Effects Of Different Freezing Processes On The Quality Characteristics Of Litopenaeus Vannamei During Frozen Storage

Litopenaeus vannamei is one of three highest breeding production shrimp in the world.Because of its delicious and high protein low fat nutrition characteristic, it is very popularby consumers. The high moisture and rich enzyme in Litopenaeus vannamei lead that itwas easy to deterioration. The most of Litopenaeus vannamei were stored, transported andsold in the form of frozen, while freeze storage can not completely inhibit the effect ofmicroorganism and enzyme. Quality loss is also caused in the long storage and then thetaste and flavor are affected..This paper studied the change of physical and chemical properties and theorganizational structure in Litopenaeus vannamei during storage period after frozen byliquid nitrogen quick-frozen,-75℃quick frozen and-18℃directly frozen three freezingprocess used the mixture solution of trehalose and tripolyphosphate as a low phosphorusabsorbent agent. The difference of flavor substances’content of Litopenaeus vannameiwhich were storaged five months was also studied in this paper. Hoping to provideteoretical drection and experience basis for quick-frozen process for a higher aquaticproduct.The main research contents and conclusion in this paper were as follows:1. Taking the water loss rate as index screen out the best soak solution type is themixture solution of trehalose and tripolyphosphate. Through the study of concentration,time and proportion of trehalose and tripolyphosphate in the solution, we found the bestsoaking technology as follows: mix trehalose and tripolyphosphate in the ratio of1to1,prepared the mixture solution whose concentration is3%, kept in4℃sokingfor2h. Thedriage in thaw was the minimum at that moment,1.78%.2. Taking texture, content of salt soluble protein and total sulfydryl，activity of ATPaseand pH as indexes, liquid nitrogen quick-frozen,-75℃quick frozen and-18℃directlyfrozen three freezing processes impact on the physical and chemical properties inLitopenaeus vannamei were studied. The results showed: there was no significantdifference for texture in liquid nitrogen quick-frozen; the content of salt soluble protein and total sulfydryl，activity of ATPase and pH in the group frozen by liquid nitrogen was lowerthan the fresh group significantly, while it is higher than the groups frozen by-75℃quickfrozen and-18℃directly.Taking pH, the water loss rate, texture, cotent of salt soluble protein and totalsulfydryl, activity of ATPase and organization structure as indexes, the changes of physicaland chemical properties in Litopenaeus vannamei which were frozen by three differentfreezing processes were studied. The results showed:The variation tendency of pH in Litopenaeus vannamei was decreasing firstly, andthen rising, while the group frozen by liquid nitrogen was later than the other groups toreach the lowest point.The water loss rate in Litopenaeus vannamei was more and more larger with theextension of storage time, while the change of water loss rate in the group frozen by liquidnitrogen was slowly, lower than the other group all the time.The hardness first increased and then decreased with the extension of storage time;adhesion were lager and lager with the extension of storage time, while the elasticity andcohesion were smaller and smaller with the extension of storage time; the changes oftexture in Litopenaeus vannamei was slowly in the group frozen by liquid nitrogen, whichcan keep a better character than the other groups.The content of Litopenaeus vannamei was less and less, the active of Ca2+-ATPase、Mg2+-ATPase、Ca2+-Mg2+-ATPase and Mg2+-EGTA-ATPase were smaller and smaller, thechange in the group frozen by liquid nitrogen was gently and smaller, so it is better to keepthe quality of Litopenaeus vannamei.Through the paraffin section for the muscular tissue of Litopenaeus vannamei wefound there was hardly any injury in the group frozen by liquid nitrogen. The ices crystal inthis group are growing slowly and having no significant injury. So frozen by liquidnitrogen is better to protect the organization structure of Litopenaeus vannamei.Taking TVB-N, K-value and total bacterial count as index, the changes of freshness ofLitopenaeus vannamei stored at-18℃refrigerator were studied. Each index indicated thatthe Litopenaeus vannamei frozen by liquid nitrogen was at the levelⅠ of freshness afterstored five monthes, while the other groups were at the level Ⅱ.Taking texture and the content of flavor substances as indexes, the effect of differentfreezing processes for the character，taste and the content of flavor substances ofLitopenaeus vannamei were studied. The results as follow:The color, character and aroma in the group frozen by liquid nitrogen are better thanthe other groups. There is no significant difference with fresh group in sensory scoring; The chewiness in the group frozen by liquid nitrogen was a little lower than the freshgroup, while there were no significant difference with fresh group in hardness, elasticityand adhesion. It was superior to the other groups;There were a higher content of free amino acid, betaine, ATP and its relevantcompounds, organic acids and inorganic, which were the important flavor substances inLitopenaeus vannamei, in the group frozen by liquid nitrogen. It was no significantdifference with fresh group.