Effects Of Cryoprotectants On Microwave Thawing Characteristics Of Minced Antarctic Krill(Euphausia Superba)

Antarctic krill is rich in high quality protein.Krill protein is considered as a good source of human protein because of its good amino acid composition and high biological value.However,due to its high content of fluoride and high activity of endogenous enzymes,Antarctic krill must be processed before eating.Rapid and volumetric heating of microwave energy can help overcome the problems of slow heat transfer rate and long heat treatment time in conventional heat treatment process,but there are also some phenomena of uneven heating and local overheating in microwave heating.Based on the above problems,this study mainly focused on the following aspects:the effects of freeze-thaw cycles and cryoprotectants on the quality of minced Antarctic krill after microwave thawing;the effects of cryoprotectants on dielectric properties of minced Antarctic krill during microwave thawing;the effects of different cryoprotectants on temperature distribution during microwave thawing were investigated,and the coupled model was established by COMSOL Multiphysics software for temperature distribution.1.In this study,minced Antarctic krill was studied.The effects of freezing and thawing cycles on changes in the content of adenosine triphosphate(ATP)and related compounds and free amino acids in minced Antarctic krill were investigated.The fresh/bitter ratio and K value of minced Antarctic krill were calculated to determine the optimal freezing-thawing cycle times.The effects of different concentrations(0%,3%,5%,7%w/w)of trehalose,sorbitol and glycerol aqueous solutions(1:1 v/v)on quality of minced Antarctic krill after microwave thawing were studied based on pH value,drip loss rate,texture characteristics,color(L~*)and related indicators of protein denaturation(myofibrillar protein content and total sulfhydryl content).Results showed that with increasing freezing and thawing cycle times,the degradation of ATP and related compounds increased rapidly,the content of umami nucleotides gradually decreased,and the content of bitter nucleotides gradually increased.The flavor/bitter ratio was 3.32at the first freeze-thaw cycle,and then after two freeze-thaw cycles the flavor/bitter ratio decreased significantly(P<0.05).At the same time,the K value increased rapidly,which indicated that the freshness had dropped sharply.After a third freeze-thaw cycle,the K value increased significantly(P<0.05).The K value was 86.88±3.58%at the fourth freeze-thaw cycle,meaning that the minced Antarctic krill was no longer fresh.After the fourth freeze-thaw cycle,the K value did not significantly change.Therefore,trehalose and sorbitol could delay the increase of pH value and drip loss rate effectively,maintain the texture and color better and delay the decrease of myofibrillar protein content and total sulfhydryl content.Moreover,the addition of 5%trehalose can inhibit the protein denaturation to a maximum extent.The addition of glycerol aqueous solution has a negative effect on texture and color after microwave thawing.2.The dielectric properties of minced Antarctic krill during microwave thawing were measured at 300 to 3000 MHz and from-20 to 5°C.The effects of trehalose,sorbitol and glycerol aqueous solutions(1:1 v/v)with different concentrations(0%,3%,5%,7%w/w)on dielectric properties were studied.The results showed that the dielectric properties of minced Antarctic krill gradually decreased with the increase of frequency in the frequency range of 300-3000 MHz.The change of dielectric properties in the phase transition region was significantly higher than that in other temperature ranges,and there was no significant difference in dielectric constant and dielectric loss rate above 0°C.The dielectric properties of minced Antarctic krill were significantly improved by adding more than 5%trehalose and sorbitol(P<0.05),but there was no significant difference between adding 5%and 7%cryoprotectants(P>0.05).The dielectric properties of minced Antarctic krill were negatively affected by the addition of glycerol aqueous solution.The dielectric constant of minced Antarctic krill decreased with the increase of glycerol solution concentration(P<0.05).In addition,the dielectric constant of minced Antarctic krill with and without cryoprotectants was compared.The dielectric properties of minced Antarctic krill with trehalose showed an inflection point at-3°C,while that with sorbitol was 0°C.The dielectric constant and dielectric loss of minced Antarctic krill reached the maximum at 0°C.3.In this study,the effects of 5%trehalose and 5%sorbitol on the thawing time,dielectric properties and penetration depth of minced Antarctic krill during microwave thawing were investigated,and the changes of temperature distribution at different cryoprotectants and cross sections were investigated.The electromagnetic heat transfer coupling model of temperature distribution was established by COMSOL Multiphysics software.The results showed that the addition of trehalose and sorbitol prolonged the thawing time and decreased the penetration depth of minced Antarctic krill.The thawing time of minced Antarctic krill reached the maximum of 51 s after adding 5%trehalose.The addition of 5%trehalose can effectively improve the temperature distribution of the sample.The temperature distribution at different cross sections of the sample is bottom>central cross section>top.The predicted transient temperature distribution is consistent with the measured temperature distribution.