Study On Freshness Preservation Technique For Scapharca Subcrenata

Scapharca subcrenata is an important economic shellfish widely bred in coastal areas of China. Consumers like it for its characteristics of high contents of protein and amino-acids, low fat content, and its remedy effects. Freshness is a primary quality index of shellfish, and a primary factor to set up its price. It is especially important to keep shellfish fresh, because it is apt to degrade and decay after death. Now that most of studies focus on breeding and life habits of Scapharca subcrenata, it is of great significance to explore the way of freshness preservation applying biological preservatives, coating preservatives and modified-atmosphere technology. Scapharca subcrenata appers some changes during its cold storage which degrade its freshness and value as food.Bred Scapharca subcrenata is object of study in this dissertation. Through analysing pH value, total volatile basic nitrogen (TVBN) and thiobarbituric acid (TBA), combinations of preservatives, conditions of modified-atmosphere combined with low temperature storage have been studied, meantime, the freshness of Scapharca subcrenata during the process has been assessed. The study provides freshness preservation for Scapharca subcrenata and other shellfish with theory and technical guidance.Content and conclusion of the study are as follows:1. Applying biological preservatives such as Nisin, lysozyme, freshness preservation effect of them on Scapharca subcrenata under different temperatures (0℃,-3℃, -20℃) have been analyzed. The results show that biological preservatives combined with low temperature storage can keep well the freshness and quality of the Scapharca subcrenata and biological preservatives present significant freshness preservation effect on Scapharca subcrenata. Under temperature of 0℃, freshness preservation for control samples only sustain 8 days, while the samples treated with biological preservatives extended 4 days more. Analysis after 12 days show best preservation result for combination of Nisin and lysozyme as preservatives, furthermore, total volatile basic nitrogen (TVBN) was examined as 10.10 mg/100g after 12 days storage; thiobarbituric acid (TBA) was examined as 0.71mg/kg; ATP enzymatic activity is about (Ca²⁺-ATPase) 0.041μmolpi/(mg·hour), and free fatty acids (FFA) as 8.12% after 12 days storage. Under mild frozen condition of -3℃, treated with combination of Nisin and lysozyme as biological preservatives, the samples still keep good organoleptic characters after 30 days storage, the meat is elastic and has no unpleasant odor. Freshness index of pH value is 6.19, TVBN is around 15.71 mg/100g, slightly over threshold of national norm (GB) of 15 mg/100g, TBA keeps below 6.08 mg/kg, ATP enzymatic activity (Ca²⁺-ATPase) is about 0.049μmolpi/(mg·hour), FFA is 10.18%. One year storage under temperature of -20℃, treated with biological preservatives, the samples were examined for pH as 6.67 at most, TVBN as 13.31 mg/100g at most, TBA as 1.82 mg/kg at most, ATP enzymatic (Ca²⁺-ATPase) activity as 0.012μmolpi/(mg·hour) at least, free fatty acid (FFA) as 13.91% at most. The freshness indexes vary slightly. Among biological preservatives, combination of Nisin and lysozyme achieve better preservation result.2. Applying sodium alginate and chitosan as coating preservatives, freshness preservation effects of them on Scapharca subcrenata have been analyzed under mild frozen condition of -3℃, the results show that treated with sodium alginate, the samples achieve good result of 20 days storage. After 20 days, TVBN was examined as 14.48 mg/100g, TBA as 2.43 mg/kg, Ca²⁺-ATPase as 0.041μmolpi/(mg·hour), FFA as 8.35%. Under temperature of -20℃, applying such two coating preservatives as chitosan and chitosan combined with tea polyphenol, after 11 months storage, pH value increase to 6.49, TVBN is 7.17 mg/100g,TBA is 0.87 mg/kg, Ca²⁺-ATPase is 0.080μmolpi/(mg·hour) at least, FFA is 14.81% at most. The two coating preservatives have similar preservation effect. In consideration of economic aspect, chitosan as single coating preservatives achieves better preservation effect.3. Applying modified-atmosphere of N₂ and CO₂, freshness preservation effects of them on Scapharca subcrenata have been analyzed under mild frozen condition of -3℃, the results show that after 30 days storage, pH value of sample was examined as 6.69, TVBN of sample nearly reach the limit of 15 mg/100g except for N₂ modified sample, which TVBN is over the limit of 15 mg/100g after 15 days storage. Applying modified-atmosphere of N₂ and CO₂ combined with biological preservatives obviously outmatch solely application of modified-atmosphere in effect of preservation of Scapharca subcrenata, after 30 days storage, pH value of sample is 6.34, TVBN is 15.12 mg/100g, TBA is 4.19 mg/kg, Ca²⁺-ATPase is 0.052μmolpi/(mg·hour), FFA is 8.90%.