Study On Liquid Nitrogen Fast Freezing And Ultrasound-assisted Thawing On Quality Of Bighead Carp Fillets

With the increase of freshwater fish production year by year,the phenomenon of unbalanced supply and demand begin to appear.In order to solve the problem of overproduction,the industry begin to freeze the freshwater fish.Bighead carp(Aristichthys nobilis),one of the four major Chinese carp,is an important commercial freshwater fish.Its head is very popular with consumers,while its body utilization is quite low,which have promoted the development of frozen cut bighead carp market.The main factors affecting the quality of frozen fish are the freezing rate and the thawing process.Liquid nitrogen fast freezing has been widely used in the industrial production of food,but some studies have shown that too fast freezing rate may cause the mechanical cracking of food with high water content.The traditional thawing methods are inefficient and easy to cause quality deterioration.Ultrasound-assisted thawing can avoid the above problems to a certain extent.Therefore,in this article,appropriate liquid nitrogen fast-freezing rate was obtained,the effects of different thawing methods on slow/fast freezing bighead carp fillets were compared.The dehydration of fast-frozen bighead carp fillets after different thawing conditions was observed,and finally optimized the ultrasound-assisted thawing process by response surface solving the dehydration problem.The main research contents and results are as follows:(1)The effects of different liquid nitrogen fast freezing rates on the quality of bighead carp fillets were studied.By controlling the environment temperature of the fast-freezer(-60,-80,-100,-120 ?),the fast freezing rates were adjusted to0.37 ?/min,0.81 ?/min,1.34 ?/min,1.79 ?/min,respectively.As the fast freezing rate increased,the phase transition time was shortened,and the water holding capacity becomes stronger(closer to the control),and when the quick freezing rate increased to1.79 ?/min,the changes were not significant(p > 0.05).For the microstructure and ice crystal morphology,the acceleration of the fast-freezing rate caused the ice crystal size(ice crystal area,circumference,equivalent diameter)and the degree of tissue damage to decrease significantly(p < 0.05),and reached the minimum at 1.34 ?/min.However,the effects of freezing rates on the pH value and salt-soluble protein content of bighead carp fillets were not significant(p > 0.05).In addition,combined with the results of the Pearson correlation analysis,the fast freezing rates were significantly related to the water holding capacity,and the area,equivalent diameter and circumference of the ice crystals of bighead carp fillets.The fast freezing rates were irrelated to the roundness and elongation of ice crystals,pH,and salt-soluble protein content.The above results showed that 1.34 ?/min(the environment temperature of the fast-freezer is-100 ?)is the most suitable fast freezing rate for bighead carp fillets.(2)Comparison of air thawing,water immersion thawing and ultrasound-assisted thawing(25 ?)on the quality were studied.Ultrasound can significantly shorten the thawing process of bighead carp fillets without causing additional quality damage.Fast freezing/ultrasound-assisted thawing could avoid the changes of color,pH values and fat oxidation of bighead carp fillets during the freezing/thawing process to a certain extent.In addition,freezing and thawing treatments would not affect the protein primary structure of bighead carp fillets,and different freezing and thawing treatments had little effect on volatile flavor components of bighead carp fillets.The results showed that although the slow-frozen bighead carp fillets showed slight tissue damage,poor texture and water holding capacity,a large amount of dehydration occurred of quick-frozen bighead carp fillets after water immersion thawing and ultrasoundassisted thawing,resulting in texture deterioration and serious tissue damage.Therefore,fast freezing and ultrasound-assisted thawing showed the advantages in the biochemical properties for bighead carp fillets.The fast-frozen samples after air thawing showed good water holding capacity and integrity of tissue.Fast-frozen bighead carp fillets might be dehydrated(beyond slow-frozen samples)after thawing,and the occurrence of this phenomenon was closely related to the thawing conditions.(3)In order to reduce the thawing rate and to solve the dehydration problem of fast freezing bighead carp fillets,a response surface optimization experiment was carried out on the ultrasound-assisted thawing conditions.By analyzing the thawing loss,microstructure and deformation degree of different temperatures and ultrasonic intensities,the temperature range of 5-15 ? and the ultrasound intensity range of0.041-0.108 W/m L as the response surface design were determined.The thawing loss and thawing time were response values,and then the response surface regression fitting on the experimental data were performed.With the thawing loss ? 3.5%(the thawing loss of slow-frozen bighead fish fillets)as the screening condition,the process conditions with relatively high thawing efficiency were selected: temperature of 7 ?,ultrasound intensity of 0.082 W/m L,and experimental verification was carried out.Finally,by comparing the moisture distribution and NMR imaging between the thawed samples by optimal process condition and the dehydrated sample,the optimal thawing condition obtained by the response surface method was verified again to solve the dehydration problem of the fast-frozen bighead carp fillets.