Study On Gelation And Gel-forming Mechanism Of Silver Carp Mince Induced By Fermentation

Surimi products are very popular among people throughout the world. Fermented fish mince is receiving increased attention recently due to its high nutrition, security, convenience, storage stability, health benefits and gel strength. Our previous studies found that lactic acid fermentation was able to enhance the gel strength of freshwater fish surimi significantly, but the gelation mechanism of fermented fish mince is not well understood. Therefore, silver carp which is one of the main freshwater fish species in China was selected as raw material. On the basis of preparation of fermented fish mince with high gel strength, structural changes and protein molecular interactions, as well as the protein constituents involved in the gel network formation of silver carp mince induced by fermentation were investigated in order to elucidate the gel-forming mechanism of fish mince induced by fermentation at molecular levels. The present study is of great importance for understanding the biochemical changes and gelation of muscle proteins during fermentation, and expanding the theory of protein gelation. And it also provides a solid theoretical basis for establishing a new bio-fermentation processing technology system in the surimi processing industries.The key factors influencing fermentation of fish mince were first investigated and the optimum fermentation conditions were determined by analyzing the effect of fermentation temperature on the microbial and physicochemical changes, as well as sensory quality, textural properties and biogenic amines content during fermentation. Results showed that silver carp mince exhibited superior sensory quality and safety and formed compact fibrous network with higher gel strength (1242.4 g*cm) after 48 h of fermentation with Pediococcus pentosaceus at 23℃. Lactic acid bacteria grew fast at 23℃during fermentation and to be the dominant species, resulting in a rapid decline in pH to 4.5 within 48 h, which consequently suppressed the growth of Pseudomonas, Micrococcaceae and Enterobacteriaceae and the production of total volatile base nitrogen (TVBN) and biogenic amines.The protein molecular interactions and the role of different protein constituents involved in the gel network formation during fermentation of silver carp mince were investigated by analyzing protein compositions, proteolysis, disulfide bonds content and protein selective solubility. Results showed that as fermentation progressed, the amount of salt-soluble and water-soluble proteins decreased gradually with a concomitant increase in insoluble proteins due to acid-induced denaturation and aggregation. Hydrophobic interactions, disulphide bonds and non-disulphide covalent bonds were mainly responsible for the formation of gel network of fermented silver carp mince, and hydrophobic interactions were particularly important during the initial stage of gel forming. Extensive formation of disulfide bonds occurred during the later stage of fermentation, intensifying the gel network. SDS-PAGE showed that myosin heavy chains were the main protein constituents for the gelation of fermented fish mince, and actin and low molecular weight proteins probably produced by proteolysis were also involved in the formation of gel network.Effects of pH and ionic strengths on the gel properties, aggregation forces and structural changes of silver carp actomyosin and myofibrils were studied using D-gluconic acid-δ-lactone (GDL)-induced acidification by analyzing gel strength, protein selective solubility, turbidity, ATPase activity, sulphydryl contents, surface hydrophobicity, CD spectra, UV spectra, intrinsic fluorescence intensity, Native PAGE and SDS-PAGE in the presence and absence ofβ-mercaptoethanol. Effect of organic acids produced during fermentation on the gel forming of fermented fish mince was also investigated. Results showed that structural changes occurred during GDL-induced acidification of silver carp actomysoin, which caused the exposure of hydrophobic and sulfhydry groups buried inside the protein molecule to the surface, resulting in a sharp reduction in Ca2+-ATPase activity and the formation of insoluble protein polymer as indicated by the decrease in solubility and increase in turbidity. Hydrophobic interaction was the dominant force involved in acid-induced aggregation of silver carp actomyosin, although hydrogen bonds, ionic bonds and disulfide bonds also played a role to a less extent. Ionic strength had a significant influence on acid-induced structural changes, aggregation and gel properties of silver carp myofibrils. The increasing in ionic strength could unfold protein partially, causing the exposure of active groups to the protein surface. Such conformational changes promoted interactions of protein molecules during acidification. Gel strength of acid-induced silver carp myofibrils increased with increasing of NaCl concentration up to 0.3mol/L, and further increasing salt concentration resulted in a gradual decrease in gel strength. Silver carp mince could form superior gels by GDL-induced acidification, and the gel strength and the protein molecular interactions involved in the GDL-induced gels were in accordance with those involved in fermented silver carp mince, convincing that gelation of silver carp minced during fermentation was mainly caused by organic acids produced.The gel-forming dynamic process of silver carp protein during acidification was investigated by means of chymotryptic digestion of silver carp myofibrils to prepare S-1, rod and HMM, LMM, and subsequent electrophoresis and structural analysis. Results indicated that the head-head interactions occurred firstly during acidification, followed by denaturation of tails and cross-linking through tails of myosin, and at the same time the heads-linked oligomers aggregated further, resulting in the formation of sequence-directed gel network.In present study, the protein molecular interactions and the protein constituents involved in the gel network formation during fermentation of silver carp mince were determined. It was convinced that gelation of fermented mince was mainly caused by organic acids produced during fermentation. The involvement of structural changes and forces in acid-induced gelation of silver carp proteins were also cleared at molecular levels, elucidating the gel-forming mechanism of fish mince induced by fermentation.