| Our country is one of the leading nations in the fishing industry, rich in freshwater resources. Parabramis pekinensis is one of the Chinese major freshwater fish for its tender meat and delicious taste. Since fish products are easy to spoil, to solve the problem of fresh fish, the fishermen will apply salt to the surface of fish and make fish air-dried naturally. High salt content, a strong cure flavor, and an unstable quality are characteristics in traditional workshop-made wind fish; meanwhile large-scale production generally use cold salted-dried mature technology, which make the fish have a low salt content, a stable quality, but lack the cured flavor. How to reduce the salt content while increasing the cured flavor has become a hot topic in the meat processing field recently. Numerous studies have confirmed that the cured meat flavor from the fat is mainly composed of unsaturated fatty acids, oxidation and further reaction of oxidation products formed. Current researches mainly add enzymes and optimize the process parameters to improve the cured flavor. The effects of curing methods and the amount of salt on cured flavor of cured meat are not yet clear. In this study, Parabramis pekinensis was used as raw material, and automatic temperature and humidity control system was used for the air-dry conditions, the effect of different curing methods on physicochemical index and sensory scores of air-drying Parabramis pekinensis was investigated to screen the best curing method for producing the most cured flavor. Furthermore, the effects of different mounts of salt based on the above curing method, on lipolysis and lipid oxidation of air-drying Parabramis pekinensis and the volatile flavor compounds in the final product were analyzed, which will provide theoretical support for the impact of salt concentration on the flavor of cured meat. The contents and results are as follows:1. The effects of different curing methods on physicochemical index and sensory quality of air-drying Parabramis pekinensisParabramis pekinensis was cured with three traditional methods, i.e. dry salted, wet salted and mixed curing, and air-dried with the same kinds of drying conditions. The effects of different cured methods on physicochemical index and sensory evaluation were investigated. The results showed that the water content of different groups all showed a downward trend during the whole process, in which the fastest rate of decrease in water content was observed in dry-cured group, i.e.33.94%. The salt content showed an upward trend and there was significant difference (p<0.05) in salt content among three groups at the end of curing. Salt content of dry-cured samples rised highest and final salt content reached 7.80%. TBA showed an upward trend and the value of dry-cured group was significantly higher than those of wet cured and mixed cured (p<0.05) at the end of curing, and the final TBA value of dry-cured group reached 5.64 mg/kg. The hardness of three groups all decreased firstly then increased, but the difference was not significant (p> 0.05) among three groups. The pH changes were not significant (p> 0.05) during the whole process. The score of sensory evaluation of dry-cured fish products was highest, followed by mixed salted group, wet salted group, respectively, i.e.91.29,86.24,84.50. Comprehensive comparing the curing speed and product flavor, dry salted method was better than wet salted and mixed methods.2. The effects of different amounts of salt on lipid lipolysis and oxidation of air-drying Parabramis pekinensisOn basis of experiment one, the effects of salt amount of of dry-cured method on lipolysis and lipid oxidation of air-drying Parabramis pekinensis during the maturation process. Parabramis pekinensis was dry salted with 2%,5%,8%,12% amount of salt respectively and air-dried under the same conditions, and the water content, salt content, pH value, acid value, free fatty acid, lipase activity, TBA value, lipoxygenase enzyme activity and sensory evaluation were analyzed. The results showed that the change trends of some index were different among four amounts of salt:neutral lipase activity of 2% salt group increased and phospholipase decreased during the whole process, while acid lipase activity reached a maximum at the end of cured and then decreased; the activities of three kinds of lipase of 5%,8%,12% salt groups all showed a downward trend, and lipase activity of 12% group was lowest at the end of the experiment; TBA values of 2% and 5% salt groups showed an upward trend, while those of 8% and 12% salt groups increased firstly and then decreased. The change trends of other index were similar among four groups:water content of four kinds of salt groups all decreased significantly (p<0.05), salt content increased significantly (p<0.05), pH value decreased, the acid value increased, free fatty acid arachidonic acid content decreased, lipoxygenase increased and reached the maximal level at the end of curing and then decreased. The results indicated that different amounts of salt had different effects on the lipolysis and lipid oxidation of air-drying Parabramis pekinensis during the whole process.Comparing the lipase of final products of air-drying Parabramis pekinensis made with 2%,5%,8%,12% salt amounts, the activities of neutral lipase, acid lipase and phospholipase decreased with the amount of salt increasing, and the activities of three lipase of 12% salt group were very low, especially phospholipase activity, was almost zero; while lipoxygenase activity increased with the amount of salt increasing. High lipoxygenase activity can promote oxidation of lipid with the lipase activity exiting. The score of sensory evaluation of 5% and 8% groups was higher than other two,Comprehensive considering lipase and lipoxygenase activity and the results of sensory evaluation,8% amount of salt was better for lipolysis and lipid oxidation of air-drying Parabramis pekinensisr.3. The effects of different amounts of salt on volatile flavor compounds of air-drying Parabramis pekinensisParabramis pekinensis was dry salted with 2%,5%,8%,12% amount of salt respectively and air-dried under the same conditions, and the volatile components of air-drying Parabramis pekinensis were further detected by headspace solid phase micro extraction-mass spectrometry technique and the characteristic flavor substances were analyzed and identified. The results showed that 28,34,37,37,41 kinds of volatile substances were detected in fresh samples and final products of 2%,5%,8%, 12% salt amount groups, including aldehydes, ketones, alcohols, hydrocarbons, among which alcohols and aldehydes contents were relatively higher, and their threshold levels were lower. The total amount of volatile substances of final products of 8%,12% salt mounts was higher than that of 2% and 5% groups, and the total content of alcohols and aldehydes of 12% salt amount group was very low. Comprehensive analysis indicated that 8% salt amount was better for the volatile components of air-drying Parabramis pekinensis. By further analyzing the composition and content of the volatile flavor compounds and the contribution level to the flavor of air-drying Parabramis pekinensis, preliminary identified the characteristic flavor subtances of air-drying Parabramis pekinensis were hexanal, nonanal, octyl aldehyde,2,5-octanedione, 1-octene-3-o1. |
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