Using pulsed magnetic fields to improve the quality of frozen blueberry: A bio-impedance approach

Freezing assisted by pulsed magnetic field (PMF) is an emerging technology that could be used in food preservation. This paper evaluates how PMF-assisted freezing affects the preservation of blueberry and its bioactive compounds. Blueberries were subjected to 8 PMF-assisted freezing treatments: T1 (36.8 mT/30 Hz), T2 (36.8 mT/60 Hz), T3 (36.8 mT/90 Hz), T4 (36.8 mT/20 Hz), T5 (44.7 mT/30 Hz), T6 (44.7 mT/60 Hz), T7 (44.7 mT/90 Hz), and T8 (44.7 mT/120 Hz). In treatment T9 (control), the blueberries were subjected to conventional freezing to −35 °C; T10 represents fresh blueberries. Compared to conventional freezing (T9), PMF-assisted freezing (T1 to T8) gave different parameters of temperature, nucleation time, degree of supercooling, and phase change time. The parameters achieved with T7 evidenced better behavior: smaller crystals were formed, allowing the cellular structure to be preserved, as confirmed by the electrical parameters (Re, Ri, and Cm) obtained from electrical impedance data. Moreover, T7 preserved anthocyanins and polyphenols, promoting the highest antioxidant capacity among the blueberries subjected to PMF-assisted freezing. Meanwhile, conventional freezing and PMF-assisted freezing reduced the polyphenol oxidase and peroxidase activities. In conclusion, at the laboratory level, PMF-assisted freezing preserves the blueberry cellular structures and bioactive compounds.