Removing The Off-flavor Of Silver Carp And The Preparation Of Low Salt Surimi

Silver carp (Hypophthalmichthys molitrix) is widely raised in China, due to its quick growth and resistance to stress, diseases and rough handling. In order to improve the added-value of silver carp, it is economic to utilize the silver carp as raw material of surimi. However, the development of silver carp surimi is limited due to its fishy off-flavor and low gel strength.In the present thesis, the silver carp is evaluated to prepare into fishy-free, low salt high quality surimi. The main content of the present study includes the detection of off-flavor of silver carp mince, the mechanism of fishy odour formation, the methods of deodorization, and improving the gel strength of the low salt gel.The off-flavors of silver carp mince were detected using sensory evaluation, solid micro-extraction- gas chromatography- mass chromatography (SPME-GC-MS), simultaneous distillation extraction- gas chromatography- mass chromatography (SDE-GC-MS), and electronic nose. The fishy odour compounds of silver carp mince were identified as E, E-2, 4-heptadienal, E, E-2, 4-decadienal, and E-2-decenal. The microwave assistant distillation-SPME-GC-MS is suitable for detection of earthy-muddy off-flavor compounds of geosmin and 2-methylisoborneol of silver carp mince, the optimized conditions for microwave assistant distillation were: microwave power output 350 W and distillation time 8 min.The electronic nose could detect the fishy, rancid, earthy-muddy odour. And the results of electronic nose for rancid odour and earthy-muddy odour were consistent with the results of sensory evaluation, the R2 between them were higher than 0.98.The mechanism of the lipid oxidation and fishy odour formation was studied. The main lipoxygenase (LOX) in silver carp muscle was identified as 12-LOX. Among the LOX, Fe³⁺, and hemoglobin, the LOX caused the fastest lipid oxidation, while the latter two catalyzes mainly decompose the peroxides during the initial phase of lipid oxidation. The fishy odour of silver carp mince was mainly caused by the E, E-2, 4-heptadienal, which stem from the oxidation of linolenic acid catalyzed by 12-LOX.The effects of water-washing, acid treatment, alkali treatment, and yeast cytosol treatment on the removal of off-flavor of silver carp mince were compared. Alkali treatment deodorizing process obtained a high recovery yield of protein, good effect of the removing of fishy and earthy-muddy odour, the retained earthy-muddy odour compounds were less than 0.5 ug/kg. The cytosol of yeast could reduce the TBARS of silver carp mince significantly (p<0.05), and the grape wine yeast obtained the best result. The optimized conditions for deodorization induced by grape wine yeast cytosol were 25 oC, pH 6.5, for 2 h. However, their mechanism of deodorizing were different, the alkali process reduced the affinity constant (K) between the off-flavor compounds and surimi, thus, removed the off-flavor. The alcohol and aldehyde dehydrogenase of the yeast cytosol were responsible for the removing of fishy odor of silver carp mince.Acid process denatured the myosin fully, and reduced the content of actin; while alkali process remained the nature of myosin and recovered the most actin. Gel of water washed surim forming well-regulated three-dimension network, resulting in good elasticity and high gel strength. Acid deodorized surimi could not form three-dimension network, thus, its gel strength was the lowest. Alkali deodorized surimi formed three-dimension network but bad-regulated, and the protein aggregated as particles, thus, the gel showed the highest hardness.The myofibril of silver carp underwent considerable degrading and aggregation during the acid and alkali treatment, that were the main reasons of the deterioration of the gel qualities of acid and alkali deodorized surimi. Especially, after the acid treatment, the myosin was denatured absolutely, the surface hydrophobility increased, the solubility decreased, and aggregated too fast to interact with each other to form well-regulated three dimension network, resulting in the lowest gel strength.The optimized conditions for preparing low salt gel from alkali deodorized surimi of silver carp using microwave heating are: adding isolated soy protein 3% (w/w); setting at 40 oC for 1 h, followed by microwave heating at middle power intensity for 60 s, the obtained gel strength was 2077.4±25.5 g×cm. Microwave heating effectively expanded the aggregated proteins, and the expanded proteins interacted with each other adequately, forming well-regulated three dimension network; furthermore, the microwave heating protected the surimi from degrading, and there were more covalent bonds formed during microwave heating; therefore, the microwave heating improved the gel strength of low salt gel significantly (p<0.05).The gelling mechanism of low salt gel induced by microwave heating was concluded as: in the first stage, big protein aggregates (diameter: 1-2μm) were expanded because of the high intensity of microwave, forming the filament-like protein aggregates (diameter: 0.1-0.2μm), they enwind with each other and formed very dense, mussy network; in the second stage, the neighboring filament-like protein aggregates recombined via hydrophilic interaction, hydrogen bonds, and S-S bonds, forming well-regulated three dimension networks with thicken wall (diameter: 1-2μm).