Temperature and concentration effects on the flow behaviour of stickwater

Condensed stickwater (SW), a liquid byproduct generated on a large-scale in the fish meal industry, has a high protein content (≈20% w/w) and non-protein-nitrogen-compounds (NPNCs). Processing of large amounts of SW requires efficient engineering, especially where its handling is problematical. To this end, the rheological properties of SW were partially investigated by a simple viscometric technique at different levels of concentration of solid matter (6–44•3% w/w) and temperature (26–80°C). The ‘power law’ coefficient of consistency (K) and flow index (n) were obtained from best-fit lines of angular-velocity vs torque. A generic linear relationship between ln-K and concentration was appreciated throughout the temperature range studied, similar to the characteristic concentration dependence of viscosity documented for globular proteins. The effect of temperature on K was highly dependent upon concentration of total solids. At 6% (w/w) it had a marginal effect as well as at 23•9% (w/w) up to 65°C, yet at 80°C an increase in K was observed; at 44•3% (w/w) the effect of temperature followed an Arrhenius type of behaviour, seen as decreasing ln-K values as temperature increased (activation ENERGY = 8•53 × 103J). Although non-Newtonian behaviour (pseudoplastic or ‘shear-thining’ with n < 0•85) was common to all SW solutions, it was accentuated at ca. 24% (w/w; n < 0•43), and persisted to low total solids content (6% w/w) throughout all temperatures. Above 50°C and 24% (w/w), the n index increased monotonically with both variables. Experiments at steady-shear over prolonged periods (up to 2h), revealed typical thixotropic behaviour as concentration and temperature of SW dispersions increased. The complex rheological behaviour is suggested to stem from irreversible protein denaturation probably involving soluble sarcoplasmic muscle proteins as well as structural breakdown of large colloidal species.