Influence of different freezing regimes on bioelectrical properties of Atlantic chub mackerel (Scomber colias) Sanja Vidacek, Helga Medic, Nives Marusic, Božena Surić and Tomislav Petrak Faculty of Food Technology and Biotechnology, University of Zagreb, Pierottijeva 6, Zagreb, Croatia corresponding author: Materials and Methods Results Conclusions The relationship between frequency and bioelectrical parameters follows the same pattern for all the fish species studied previously. Mann-Whitney U test shows that that the reactance is the only parameter that can differentiate fast frozen from slow frozen fish and double fast frozen from double slow frozen fish. The results of testing the differences among unfrozen, frozen and double frozen samples (ANOVA) show that the resistance measured at 500 and 1000 kHz, reactance measured between 0.1 and 15 kHz, moisture content and WHC are significantly different for different tissue conditions. Therefore, the reactance is the only variable that can distinguish fast frozen from slow frozen fish samples (at higher frequencies) and that may differentiate between double and single freezing cycle (at low frequencies) which supports our previous results on farmed fish. CHILLING (+4  C,12 hours) CUTTING THE EPAXIAL MUSCLE (10 x 2 x 1 cm) 1.group of samples UNFROZEN (CHILLED) 2. group of samples UNFROZEN (CHILLED) SLOW FREEZING (AIR,-18  C) FAST FREEZING (LIQUID N 2 ) FROZEN STORAGE (-18  C, 14 days) THAWING BY AIR (+4  C,12–16 hours) CLEANING, FILLETING STATISTICAL ANALYSIS (SPSS, 12.0) Confirmation of the normality of data EXPERIMENTAL PLAN Figure 1 Resistance (R) and reactance (X) spectra of slow frozen (a) and fast frozen (b) groups of Atlantic chub mackerel - Table 1 Differences between slow and fast frozen samples after one and two freezing cycles (Mann Whitney U test) Table 2. ANOVA among unfrozen, frozen and double frozen samples Bioelectrical measurements Resistance (R) and Reactance (X) measured by HPLCR Meter at 19 frequencies from kHz; constant current method Physical and chemical measurements Water holding capacity (WHC), water content, pH Repeated bioelectrical, physical and chemical measurements SLOW FREEZING (AIR,-18  C) FAST FREEZING (LIQUID N 2 ) Thirty (30) samples of Atlantic chub mackerel DIVIDING THE SAMPES Testing the difference between -fast and slow frozen samples - double fast and double slow frozen samples Mann Whitney U test Kolmogorov Smirnov test -unfrozen, frozen and double frozen samples ANOVA ParameterDifference Significance (P<0.05) R S f and F f No ( kHz) S fII and F fII No ( kHz) X S f and F f No ( kHz) Yes ( kHz) S fII and F fII No ( kHz) Yes ( kHz) Moisture, pH, WHC S f and F f No S fII and F fII No Parameter Unfrozen samples Frozen samples Double frozen samples Significance (P<0.05) R (0.1 kHz)656.7± ± ±92.1No R (1 kHz)579.9± ± ±75.1No R (50 kHz)553.6± ± ±73.1No R (500 kHz)512.7± ± ±73.6Yes R (1000 kHz)501.1± ± ±73.7Yes X (0.1 kHz)-424.3± ± ±50.2Yes X (1 kHz)-60.3± ± ±8.4Yes X(15 kHz)-14.5± ± ±1.5Yes Moisture content (%)75.1± ± ±1.4Yes WHC18.2± ± ±3.4Yes pH6.3±0.36.3±0.26.2±0.3No a b Introduction In recent years, bioelectrical properties of frozen-thawed fish have attracted much attention as possible indicators of freezing history. In our previous studies, the resistance and reactance of frozen-thawed sea bass and rainbow trout in a frequency range 100 Hz-1 MHz were measured. The results revealed that the reactance higher than 500 kHz could differentiate the fish with different freezing history. In this work we assessed whether frozen-thawed, commercially important wild fish Atlantic chub mackerel had similar bioelectrical properties as farmed species. Legend Sf-slow frozen S fII-double Sf F f – fast frozen F fII--double Ff SLOW FROZEN-THAWED SAMPLES DOUBLE SLOW FROZEN- THAWED SAMPLES DOUBLE FAST FROZEN- THAWED SAMPLES Repeated bioelectrical, physical and chemical measurements FROZEN STORAGE (-18  C, 14 days) THAWING BY AIR (+4  C,12–16 hours) FAST FROZEN-THAWED SAMPLES