A Study of Ultra-Low Temperature Application for Fish Freezing and Storing
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This work was performed in order to get information about the economic benefit of maintaining the quality and increasing the shelf-life of fish using ultra-low temperature (between –30.0 and –85.0 °C). The scope of the study was the validation of the ultra-low temperature application for the industrial freezing and storage of fish. The highest quality of the fish during long term storage can be achieved when all the compounds of the fish are immobilized in a crystal and/or in a glassy state (usually below –56.0 °C in the literature). Thus, the temperatures, at which the immobilization of fats, proteins and water occur, are sufficient for maintenance of a high quality shelf-life during long-term storage. The analysis of the fish stability at different temperatures was achieved using two methods:the long-term storage experiments with analyses of quality parameters (fat oxidation, colour, gaping score, drip loss, etc.) after certain periods of time. This method was conventional but consumed time and resources; the DSC analysis of the thermal properties of the fish at different temperatures. This knowledge of the physical state of the fish and its compounds (freezing point and end of freezing, glass transition, unfreezable water, liquid fraction of fat, etc.) was used for the estimation of the deteriorative reactions in fish tissues during long-term storage. Thus, it can be concluded that the stability of the quality can be achieved.The Atlantic salmon and Atlantic herring fillets were used for long-term storage experiments with analyses of their quality parameters. It was found that decreasing the temperature does not correlate with the inhibition of the deteriorative reaction in fish tissues in the temperature range between –45.0 and –60.0 °C (for Atlantic salmon). The fat oxidation was observed at approximately the same rate at –60.0 °C storage temperature, as at –45.0 °C. The effect of packaging material with the oxygen barrier was significant, especially for Herring fillets. The packaging material with medium barrier properties showed good preservation of fish fat oxidation during storage even at –25.0 °C. The concentration of oxygen in the fish influenced the oxidation significantly. It was concluded that a presence of a fluid phase was responsible for the occurrence of the oxidative reaction (based on the statement that oxidation is a chain reaction). At the same time, the other quality parameters did not depend significantly on the decreasing of the temperature in the range between –25.0 and –60.0 °C. This study showed that knowledge of the physical state of the product at different temperatures will be important for further quality prediction and experiment design in future studies. The thermal transitions were investigated for five commercial fish and their oils. This study revealed that all fish had a significant amount of unfreezable water (from 5.1 to 8.6 % wet basis). The ice formation was stopped at a temperature below –33.5 °C due to the occurrence of the maximal-freeze concentration (approximately 75.0 % of solids). The glass transition appeared in the protein-unfreezable water system in the temperature range between –86.0 and –68.0 °C. The liquid fraction associated with proteins will be immobilized below this temperature range. Thus, two levels of fish protein stability were found. The first level was below the “end of freezing”, when a significant amount of ice was formed, but unfrozen water existed in the system. Thus the deteriorative reactions were inhibited, but were still possible. When the unfrozen solution transitioned into the glassy state, the second quality level situated below the glass transition level occurred yielding the highest stability of the protein-water system. The storage temperature should be chosen slightly below the “end of freezing” or glass transition region. The significant decreasing of the temperature below the “end of freezing” region will not give economic benefits, because the quality alteration for that range will be insignificant in comparison with the running costs of the storage process. It was also found that the glass transition caused significant changes in the quality of fish muscles. A brittleness was detected at temperatures below –86.0 °C, which makes the industrial storage of fish more complicated. Several refrigeration stages can be required for decreasing the temperature of the fish to such levels. The oil extracted from the fish tissues showed a glass transition event in the temperature range between –115.0 and –120.0 °C, this event was investigated. It was found that triacylglycerides (TAGs) which contain essential fatty acids (EPA, DHA and some others) do not freeze, but show a glass transition in the temperature range between –105.0 and –112.0 °C. Thus, a high amount of fish oil will be in a liquid state at temperatures below –25.0 °C, and the chain reactions will be possible. The dependence between the heat of fusion of pure TAGs and their melting temperatures was used for the determination of the liquid fraction in fish oil at different temperatures. The results showed that the liquid fraction at –25.0 and –45.0 °C was much more than 50.0 %. The oxidative products can be formed even when the protein-water solution will be in a glassy state (below –86.0 °C). The complete immobilization of fish oil will be achieved below the glass transition region. Thus, a proper packaging material is important for the preservation of fish quality, when the fish is stored at temperatures above the TAG’s glass transition temperature. The outcome of the study was the following: the maintenance of a high quality of fish is possible at temperatures slightly below –33.5 °C, when the free water is completely frozen. The industrial application of ultra-low temperatures in the range between –69.0 and –35.0 °C is not necessary for long-term storage of fish, because the amount of unfrozen water is constant and significant amount of fat is in a liquid state in this temperature range. The inhibition of fat oxidation (in the case of fatty fish) can be achieved by the use of packaging material with medium or high oxygen barrier than by the decreasing of the temperature. Such operations will help to increase the shelf-life significantly. For example, in the case of herring the shelf-life was extended up to 2 years.