Luo Yongkang

Effect of freeze-thaw cycles on the quality of silver carp(Hypophthalmichthys molitrix)

We evaluated the changes in silver carp(Hypophthalmichthys molitrix) tissue quality during freezing and thawing processes.Silver carp is one of the primary freshwater fish species in China.With high nutritional values,fast growth rate,and high yield,this species forms the basis of an important industry.Because of the nega-tive effects associated with microbiological growth and biochemical processes,frozen storage is widely used to preserve fish and fish products.However,the melting and reformation of ice crystals causes osmotic removal of water,mechanical damage,and denaturation of proteins.The extent of the loss in tissue quality is dependant on the rate of freezing,thawing methods,storage temperature and temperature fluctuations during storage,transportation,retail display,and consumption.Repeated freezing and thawing processes are common in storage and at retail outlets,in homes,and restaurants.Despite this,the effects of freezing/thawing on changes of color,muscle texture and protein physicochemistry have not been fully investigated.We stored fresh silver carp at –18?C for 10 days.After being thawed by flowing water,the thawed carp were refrozen at the same temperature for another 5 days.This freeze-thaw cycle was repeated five times.After the first,third,and fifth cycles,we randomly selected three silver carp and measured changes in thawing loss,cooking loss,hardness,chewiness,resilience,thiobarbituric acid(TBA),L* value,b* value,salt soluble protein(SSP) content,Ca2+-ATPase activity,total sulfhydryl groups(SH) content,surface hydrophobicity,and sensory scores.During the first freeze-thaw cycle,hardness,chewiness,resilience,salt soluble protein(SSP) content,and Ca2+-ATPase activity decreased significantly(P0.05) whereas b* and surface hydrophobicity increased significantly(P0.05).After the third freeze-thaw cycle,thawing loss,L*,and TBA levels significantly increased(P0.05).Total SH content decreased significantly(P0.05) after the fifth freeze-thaw cycle.In contrast,there was no significant change(P0.05) in cooking loss following five freeze-thaw cycles.After the fifth freeze-thaw cycle,the sensory scores were 47,which were unacceptable.Our results confirm that the freeze-thaw process causes thawing loss,discoloration,softening of muscle tissue,lipid oxidation,de-crease in SSP content,and protein conformational changes.These changes have a detrimental effect on the quality of silver carp tissue.So it is important to prevent fluctuation of temperature during storage,transportation,and retail to avoid the negative effects of freeze–thaw cycles on silver carp tissue quality.