## 4 Discussion
The right combination of these two factors (Figure 3a) in the superchilling process can provide a minimal rate of formation of TVB-N in the fish during its storage [35]. The air temperature of \(-20\)\({}^{\circ}\)C and the airflow velocity of 3 m s\({}^{-1}\) could result in the formation of relatively large ice crystals inside the cells. They become the main cause of future denaturation of muscle proteins [38]. The established changes in sensory characteristics and microbiological parameters were in good agreement with the data obtained on the indicator of fish freshness--TVB-N. According to obtained results presented in Figures 2b,c and 3b,c, the airflow velocity (v) is most important for the TVB-N content, and the formation of TVC of microorganisms as the factor air temperature (T) is the second most influential factor. Similarly, [PERSON] and [PERSON] [39] supposed that the superchilling and storage of the Atlantic cod fillet at \(-1.1\)\({}^{\circ}\)C extends the shelf life to about 20 days as the formation of ice crystals in the muscle tissue is negligible. Decreasing the temperature to \(-2.2\)\({}^{\circ}\)C keeps the fish at an acceptable freshness until 26 days, and the formation of ice crystals is more accelerated but does not exceed the acceptable limit. By decreasing the temperature to \(-2.8\)\({}^{\circ}\)C, the shelf life is extended to 35 days, but damage caused by ice crystals makes the cod unfit for filleting. The airflow velocity, along with the air temperature, plays a major role in optimizing the regime of superchilling from the aspect of the sensory characteristics of the grilled golden rainbow trout (Figure 2c). The packaging material with a thickness of 80 \(\upmu\)m retained fish freshness, preventing the tender skin of the golden rainbow trout from drying out under the influence of the relatively high cooling airflow velocity. At the same time, high cooling airflow velocity allows for a reduction in the period of time required to reach the desired temperature in the center of the fish. Therefore, shorter superchilling time preserves the structure of the fish muscle tissue, too. That is due to the formation of many in number and small in diameter ice crystals in fish musculature, which cannot harm and physically damage fish flesh [40]. The results of the sensory analysis are in agreement with the data obtained of the indicators, TVB-N, and biogenic amines, as well as with the TVC estimated in the superchilled golden rainbow trout. Our results are similar to those of [PERSON] et al. [41], who studied rainbow trout fillets, and those of [PERSON] and [PERSON] [42], who studied cod fillets (_Gadus mortua_) stored at \(-2.2\)\({}^{\circ}\)C.  
Different regimes of superchilling result in a different quality of golden rainbow trout after 21 days of storage from \(-2\) to \(-3\)\({}^{\circ}\)C. The results presented above (Figure 3) allow us to conclude that the change in each of the three analyzed variable factors affects differently the quality parameters of the fish. It was shown that the most significant influence of the variable factors on the sensory, physicochemical, and microbiological quality parameters turns out to be the air temperature (T), followed by the airflow velocity (v). The temperature intervals of superchilled storage are the main step to preserve muscle structure and minimize drip loss and muscle softening, which causes deterioration in the sensory characteristics of fish and accelerates the growth of surface microorganisms [43]. The process of ultra-fast air superchilling is very aggressive to the tender appearance of golden trout, so packaging, air velocity, and temperature play major roles in the deterioration of the initial qualitative characteristics, which lead to lower product quality during storage due to the more recently occurred proteolytic degradation. Small variations in temperature can cause large variations in the formation and size of ice crystals, which can lead to bigger liquid loss due to the freeze denaturation of muscle proteins [42].