Industry measures and presents the amount of glazing as the percentage from the glazed product that is actually glazing water. Although this value can be important when defining the product price, it may be deceiving regarding the ability to protect the product during frozen storage. If glazing works as a barrier to separate the frozen fish from the cold air, the critical parameter should be its thickness. In opposition to the percentage of glazing, this value is independent of the kind of product (or its size and shape) and will clearly indicate the capacity of glazing to protect any product according to a set of storage conditions (e.g. storage temperature, temperature fluctuations, and storage period).
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Contrary to what many may think, glazing loss is slow especially at low temperatures and when temperature fluctuations are avoided. When salmon was stored during 37 weeks in an industrial freezing chamber at -21.4 ± 1.6 degrees Celsius (-6.5 ± 2.9 degrees Fahrenheit), only 7.1 percent of it glazing was lost at the end of the experiment. But when the product was stored at -5.0 ± 0.6 degrees Celsius (23.0 ± 1.1 degrees Fahrenheit), a similar percentage of glazing was lost (6.9 percent) just after seven weeks. At the end of the research period (14 weeks), it reached a loss as high as 17.1 percent. When a chitosan coating (0.5 percent weight/volume, or w/v) was used to protect the product, the improvement at the end of the experiments was noteworthy, reducing the amount of loss to about half of the obtained with water glazing.
Although glazing may not be uniformly distributed on the product (especially in corners it can be thinner), it is possible to assume that with such low glazing thickness losses after 37 weeks at usual storage temperatures, the product should be safe from exposure to cold air during the typical shelf life period (52 to 104 weeks). As mentioned before, the only way to guarantee that the product is protected is to think in terms of thickness and not in percentage of glazing. To better study this problem it is necessary to understand the correlation between glazing percentage and its thickness and how the variables of glazing application affect its initial value.
Glazing Thickness and Variables
It is empirical and of common sense that the amount of glaze that is formed when a frozen fish product is immersed in a cold solution is dependent on the temperatures of the product and of the solution, the immersion time, and the product itself. More challenging are answers to questions like: How each variable impacts the amount of glaze? What are the limits to glazing uptake? How glazing uptake translates into glazing thickness?
These issues started to be addressed in recent research. When salmon at -25 degrees Celsius (-13 degrees Fahrenheit) was dipped in water at 0.5 degrees Celsius (32.9 degrees Fahrenheit) during 10, 20, 30, 40, 50, and 60 seconds, the coating thickness obtained increased between 0.57 millimeters (mm) for 10 second dipping and 0.84 mm for 60 second dipping. Raising the temperature of the salmon in 10 degrees Celsius (18 degrees Fahrenheit) resulted in an average reduction of the glazing thickness of 27 percent. Likewise, when water temperature was raised by 2.0 degrees Celsius (3.6 degrees Fahrenheit), the thickness was also reduced, but only by 13.6 percent (on average). This last experiment clearly showed that the reduction was greater for 10 second and 20 second dipping times, where on average the reduction was 23.1 percent; the longer dipping times (30 to 60 seconds) had only 8.8 percent average reduction.