Viscosity, mixing regime and power consumption analysis during enzymatic hydrolysis of fish (Sardina pilchardus) processing by-products, through development of power number and Reynolds number correlations

Abstract

The influence of processing conditions on fluid rheology and mixing regime is of considerable importance during enzymatic hydrolysis of fish processing by-products. Solids concentration ranging from 30 to 70 % (wet basis) and mixing speed from 100 to 400 rpm were investigated, with apparent viscosity, power consumption, power number (Np) and Reynolds number (Re) as response variables of interest. The fish processing by-products at different solids concentration showed power-law pseudoplastic behaviour. Apparent viscosity ranged from 4.35 to 8.51 mPa·s, and power consumption ranged from 53.0 to 492 W/m3. Np―Re curves showed the typical relationship where Np decreased from 20.4 to 1.99 when Re increased from 425 to 3 317. Poor mixing was observed at high solids concentration and low mixing speed, with cavern formation in the early stages of mixing. Liquefaction of solids led to the disappearance of caverns, resulting in good degree of mixing during the hydrolysis process. Contrary to expectations, our study indicated that liquefaction of solids and reduction in molecular weight of proteins had a significant effect on power consumption more than increase in soluble protein concentration, gel formation and/or emulsion. Liquefaction and molecular weight reduction caused a decrease in fluid viscosity while increase in protein concentration, gel formation and/or emulsion contributed to an increase in viscosity. The net result was a decrease in power consumption from 0 to 60 min of enzymatic hydrolysis. The practical implications of the results presented in this study are that one Np―Re curve can be used for any solids concentration between 30 and 50 %. Beyond 50 %, low degree of hydrolysis, protein recovery and significantly high power consumption are experienced.