Samantekt:
Various studies have shown that two-phase slurry ice is more efficient than ordinary flake ice for chilling fresh fish. In most of the studies only one type of slurry ice has been applied, most often prepared in commercial ice-machines. The objective of this work was to investigate both chilling and maintenance of low temperature utilising flake ice and different kinds of ice slurries, both from commercial ice-machines and also manually prepared by mixing crushed ice and brine. The slurry ice particles are smaller when produced in the ice machines than in the latter method and this small size of the ice particles is widely accepted as one of the predominant factors for rapid cooling of fish. Both saithe and a cylinder made of agar were used as specimen in the experiments. As in other similar studies the cooling rate of all of the different slurry ice types was superior compared to flake ice. Very similar cooling rates were gained using different ice slurries of the same temperature. Therefore, the most important property of the chilling medium was concluded to be temperature since the size of the ice particles seemed to have only minor influence on the cooling rate. The importance of distributing the ice medium evenly when packing fish and ice medium in fish tubs became evident in this work. In order to maintain low temperature during storage, ice slurries are only better than ordinary flake ice for the first few days of storage. After a few days the faster melting of the ice slurries results in inferior cooling capacity so the flake ice, in general, maintains lower temperature in fish through long storage. 1 Introduction Numerous papers showing comparison of chilling of fish with flake ice vs. chilling of fish with two-phase ice slurries have been published. Figure 1 shows the results of an experiment in which a 350 L insulated container was half filled with slurry ice and 30 kg of small to medium sized cod (Davies, 2005). The slurry ice was made of 3 wt. % salt brine with an ice fraction of 35 wt. % and a temperature of-2.6 °C. A second container was packed with cod and flake ice and the temperature evolution in the centre of fish specimen was recorded as in the first container. The performance of the slurry ice is obviously much better. However, the initial temperature of the flake iced fish actually seems to have been higher in the flake iced fish than in the slurry iced fish. More rapid cooling and higher heat transfer rates are explained by larger contact area between the fish and ice particles but also by the fact that the temperature difference between the chilling medium and the fish is larger in the slurry ice case. Similar studies have been done with plaice (Paul, 1998). The fish was cooled in boxes and the results, presented in Figure 2, show that the time required to chill the plaice below 2 °C was more than three times shorter for the slurry ice than for the flake ice. Other similar studies (e.g. Egolf el at., 2005) also show a superior cooling rate of slurry ice compared to flake ice for chilling fish. Figure 1. Cooling of cod using flake ice and slurry ice (Davies, 2005). Figure 2. Cooling of plaice using flake ice and slurry ice (Paul, 1998). Comparison between different slurry ice-and liquid ice types was not found in the literature. Neither has much been published about the cooling capacity of the different ice media. Therefore it was considered useful to include more than one type of slurry ice in the model studies in the present study and not only investigate cooling, but also the maintenance of low temperature during storage.