Physicochemical changes of myosin and gelling properties of washed tilapia mince as influenced by oxidative stress and microbial transglutaminase.

Physicochemical properties of myosin from tilapia subjected to oxidation via Fenton's reaction using H2O2 (0, 0.05, 0.1, 1 and 5 mM) were determined. With increasing H2O2 concentrations and times (from 0 to 12 h), sulfhydryl group content and Ca(2+)-ATPase activity decreased, while carbonyl content and surface hydrophobicity increased to a higher extent. After being subjected to oxidation, cross-linking via disulfide bond along with increased storage modulus (G´) was observed. Microbial transglutaminase (MTGase) induced polymerization of myosin in both non-oxidized and oxidized forms and increased gel G´. Gel properties of washed mince and oxidized washed mince were determined in the presence and absence of MTGase. A stronger gel was observed when 0.3 unit MTGase/g was added, regardless of oxidation process. Nevertheless, the gel strengthening effect of MTGase was hampered when mince was subjected to severe oxidation. Excessive protein aggregation of oxidized samples prior to gelation resulted in the reduction of gel strength and water-holding capacity. Negative effect of protein oxidation on gelation could therefore be alleviated to some degree by MTGase addition.

Impact of microbial transglutaminase on gelling properties of Indian mackerel fish protein isolates.

Impacts of microbial transglutaminase (MTGase) (0-0.6 units/g sample) on gel properties of Indian mackerel unwashed mince, surimi and protein isolates with and without prewashing were studied. Generally, lower myoglobin and lipid contents were found in protein isolate with and without prewashing, compared to those of unwashed mince and surimi (P<0.05). Protein isolate had the decreased Ca(2+)-ATPase and protein solubility, indicating protein denaturation. When MTGase was incorporated, breaking force and deformation of all gels markedly increased, especially as MTGase levels increased (P<0.05). At the same MTGase level, gel from protein isolate with prewashing exhibited the highest breaking force and deformation (P<0.05). The addition of MTGase could lower the expressible moisture content of most gels. No change in whiteness of gel was observed with the addition of MTGase (P>0.05), but gel from protein isolate gels had decreased whiteness as MTGase at high level was added. The microstructure of protein isolate gels without prewashing showed a similar network to unwashed mince gels, whilst a similar network was observed between surimi gel and gel from protein isolate with prewashing. Nevertheless, a larger void was noticeable in gels from protein isolates. All gels incorporated with MTGase (0.6 units/g) showed a slightly denser network than those without MTGase. Thus, gel with improved properties could be obtained from protein isolate from Indian mackerel with added MTGase.

Comparative study on protein cross-linking and gel enhancing effect of microbial transglutaminase on surimi from different fish.

BACKGROUND: Microbial transglutaminase (MTGase) has been used to increase the gel strength of surimi. Nevertheless, its effectiveness varies with fish species. The aim of this study was to elucidate the effect of MTGase at different levels on protein cross-linking and gel property of surimi from threadfin bream, Indian mackerel and sardine in the presence and absence of endogenous transglutaminase. RESULT: Breaking force of all surimi gels increased as MTGase levels (0-0.6 U g⁻¹) increased except for threadfin bream surimi gel, where the breaking force decreased at 0.6 U g⁻¹ (P < 0.05). In the presence of EDTA, the gel strengthening effect was lower, suggesting the combined effect of endogenous transglutaminase with MTGase. With the addition of MTGase, the gel with the highest increase in breaking force showed highest decrease in myosin heavy chain. When cross-linking activity of MTGase on natural actomyosin (NAM) was determined, the highest decreasing rate in ε-amino group content with the concomitant increased formation of cross-linked proteins was found in NAM from threadfin bream. The reactivity of muscle proteins toward MTGase-induced cross-linking was in agreement with surimi gel strengthening. CONCLUSION: The composition and properties of muscle proteins of varying fish species more likely determined protein cross-linking induced by MTGase, thereby affecting their gel properties.