Study On Salting And Air-drying Processes Of Grass Carp And Lipid Oxidation During Processing

In order to study the processing of the dry grass carp, the frozen grass carp was used asraw material. The research mainly includes three parts: firstly, influence of different curingconditions on mass transfer kinetics of grass carp during wet-curing. At the same time, aseries of kinetic parameters which provides the theory basis for the enterprise were got.Besides, these kinetic parameters can be used to predict salt content in the grass carp.Secondly, to find the best air drying process parameters, the air drying process of grass carpwas optimized by orthogonal experiment. Finally, on the basis of the optimal processing, therule of lipid oxidation in grass carp during the processing was researched. This was helpful toknow the relationship between the lipid oxidation with the flavor formation better. Theresearch content and results are as follows:1Influence of different curing conditions on mass transfer kinetics of grass carp duringwet-curingThe influence of different curing conditions on mass transfer of water and salt duringwet-curing of grass carp was studied. The different curing conditions include three factors:brine concentration (5、10、15%), ratio of fish to brine (1:2、1:3、1:4), temperature(5、10、20℃). Intended to obtain the first data of mass transfer kinetics forgrass carp under differentcuring conditions, the changes of salt, water content and total weight were determined.Results indicated that weight increase significantly reduced (p<0.01), salt contentsignificantly increased (p<0.01), water content significantly decreased (p<0.01) with theincrease of salt concentration. With the increase of ratio of fish to brine, physical andchemical indicators of grass carp had no significant difference at the early stage of the salting.When the ratio of fish to brine was1:2, moisture content was significantly higher than theothers (p<0.05) after salting24h. After salting36h, weight increase of grass carp wassignificantly lower than other two groups (p<0.05) when ratio of fish to brine was1:4.Besides, the salt content was significantly higher than other two groups (p<0.05). With theincrease of temperature, the salt content increased significantly (p<0.05). Under the conditionof5℃and10℃, weight increase and moisture content of grass carp had significant difference (p<0.05). The diffusion rate of De value increased significantly with the increase ofbrine concentration (p<0.01). There was no significantly impact between ratio of fish to brineand diffusion rate. When the ratio of fish to brine was1:2, the De value arrived the highest;when temperature was10℃, the De value was maximum. Besides, the date calculated fromthe prediction model of salt content and diffusion formula about the grass carp wet salted havea good linear relationship. The relative error was low between the test value and the theorypredicted value by confirmatory analysis. This indicated that data were reliable. Theprediction model can be used to predict the salt content of grass carp and provide theoreticalbasis for practical production.2The process optimization of grass carp air dryingGrass carp was used as a raw material for air drying. Under the condition of10%brineconcentration,10℃, grass carp were salted for10h. After that, grass carp were dried.Theinfluence of drying temperature, relative humidity, drying time on the quality and lipidoxidation was studied. The results showed that the sensory score initially increased and thendecreased with the extended drying time. At the fourth day of air-drying, the Grass carp gotthe highest sensory evaluation. When the drying temperature rose to15℃, the sensory scorereached its maximum. After that, it rapidly reduced. Besides, the sensory score showed atendency to initially increase and then decrease. The sensory score was the highest at50%relative humidity. Drying time had significant effect (p<0.05)on sensory evaluation. Therewas highly significant negative correlation (p<0.01)between sensory evaluation and fat content.TBARS significantly correlated with salt content (p<0.05). The optimum process conditionsof Grass carp were obtained aiming at maximum sensory evaluation. They were temperature15℃, relative humidity50%, drying time4d. Under this condition, the sensory score was75.10.3Lipid oxidation of grass carp during curing and air drying ripeningGrass carp was used as the material. The best process of salted and air-drying was used.In order to explore the changes of lipolysis and lipid oxidation of grass carp during thepreparation process, sulfur barbituric acid value (TBARS), lipoxygenase activity, fatty acidcomposition were determined. The results showed that fat content of dry basis significantlyreduced (p<0.05), TBARS rose gradually and significantly increased later (p<0.05), activityof lipoxygenase rose first and then fell and reached the maximum value on the first day of airdrying (p<0.05), all of these explained that grass carp fat oxidation reaction was very intensein the late processing. The content of saturated fatty acid and monounsaturated fatty acid bothincreased significantly (p<0.05). However, the content of polyunsaturated fatty acidsdecreased significantly (p<0.05). Palmitic acid (c16:0) stearic acid (c18:0), oleic acid (c18:1) acid (c16:1) linoleic acid (c18:2) arachidonic acid (c20:4) were the main components of thefatty acid composition. The fat content of dry basis correlated significantly with TBARS andsalt content of grass carp (p<0.05). Saturated fatty acid content had a significant relativitywith TBARS and salt content of grass carp (p<0.05)...