The Changes Of Quality And The Influencing Factors Of Melanosis During Frozen Storage Of Shrimp(Metapenaeus Ensis)

Metapenaeus ensis are popular with consumers due to their delicious taste and high nutritional value.The fresh shrimp are difficult to be preserved because of their perishable nature,therefore,shrimp usually need to be frozen storage.At present,thegenerally used freezing temperature of household refrigerators is-18℃,which can meet the basic requirements of some consumers to prolong the shelf life of food.However,even during frozen storage,protein denaturation and oxidation,reduced water-holding capacity,melanosis and other phenomena easily occur in shrimp.Therefore,it is necessary for consumers who pursue high quality food to further reduce storage temperature to inhibit the quality deterioration of shrimp.At present,ultra-low temperature storage（e.g.-60℃～-80℃）is often used to preserve precious biological samples or foods,with the development of freezing technology,it is expected to become a reality to reduce the freezing temperature of domestic freezers at a reasonable cost of energy consumption.The influence of ultra-low temperature storage on the quality of Metapenaeus ensis is not clear,this study will focus on the quality change and the influence of different factors on melanosis of shrimp frozen at-18℃and-60℃.The TMT labelling method was used to study the changes of proteome during freezing at different temperatures.Fresh shrimp undergo freezing,storage and thawing before consumption,the melanosis of shrimp is easily affected by freezing rate,storage temperature,storage time and thawing.In view of the above mentioned factors,the melanosis of shrimp was studied in this paper,the contents and results are as follows:（1）To compare the effect of different storage temperature on the quality of shrimp,the freshness,water-holding capacity and protein changes were studied of the shrimp stored at 4℃,-18℃and-60℃.The results showed that the freshness,water holding capacity and protein of shrimp changed slowly with the storage temperature decreased.The freshness deterioration of the shrimp were the fastest at 4℃,and the TVB-N was 33.29mg/100g when stored at 7d,which exceeded the maximum limit of 30mg/100g in the national standard.The freshness of shrimp stored at-60℃are the best.As for water-holding capacity,the shrimp stored at-60℃was better than-18℃,which was mainly manifested in the low drip loss rate,sublimation loss rate,compression loss rate and better water-holding capacity of myofibrils.For protein,the protein denaturation and oxidation of the shrimp stored at-60℃was low.There was no significant difference in Ca²⁺-ATPase activity between-60℃and-18℃groups at 90 d,but-60℃group was significantly higher than-18℃group at 210 d.In addition,the content of sulfhydryls in-60℃group was significantly higher than-18℃at 90d and 210 d,and the content of carbonylgroup was significantly lower than-18℃.The data of TMT labeled proteomics showed,a total of 75 differential abundant proteins（DAPs）were determined in F18-CON（-18℃vs.control）,F60-CON（-60℃vs.control）,F18-F60（-18℃vs.-60℃）comparisongroups.There were 64 DAPs in F18-CONgroup,32 DAPs in F60-CONgroup.The results showed that the protein changes of shrimp in-18℃group were probablygreater than-60℃.In DAPs,the projectin,β-thymosin 3,heat shock protein were significantly correlated with the freshness,protein denaturation and oxidation indicators of shrimp,the downregulation rate of the above three proteins and ribosomal protein L44,60S ribosomal protein L7a,histone H 1.0at-18℃group was higher than-60℃group,indicating that the above proteins were unstable at-18℃and may be sensitive to a higher frozen storage temperature.（2）In order to study the effect of freezing rate,storage temperature,storage time,thawing on the melanosis of shrimp,appearance,black spots area,relative activity of PPO（polyphenol oxidase）,dopamine and melanin content were determined.The results showed increasing freezing rate,decreasing storage temperature,shortening storage time and thawing time can delay the melanosis of shrimp.During the freezing process,increasing freezing rate was beneficial to inhibit the melanosis of shrimp.Black spots area and dopamine content of shrimp frozen in liquid nitrogen were significantly lower than those of-18℃.Reducing the storage temperature was beneficial to inhibit the melanosis of shrimp.Under the condition of-60℃,there was no significant difference of the black spots area and relative acticity of PPO compared to controlgroup.The activity of PPO increased first and then decreased in 4℃and-18℃groups,the change rate of PPO activity was the fastest in 4℃group.Moreover,the dopamine content in 4℃and-18℃groups increased first and then decreased,while in-60℃group increased all the time.The melanosis of shrimp became more serious with storage time,which was more obvious at-18℃.The black spots area of shrimp stored for 90d was significantlygreater than30,60d,and the content of dopamine decreased significantly,indicating that the consumption wasgreater than accumulation,the activity of PPO increased first and then decreased during storage.Thawing aggravated the melanosis of shrimp,the melanosis was more obvious with thawing time.Black spots area of shrimp in frozengroup was significantly lower than thawinggroup,there was no significantly difference of the relative activity of PPO in frozengroup compared to controlgroup.The effect of thawing time on melanosis of shrimp was the most significant,the black spots area of shrimp thawed for 24 h was significantlygreater than thawed for 12h,the relative activity of PPO was significantly decreased,the content of dopamine decreased significantly,the content of melanin increased significantly.In conclusion,improving freezing rate,decreasing storage temperature,shortening storage time and thawing time can inhibit the melanosis of shrimp.