| Shrimp belongs to arthropod, crustacea, malacostraca, decapoda, parade suborder, shrimp faction, shrimp branch. Shrimp is one kind of precious aquatic product, which has tender and delicious taste as well as rich nutrition. Fresh shrimp not only can be cooked into a variety of delicious dishes, but also can be made into'the sun-dried shelled shrimp'and'the dried shrimps'and so on. The effects of preservation of shrimps under different temperature conditions (-2℃and -5℃) and biochemical characteristics of polyphenol oxidase (PPO) which causes browning of shrimps during partially frozen were studied in this paper, so the effective shrimp color fixative formula was determined. Therefore, an important recommendation for improving the quality of shrimps and extending the shelf life during storage was offered.(1) Pacific white shrimp was taken as a raw material, and the brine partially frozen preservation method was used in this experiment. In order to determine the freezing point of shrimp, the preservation effect of shrimps under different temperatures (-2℃and -5℃) was studied. It was indicated that partically frozen can well maintain the quality of shrimps, the low K value and the bacterium total, the T-VBN value, and under -5℃the shelf life of shrimps may reach 10d. Storaged under -2℃and -5℃the sensory score and muscle water holding capacity of shrimps were gradually reduced, the total number of bacteria and pH value both descended and then increased, T-VBN value, K value and salt degrees all increased, but values of different temperatures varied over the same period. It was also indicated that the preservation effect under - 5℃was better than that of - 2℃.(2) The biochemical characteristics of PPO and its influencing factors were analyzed. 0.05mol/L catechol solution was taken as the enzyme substrate, the result showed that the activity of PPO almost unchanged within the first 3min and maintained at 38units/min, with reaction time's extension, the reaction speed decreased and to the 6th minute the PPO activity dropped to 5units/min. It was indicated that the most suitable pH value of PPO is 7.0. Between pH 3.0-7.0 PPO activity increased with the increasing pH value, especially between pH 5.5-7.0 the PPO activity increased more rapidly, thus when the pH value increased to more than 7.0, the PPO activity declined with the increasing pH value. Meanwhile, the optimum temperature of PPO is 40℃. Between 20℃-40℃, the activity of PPO increased as the temperature increased, but when temperature is higher than 40℃, the enzyme activity declined with the increasing temperature. In addition, spectrophotometric method was used to study the influence rule of the substrate concentration on PPO activity. The results showed that shrimp browning reaction kinetics was consistent with the Michaelis-Menten equation, the Michaelis-Menten constant is 2.03mmol/L, and the maximum response speed is 19.4 U/min. The Michaelis-Menten equation as follows:(3) The browning of shrimps during partially frozen storage when under -5℃was studied in order to find out an effective color fixative formula for shrimp. Phytic acid, NaHSO3, citric acid, iso-ascorbyl sodium, EDTA-Na2 were taken respectively as the color fixative, and their different concentrations were studied to prevent the shrimps turing brown, which was compared with the effect of the blank group. The single factor experiments showed that citric acid, phytic acid, EDTA-Na2 and NaHSO3 all can prevent the shrimps turning brown effectively, and orthogonal experiments were also done based on the single factor experiments. During the experiment, L9 (34) orthogonal experimental table was used, and test results showed that 0.2% citric acid +0.1% phytic acid +0.02% EDTA-Na2 was the best color fixative. This color-protecting agents can not only inhibit the browning of shrimps, control the browning degree of the shrimps at 59.12, but also will not do harm to the shrimps'flavor.
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