The use of a 1.5 percent w/v chitosan solution to glaze frozen salmon resulted in a much thicker protective coat than when only water was used. For example, when the same conditions of product temperature (-25 degrees Celsius/-13 degrees Fahrenheit) and solution temperature (2.5 degrees Celsius/36.5 degrees Fahrenheit) were used, the thickness of the water glazing (0.78 mm) at the end of the experiment (i.e. after 60 second dipping time) was thinner than the one obtained with chitosan solution only after 10 second dipping time (0.83 mm). As seen in Chart 1, the thickness of the chitosan solution glazing after 60 seconds was 1.37 mm, 73 percent higher than the one obtained with water.
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Another important result was obtained by the comparison between the thickness of the glazing when salmon at -25 degrees Celsius (-13 degrees Fahrenheit) was dipped in water at 0.5 degrees Celsius (32.9 degrees Fahrenheit) with salmon at -15 degrees Celsius (5 degrees Fahrenheit) dipped in chitosan solution at 8 degrees Celsius (46.4 degrees Fahrenheit) as shown in Chart 2.
Chart 2 clearly shows that after a 40 second dipping time the results obtained with the chitosan solution are equal or better than the ones obtained with traditional water glazing. This is quite relevant since, even if only thinking in glazing as a barrier to prevent the contact of the product with cold air (which, as seen below, is not the case for chitosan glazing), the same thickness can be obtained in a much less demanding energy setup (higher glazing solution and product temperature), leading to direct energy savings or even making some equipment unnecessary.
The results obtained also confirmed that after a period of time (depending on the setup) the thickness of glazing stops increasing and even decreases. This phenomenon was clear when glazing water was maintained at 2.5 degrees Celsius (36.5 degrees Fahrenheit). After a 40 second dipping time, the thickness stopped increasing when salmon at -25 degrees Celsius (-13 degrees Fahrenheit) and -20 degrees Celsius (-4 degrees Fahrenheit) was used and started reducing for salmon temperature of -15 degrees Celsius (5 degrees Fahrenheit). In the case of glazing with chitosan solution, this phenomenon was not observed in the conditions defined for the experiments even when 60 seconds of dipping time was applied.
Temperature Profile and Safe Dipping Time Concept
The glazing thickness can be explained by the conduction of heat from the solution to the frozen fish, leading to a decrease in the temperature of the solution (changing phase) and a corresponding increase on that of the product. The impact of raised temperature in the product is often neglected but it is directly dependent on the product, dipping time, and product/glazing solution temperature. Immediately after immersion, a temperature profile will be established inside the product, with a higher temperature close to the surface and lowering as it moves to the center of the product. These temperatures can be predicted by the use of the second law of Fourier. Chart 3 presents the temperature profile when salmon at -15 degrees Celsius (5 degrees Fahrenheit) is dipped in chitosan solution at 8 degrees Celsius (46.4 degrees Fahrenheit), clearly indicating that after 50 and 60 seconds the product is in all its volume above or very close to -5 degrees Celsius (23 degrees Fahrenheit). This is particularly relevant since Vibrio spp. is a common microorganism in seafood reported to grow above this temperature.