The degenerative effect of temperature fluctuations during storage time is a critical condition that needs to be quantitatively characterized in products where drip losses are appreciable. In this work, real storage conditions were reproduced using freezers modified to cause 3 levels of temperature fluctuation (± 0, ± 3, ± 5; ± 7) during storage of Tilapia (Oreochromis sp), at temperature of -18 °C. The fast frozen tilapia muscle (freezing cabinet) was chosen to quantify the growth of ice crystals according to temperature fluctuations. The identification of crystals in the optical microscope as well as histological treatments and measurements using specific software has shown that the growth of ice crystals in the first days of storage follows an asymptote, whose final value is conditioned only by the level of temperature fluctuations regardless of initial diameter, which begins storage. It has also been found that the growth of crystals formed during rapid freezing rapidly develops according to temperature fluctuations to which the product has been subjected. This work also identified statistically significant differences in the equivalent diameter of crystals formed at the four proposed levels of temperature fluctuation with significance level of p < 0.05.Keywords: freezing; recrystallization; cold chain; temperature oscillations.Practical Application: The use of household appliances such as vertical freezers during controlled trials is an advantageous strategy because it represents problems that the food industry faces dealing with temperature fluctuations during the cold chain. In addition, a combination of scanning electron microscopy and optical microscopy techniques was efficiently used in this study to quantify the growth of ice crystals (equivalent diameter) during storage time. This combined methodology is a viable alternative to analyze numerous samples where the use of electron microscopy becomes costly.
Empirical models can be used to represent the recrystallization process in frozen food as a simple strategy (limited by the complexity of the process). Anyway, the empirical model has a better fit when used within the range of experimental values from which they were generated. On this work, an empirical mathematical model derived from the Arrhenius equation was proposed, since in previous publications it was shown that there is a direct relation between the growth of ice crystals and the temperature oscillations that occur during the storage of frozen products. Equivalent diameter data of ice crystals obtained from the storage of frozen Tilapia analyzed in the optical microscope was used as a database for the formulation of the empirical model. The developed model was acceptable to predict ice crystal growth during recrystallization in frozen Tilapia samples and had the advantage of being simple and robust enough to estimate this growth in the flotation range from-18 to-11 °C After the first 30 days of storage. The average equivalent diameter (D eq) values predicted by the model indicated that the model provides a satisfactory description of the growth of the crystals with R 2 equal to 0.930.
There are several studies in the literature, that explain how the freezing rate determines the number, distribution and size of ice crystals in frozen foods; Meanwhile the degenerative effect of temperature oscillations over the shelf life becomes a critical condition that needs to be quantitatively characterized in products of high demand such as fish meat where losses by exudation are appreciable. To reproduce real storage conditions under controlled temperature fluctuations, it is possible to predict the behavior of recrystallization in the food during storage, as well as to subsidize technical information that helps to estimate the shelf life of these products. In this work, Tilapia (Oreochromis niloticus) was chosen as a case study to quantify the relationship between the development of ice crystals and different levels of temperature oscillation.The measurements performed on the micrographs showed that the growth of the crystals still in the first days of storage follows an asymptote whose final value is conditioned only by the level of temperature oscillation imposed on the product, regardless of the initial diameter with which the stock is started. It has also been observed that the growth of the crystals formed during the rapid freezing also evolves rapidly, due to the temperature fluctuations to which they were subjected. This behavior disagrees with some authors who present the theory of a final boundary diameter. In this work, this single level was not identified, but it was verified that the growth of the crystals during the storage reaches different final limit diameters. Thus, statistically significant differences were demonstrated between the four levels of oscillation proposed in this work with a level of significance of p <0.05. Additionally, an empirical model was developed to predict the growth of ice crystals during recrystallization in the samples. The correlation coefficient R 2 = 0.917 indicated that the model provides a satisfactory description of crystal growth when evaluated at temperatures from -18 to -11 °C.
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