Seafood products are made from an important natural resource with a steadily growing consumption over the last several decades. According to a 2014 Food and Agriculture Organization report, the supply of fish food increased in the last five decades at an average annual rate of 3.2 percent, even outpacing population growth worldwide (1.6 percent). Since the 1990s this growth has been supported mainly by aquaculture production, which had a 6.2 percent annual growth rate between 2000 and 2012. Another recent trend has been the increasing demand for processed products, particularly frozen seafood products. In 2012, frozen seafood products accounted for 54 percent of the total processed fish for human consumption and for 29 percent of the total seafood market for edible purposes.
According to the Code of Practice for Fish and Fishery Products, glazing is the application of a protective layer of ice formed at the surface of a frozen product by spraying it with, or dipping it into, clean seawater, potable water, or potable water with approved additives, as appropriate. When frozen fish are to be stored, depending on packaging, they are exposed more or less to the cold air of the freezing chamber. Without glazing, the oxygen of the air will react with the fats (turning them rancid) and drying and dehydration of the product will not be prevented (which may lead to freezer burn). In addition, glazing is a physical barrier that protects the product from damage during production, packaging, transport, and retail.
The most common method of glazing is dipping, where frozen seafood products are immersed in a tank filled with cold water for a period of time, creating an ice coat that completely surrounds the product. Glazing carried out by spraying uses proper equipment to spray glazing solution over the product. Although dipping is a relatively simple and cheaper method, with more production capacity than spraying, it is more difficult to control the amount and uniformity of glaze. The amount of glaze formed is dependent on factors such as: a) product and glazing solution temperature; b) size, shape, and surface area of the product; and c) glazing time.
Glazing solution is commonly used at a temperature close to the freezing point. The amount of glazing added to the product should be considered wisely by manufactures in order to effectively guarantee the protection of the product, without being perceived by consumers as a method of deceiving and/or improving manufacturers’ profits.
Glaze Lost During Storage
Until now glaze has been viewed as a substance with the main function of acting as a barrier to protect the product surface from exposure to the (cold) environment. With that in mind, the amount of glaze should be defined according to the time it takes for it to be reduced during storage until the product’s surface is exposed to the cold temperatures. However, that is not the case since there is no information available regarding the amount of glaze needed to guarantee that. To address this goal a paradigm of the frozen seafood industry must be broken, but first let’s discuss how industry currently measures glazing.
It is common to read expressions like over glazing or excessive glazing. Unless a threshold is defined between client and manufacturer, these terms only reflect a subjective judgment since there is no threshold defined for the amount of glaze that is necessary to protect the product. On the other hand, when organizations intentionally mislabel the product weight in the pursuit of financial profit, it should be addressed as fraudulent behavior and not as excessive glazing.
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).
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.
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.
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.
Depending on the conditions of each facility and the time that it would take the product to return below -18 degrees Celsius (-0.4 degrees Fahrenheit) in all points, and taking in consideration the cumulative effect of future temperature oscillations until the product reaches the final consumer, I believe that organizations should address this issue and propose the adoption of the Safe Dipping Time (SDT) concept. SDT should be defined by each organization according to its operational conditions (e.g. temperature of product and glazing solution, production room temperature, time to product return to frozen storage and its temperature) and will represent the maximum time that a product can be dipped without the temperature raise constituting a hazard.
Antimicrobial Activity of Chitosan Solution
Freezing is a commonly used method for long term preservation since it is well known for the capacity for inhibiting microbial growth and slowing down enzymatic activity. The Fish and Fishery Products Hazards and Controls Guidance – Fourth Edition presents in its Appendix 4 a list of common pathogens in the fish industry and temperatures that enable growth and toxin formation. According to the guide, the two pathogens that are able to grow at the lowest temperatures are Yersinia enterocolitica and Listeria monocytogenes at -1.3 degrees Celsius (29.7 degrees Fahrenheit) and -0.4 degrees Celsius (31.3 degrees Fahrenheit), respectively. These temperatures are much higher than the -18 degrees Celsius (-0.4 degrees Fahrenheit recommended for storage of frozen fish and therefore, during frozen storage, temperature is the main factor to inhibit microbiological growth. In fact, in an experiment where salmon was glazed with water or chitosan solution (0.5 percent w/v) and stored at -5 degrees Celsius (23 degrees Fahrenheit), the total viable count (TVC) did not show any trend during the 14 week experiment and fluctuated between 5.0 X 102 and 1.0 X 104 colony-forming units/gram for both glazing solutions tested.
Nevertheless, as soon as the temperature rises, as is the case when fish is thawed at home before cooking, microorganisms can growth again, start spoiling the product, or even produce toxins depending on the time/temperature of exposure. The use of water glazing will not affect in any way microbial growth but the use to a 1.5 percent (w/v) chitosan solution has been proven to reduce the TVC in salmon stored at -22 degrees Celsius (-7.6 degrees Fahrenheit) during 181 days. In this experiment, two different TVC evaluations were performed. Both were based on BS EN ISO 4833:2003 but one was done immediately after storage and the other after the product was kept in a refrigerator at 5.9 degrees Celsius (42.6 degrees Fahrenheit) during 24 hours, simulating conditions of home thawing. Table 1 presents TVC results for both experiments.
The results clearly show that using a chitosan solution to glaze the product can actively reduce the microbial contamination of the product and even help ensure that it is safe during a 24-hour thawing process.
Glazing has been used too long just as a mechanical barrier. At a time where safety, added-value products, and product differentiation are so important for consumers and therefore for the fish industry, organizations should revise the use of glazing, bringing new benefits to the product and consumers.
Reducing the amount of glazing to the thickness necessary to guarantee the protection from cold air and introducing substances that can guarantee a safer product, not only during storage time but also at consumers’ home, are paradigm changes that can shape the industry in the forthcoming years and increase the consumers’ confidence regarding frozen fish.
Soares is quality manager at Vanibru, Lda. He is also author of the book Food Safety in Seafood Industry: A Practical Guide for ISO 22000 and FSSC 22000 Implementation. Reach him at firstname.lastname@example.org or @foodsafetybooks.
AUTHOR’S NOTE: The author acknowledges Tânia Mendes, Marina Oliveira, Tiago Fernandes, and António Vicente for the support and endless discussions about the subjects presented in the article and to Adriana Machado for the assistance in preparing the article.
History of Glazing
The first U.S. patent describing a process to artificially freeze and preserve fish was published in 1861. In his patent, Enoch Piper claims the invention of a new and improved method of preserving fish that includes a 24-hour freezing process and suggests glazing the fish, by a dip in cold water, forming a coat of about 1/8 inch in thickness. After being glazed, the frozen fish is transferred to an insulated chamber and cooled with a freezing mixture within vertical metallic tubes to keep the fish frozen until used.
In 1902, D. W. Davis patented a process of freezing in a rectangular pan covered with a lid and packed in an ice-and-salt mixture for freezing. In his patent, Davis describes that before placing in cold storage, the frozen cakes are to be immersed and held submerged for several minutes in a body of water, which absolutely must be at a temperature of 32 degrees Fahrenheit.
Later, in 1926, the Appendix VIII to the report of the U. S. Commissioner of Fisheries mentions that fish must be frozen in metal pans and then warmed slightly by spraying or immersing in cold water to loosen them. The report states that glazing tanks are commonly made from wooden or concrete, provided with a movable wooden platform suspended by ropes to a windlass by which it was moved up and down the tank.
Today, the glazing of frozen fish has undergone significant technological improvements, especially in terms of the equipment used (and the materials of which they are made). However, regarding the type and function of the glazing solution used, the industry is still currently stuck in the 19th century.—N.S.