Purification and characterization of a sarcoplasmic serine proteinase from threadfin bream Nemipterus virgatus muscle

Unlocking the secrets of crude myofibril-bound serine protease from grass carp: The role in degrading myofibrillar proteins

Grass carp (Ctenopharyngodon idella) are used as raw material for conventional surimi products in Southern China. However, endogenous serine proteases deteriorated the texture of the surimi gel. To unlock the mechanism behind, the present study isolated the crude myofibril-bound serine protease (cMBSP) in grass carp and studied its effects on surimi gel. The cMBSP activity was the highest at 40 °C and pH 8.0, and it remained stable at 20-55 °C neutral pH. Additionally, it was susceptible to serine protease inhibitors and high concentrations of Na+. The maximum degradation of myosin heavy chain by cMBSP was observed at 50 °C. Protein unc-45 homolog B (a myosin chaperone) is one of the apparent degradation products according to mass spectrometry. The cMBSP caused lower water holding capacity and deteriorated texture in the surimi gel. This study expanded insights about the mechanism of surimi gel degradation by cMBSP, which provided theoretical basis for enhancing surimi quality.

Characteristics of the Intestine Extracts and Their Effect on the Crude Collagen Fibers of the Body Wall from Sea Cucumber Apostichopus japonicus

Sea cucumbers Apostichopus japonicus will vomit their intestines during certain stimulations, and the collagen of the body wall will then be degraded. To define the effect of the sea cucumber intestine extracts on the body wall, the intestinal extracts and crude collagen fibers (CCF) of sea cucumber A. japonicus were prepared. According to the gelatin zymography, the type of endogenous enzymes in intestinal extracts were mainly serine endopeptidases with optimal activities at pH 9.0 and 40 °C. According to the rheology results, the viscosity of 3% CCF decreased from 32.7 Pa·s to 5.3 Pa·s by adding intestine extracts. The serine protease inhibitor phenylmethanesulfonyl fluoride inhibited the activity of intestinal extracts and increased the viscosity of collagen fibers to 25.7 Pa·s. The results proved that serine protease in the intestinal extracts participated in the process of body wall softening in sea cucumbers.

The effect of heating method on the gel structures and properties of surimi prepared from Bombay duck (Harpadon nehereus)

This study investigates the effects of heating method, setting time, and setting temperature on the gel properties, water holding capacity (WHC), molecular forces, protein composition, protein conformation, and water transition of Bombay duck (BD) surimi gel. The obtained results demonstrate that the best gel properties are obtained by two-step heating at 30°C for 120 min while the hardness was 10.418 N and the breaking force was 4.52 N. Gel softening occurs at setting temperatures greater than 40°C due to the effect of endogenous enzymes in destroying the protein structure and increasing the hydrophobic and disulfide interactions. Low-field nuclear magnetic resonance spectra confirm that high two-step setting temperatures induce gel softening and the destruction of the surimi gel structure, as evidenced by the increased water migration at these temperatures. Of all protein conformations in the gel, the β-sheet structure, decreases from 38.40% at 30°C to 11.75% at 60°C when the setting time is 60 min, is the most susceptible to gel softening. Overall, the data reported herein provide a scientific basis for the development of new BD surimi products on an industrial level.

Role of Endogenous Cathepsin L in Muscle Protein Degradation in Olive Flounder (Paralichthys olivaceus) Surimi Gel

We investigated the effect of endogenous cathepsin L on surimi gel produced from olive flounder (Paralichthys olivaceus). The amino acid sequences of six proteins predicted or identified as cathepsin L were obtained from the olive flounder genome database, and a phylogenetic analysis was conducted. Next, cathepsin L activity toward N-α-benzyloxycarbonyl-l-phenylalanyl-l-arginine-(7-amino-4-methylcoumarin) (Z-F-R-AMC) was detected in crude olive flounder extract and a crude enzyme preparation. A considerable decrease in the level of myosin heavy chain (MHC) in surimi occurred during autolysis at 60 °C. In contrast, the levels of actin, troponin-T, and tropomyosin decreased only slightly. To prevent protein degradation by cathepsin L, a protease inhibitor was added to surimi. In the presence of 1.0% protease inhibitor, the autolysis of olive flounder surimi at 60 °C was inhibited by 12.2%; the degree of inhibition increased to 44.2% as the inhibitor concentration increased to 3.0%. In addition, the deformation and hardness of modori gel increased as the inhibitor concentration increased to 2.0%. Therefore, cathepsin L plays an important role in protein degradation in surimi, and the quality of surimi gel could be enhanced by inhibiting its activity.

A novel soft surimi gel with functionality prepared using alcalase for people suffering from dysphagia

In this study, effects of alcalase on physical properties, pepsin digestibility, and antioxidative activity in the heat-induced surimi gel were investigated to develop a novel gel product for people with dysphagia. The heat-induced gelation profile of surimi showed that alcalase activity was maximized at approximately 37 °C. The surimi gel attained an appropriate texture for people with dysphagia, when a combination of 0.3-0.5% alcalase and two-step heating at 37 °C and 90 °C was used. Adding alcalase effectively promoted proteolysis, resulting in softening of the gel. Furthermore, the gel with 0.5% alcalase showed improved pepsin digestibility, when heated at 37 °C and 90 °C. Its antioxidative activity was enhanced by adding 0.5% alcalase. Therefore, a combination of 0.5% alcalase and the two-step heating at 37 °C and 90 °C was useful in improving the physical and functional properties of the surimi gel for people with dysphagia.