Acid-induced gelation of natural actomyosin from Atlantic cod (Gadus morhua) and burbot (Lota lota)

The acid-induced gelation of natural actomyosin (NAM) from burbot (Lota lota) and Atlantic cod (Gardus morhua) added with d-gluconic acid-δ-lactone (GDL) during incubation at room temperature (22–23 °C) for 48 h was investigated. During acidification, pH values of both NAMs reached 4.6 within 48 h. Both NAMs underwent aggregation during acidification as evidenced by increases in turbidity and particle size, especially after 6 h of incubation. The decreases in Ca2+-ATPase activity and salt solubility of both NAMs were observed during incubation. Decreases in total sulphydryl content with the concomitant increases in disulphide bond content of NAM from both species were also noticeable. Additionally, surface hydrophobicity of NAM increased, suggesting the conformational changes in NAM induced by acidification. The storage modulus (G′) values increased with increasing incubation time and G′ development was greater in Atlantic cod NAM, compared with burbot NAM. Differential scanning calorimetry (DSC) revealed that Tmax and enthalpy of myosin peak shifted to the lower values and endothermic peak of actin completely disappeared. In general, gel development was more pronounced in Atlantic cod NAM, compared with the burbot counterpart. As visualised by transmission electron microscopy, network strands of aggregates from Atlantic cod were finer and more uniform than those of the burbot counterpart. Acid-induced gelation of NAM from both fish species therefore involved both denaturation and aggregation processes. However, gelation varied with fish species and had an impact on the resulting gels.