| Fish meat is frequently frozen to preserve because it is rich in nutrients such as water and protein,and its muscular tissue is delicate and easily deteriorates.As an essential step prior to processing frozen fish,thawing has a significant effect on the ultimate quality of frozen fish.However,it is noted that existing conventional thawing methods have some drawbacks,including extended thawing time,thawing loss,color deterioration,protein denaturation,and lipid oxidation,and etc.Therefore,this study used quick-frozen small yellow croaker as the raw material and investigated the influence of different ultrasonic frequencies and working modes on the quality of the small yellow croaker.The influence mechanism of ultrasonic thawing on the quality change of quick-frozen small yellow croaker was systematically investigated,including the impact on the water migration law of fish meat,the structure and functional characteristics of myofibrillar proteins(MPs)and the molecular level of protein.The main research contents and results are as follows:(1)The effect of ultrasonic thawing temperature on the quality characteristics of quick-frozen small yellow croaker was studied.Employing energy consumption data,the optimum thawing temperature of quick-frozen small yellow croaker was selected.The results indicated that as the thawing temperature increased,the thawing time decreased.There was no significant difference in the color difference value(ΔE),texture index,thawing loss,cooking loss,total volatile basic nitrogen(TVB-N)value,thiobarbituric acid-reactive substance(TBARS)value,p H value and volatile components of fish meat after thawing at 6 ℃ and 10 ℃,and these results were close to the fresh sample(FS).Nevertheless,after thawing at 15 °C,the fish meat quality was determined to be the lowest.When quality and energy consumption indicators are considered,10 °C was determined to be the ideal thawing temperature for quick-frozen small yellow croaker.(2)The quick-frozen small yellow croaker was thawed by ultrasound with different frequency combinations and working modes.The frequency combination and working mode of ultrasonic thawing were optimized by quality evaluation indexes such as physical and chemical indexes,total number of colonies,microstructure and etc.The results indicated that in the mono-frequency ultrasonic(MU)thawing group,fish thawed at MU 20 k Hz exhibited the lowest thawing time with the highest quality.In the dualfrequency ultrasound thawing group,the fish thawed with dual-frequency sequential ultrasound(DSEU)20/40 k Hz were close to the FS group.Meanwhile,in the trifrequency ultrasound thawing group,the quality of the samples thawed by tri-frequency sequential ultrasound(TSEU)20/40/60 k Hz showed better results.Among them,the DSEU 20/40 k Hz was the best frequency combination for thawing quick-frozen small yellow croakers(ultrasonic power density 50 W/L,temperature 10 ℃).In comparison to fish thawed using traditional flowing water(FW),its hardness(796.53 g),elasticity(87.38%),recoverability(23.45%),and chewability(301.71%)were increased by22.57%,19.25%,29.57%,and 28.32%,respectively.The thawing time(1046.67 s),ΔE value(22.87),the thawing loss(4.76%),the cooking loss(5.01%),the TVB-N value(4.27 mg/100g),the TBARS value(0.22 mg MDA/kg),the conductivity value(1217.35μs/cm),and the total number of colonies(2.58 lg CFU/g)all decreased by 57.80%,37.62%,5.56%,54.58%,35.60%,42.10%,10.99% and 22.99%,respectively.Additionally,the results indicated that when the same frequency combination was used,the simultaneous ultrasonic mode resulted in a shorter thawing time and the sequential ultrasonic mode showed a better quality of the final products.(3)The effects of ultrasonic thawing with various frequency combinations and working modes on the water content of quick-frozen small yellow croaker were studied.Using low-field nuclear magnetic resonance technology,the possible mechanism of multi-frequency ultrasonic thawing on the quality of quick-frozen small yellow croaker was clarified from the perspective of water distribution.The results showed that in MU thawing group,the water holding capacity(WHC),water activity,and bound water content of fish meat thawed with MU 20 k Hz ultrasound were the most similar to FS.Multi-frequency sequential ultrasound thawed samples exhibit a greater WHC,more bound and fixed water,and a lower water activity than multi-frequency synchronous ultrasound thawed samples.Compared with other thawing groups,the samples thawed with DSEU 20/40 k Hz had the highest WHC,bound and fixed water content,which was more conducive to preserving the quality of quick-frozen small yellow croaker.(4)The effect of multi-frequency ultrasonic thawing on the MPs structure of quickfrozen small yellow croaker was studied by means of circular dichroism,Raman spectroscopy,fluorescence spectroscopy,scanning electron microscopy(SEM),differential scanning calorimetry and etc.Meanwhile,the possible mechanism of multifrequency ultrasonic thawing affecting the quick-frozen small yellow croaker quality has been revealed.The results indicated that when MPs were pretreated with DSEU20/40 k Hz,the primary structure(surface hydrophobicity,free amino acid content,and total sulfhydryl content),secondary structure,and tertiary structure were the most identical to FS.Moreover,the MPs pretreated with DSEU 20/40 k Hz exhibited flat surface,less aggregation and degradation and strong thermal stability.In general,the DSEU 20/40 k Hz thawing process efficiently improved the structural of MPs and the quality of quick-frozen small yellow croakers.(5)The effects of ultrasonic thawing with different frequency combinations on the degree of oxidation,aggregation,WHC and rheological properties of MPs of quickfrozen small yellow croaker were investigated.From the perspective of the MPs properties,the possible mechanism of the effect of multi-frequency ultrasonic thawing on the quality of quick-frozen small yellow croaker was clarified.The results indicated that the MPs obtained through DSEU 20/40 k Hz thawing had lower aggregation and oxidation degrees,higher WHC,and gel strength than the MPs obtained by MU 20 k Hz and TSEU 20/40/60 k Hz thawing.Additionally,the MPs generated using the DSEU20/40 k Hz thawing technique had the highest peak energy storage modulus,the best thermal gelation ability,and the lowest apparent viscosity,all of which were closest to the FS group.Therefore,by efficiently improving the functional properties of MPs,the DSEU 20/40 k Hz thawing led to a higher quality of thawed quick-frozen small yellow croaker.(6)Proteomics technology was used to investigate the possible mechanism underlying the influence of multi-frequency ultrasonic thawing on the quality of quickfrozen small yellow croakers.Gene Ontology(GO)enrichment analysis showed that the thawing by FW altered more GO function items than the thawing by DSEU 20/40 k Hz.The Kyoto Encyclopedia of Genes and Genomes(KEGG)pathway analysis revealed that the DSEU 20/40 k Hz thawing could decrease the concentration of some glycolytic enzymes in small yellow croaker and prevent the occurrence of glycolytic reactions and other catabolic processes in fish meat.Additionally,12 differentially abundant proteins(DAPs)were discovered to be highly correlated with the small yellow croaker quality indexes,which are predicted to serve as potential biomarkers for monitoring the quality of the quick-frozen small yellow croaker.The DAPs content in the DSEU 20/40 k Hz group was the closest to that in the FS group,implying that the small yellow croaker thawed by DSEU 20/40 k Hz thawing mode had better quality of than those thawed by FW thawing. |
