and is closely related to the (Potential) Burgers' Equation. We show that for certain forms of coefficient functions, the PDE converges to a stationary solution given by (fixed) boundary conditions that make physical sense. We illustrate the results with numerical simulations.

In this work, we introduce a simple model for a gravity separator based on first principles.It can be used for controller design to control the levels of oil, water and the gas pressure as well as observer design to estimate unmeasurable parameters and disturbances. Evaluation of separation efficiencies for the respective water-and oil-continuous phases are part of the model in addition. The model incorporates three dynamic state equations describing the levels of the overall liquid (water plus oil) and water as well as the gas pressure subject to the in-and outflow dynamics. Furthermore, algebraic equations calculating simplified droplet distributions for each continuous phase are introduced in order to determine the exchange of water and oil between the two continuous phases. The controllers are of PI-type and control the outflows of the separator. Additionally, we introduce a virtual monitoring approach for the inflows of gas and liquid as well as the effective split ratio of oil and water entering the continuous water phase. Measurements for these variables are either expensive, unreliable or even impossible to take (as for the split ratio). We demonstrate the results in simulations.

BACKGROUNDThe catch of marine whitefish is typically seasonal, whereas the land‐based processing industry has a need for all‐year stable supply of raw materials. This challenge can be met by applying fish frozen at sea. When using frozen fish, the methods employed for thawing may influence the safety and quality of the final product. This study aimed to investigate the applicability of novel thawing strategies in order to provide an all‐year supply of high‐quality and safe cod products.RESULTSComparative investigations of quality and safety factors after thawing in water, with and without air circulation, and contact thawing were performed. The parameters included water‐holding capacity, thawing loss, drip loss, cooking yield, sensory evaluation and microbiological analyses (including total volatile bases nitrogen). Water thawing with air circulation provided faster thawing than water thawing without air circulation and contact thawing. For all three methods, the quality of the thawed fish was acceptable and the shelf life of the fillets during chilled storage was between 10 and 14 days post‐filleting.CONCLUSIONThe results show that controlled freezing of cod, followed by appropriate thawing, may provide the processing industry with an all‐year delivery of raw materials, without compromising quality and safety of the final product. © 2017 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

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In this work the stability properties as well as possible applications of a partial differential equation (PDE) with state-dependent parameters are investigated. Among other things, the PDE describes freezing of foodstuff, and is closely related to the (Potential) Burgers' Equation. We show that for certain forms of coefficient functions, the PDE converges to a stationary solution given by (fixed) boundary conditions that make physical sense. These boundary conditions are either symmetric or asymmetric of Dirichlet type. Furthermore we present an observer design based on the PDE model for estimation of innerdomain temperatures in block-frozen fish and for monitoring freezing time. We illustrate the results with numerical simulations.