Changes in textural and rheological properties of gels from tilapia muscle proteins induced by high pressure and setting

Effects of High-Pressure, Microbial Transglutaminase and Glucono-δ-Lactone on the Aggregation Properties of Skim Milk

The object in this study is to investigate the effects of high pressure and freezing processes on the curdling of skim milk depending on the presence of transglutaminase (TGase) and glucono-δ-lactone (GdL). Skim milk was treated with atmospheric freezing (AF), high pressure (HP), pressure-shift freezing (PSF) and high pressure sub-zero temperature (HPST) processing conditions. After freezing and pressure processing, these processed milk samples were treated with curdling agents, such as TGase and GdL. Pressurized samples (HP, PSF and HPST) had lower lightness than that of the control. In particular, PSF had the lowest lightness (p<0.05). Likewise, the PSF proteins were the most insoluble regardless of whether they were activated by TGase and GdL, indicating the highest rate of protein aggregation (p<0.05). Furthermore, the TGase/GdL reaction resulted in thick bands corresponding to masses larger than 69 kDa, indicating curdling. Casein bands were the weakest in PSF-treated milk, revealing that casein was prone to protein aggregation. PSF also had the highest G' value among all treatments after activation by TGase, implying that PSF formed the hardest curd. However, adding GdL decreased the G' values of the samples except HPST-treated samples. Synthetically, the PSF process was advantageous for curdling of skim milk.

Effects of oxidative modification on gel properties of isolated porcine myofibrillar protein by peroxyl radicals

AAPH-derived (2,2'-azobis (2-amidinopropane) dihydrochloride) peroxyl radicals were selected as representative free radicals of lipid peroxidation to investigate the effects of oxidative modifications on isolated porcine myofibrillar protein structures as well as their rheological and gelling properties. Incubation of myofibrillar protein with increasing concentrations of AAPH resulted in a gradual increase (p<0.05) in carbonyl content and SH→S-S conversion. Results from SDS-PAGE indicated that medium (~1 mM) and relatively high (>3 mM) concentrations of AAPH induced aggregation of myosin and denaturation of myosin, troponin and tropomyosin, respectively. These structural changes resulted in changes on gelation of myofibrillar protein. Low level protein oxidation (AAPH≤0.5 mM) had no remarkable effect (p>0.05) on the viscoelastic pattern of myofibrillar protein gelation. Moderate oxidative modification (AAPH~1mM) enhanced the water-holding capacity (WHC) and texture properties of gels, while further oxidation (AAPH>3mM) significantly reduced the gel quality.

The influence of fish age, salt level, and MTGase addition on the quality of gels prepared from unwashed mince of Farmed Meagre (Argyrosomus regius)

The potential of using the unwashed mince of farmed meagre as raw material for the preparation of heat-induced gel products was assessed taking into account the effect of age (small size <1 kg meagre vs commercial size >2 kg meagre), lower salt levels (1.0%, w/w, vs 2.5%, w/w), and microbial transglutaminase (MTGase) incorporation (0.0%, w/w, vs 0.5%, w/w). Heat-induced gel products from >2 kg fish were of superior quality. Salt reduction from 2.5% to 1.0% (w/w) was detrimental for textural quality, particularly, of gels prepared from >2 kg meagre mince. MTGase addition improved texture. Moreover, MTGase incorporation led to a greater importance of non-covalent hydrophobic bonding.