Surimi wash water treatment by chitosan-alginate complexes: Effect of molecular weight and degree of deacetylation of chitosan and nutritional evaluation of solids recovered by the treatment

Soluble surimi wash water (SWW) proteins could be recovered using chitosan (Chi) complexed with alginate (Chi-Alg) generating co-products for feed formulations. Chi with a degree of deacetylation (DD) of 84% complexed with Alg at a mixing ratio (MR) of 0.2 was used to study Chi-Alg concentration and treatment time protein recovery effects. Insoluble SWW solids were removed by centrifugation and the supernatant was then adjusted to pH 6. Flocculation at 20°C using Chi-Alg at 20, 40, 100 and 150 mg/L SWW was aided by 5 min agitation and holding for 30 min, 1h and 24h. Concentration had an effect between low (20 and 40 mg/L) and high (100 and 150 mg/L) levels. Time had an effect between 30 min and 1h but not between 1 and 24h. Turbidity reduction was affected only by concentration. 100 mg Chi-Alg/L SWW for 1h achieved 83% protein adsorption and 97% turbidity reduction while lower concentrations yielding higher adsorption required longer times. Fourier Transform Infrared (FTIR) analysis of untreated and Chi-Alg treated SWW solids confirmed protein adsorption. Amide band areas normalized against a common 3005–2880 cm−1 region confirmed the high protein recovery by 100 mg Chi-Alg/L SWW. Six Chi samples differing in molecular weight (MW) and degree of deacetylation (DD) were tested to recover soluble SWW solids using 20, 40, and 100 mg Chi-Alg/L SWW (0.2 MR, 1h). High (94%, 93%) and low (75%) DD chitosan had lower protein adsorption (73–75%) when compared to the intermediate (84%) DD chitosan (74–83%). Intermediate DD and high MW Chi seemed to perform better; however, SY-1000 with 94% DD did not follow this trend (79–86% protein adsorption, 85–92% turbidity reduction).; Insoluble SWW (P1) and soluble solids (P2) recovered using 150 mg Chi-Alg/L SWW contained 61.4 and 73.1% protein, respectively. Rat diets formulated with 10% protein substitution by P1 and 10% and 15% by P2 had acceptability and protein efficiency ratios (PER) as high as the casein control with no deleterious effects. Rat diets with 100% P2 protein substitution showed higher PER and net protein ratio than the casein control with no deleterious effects. Protein recovered from SWW using Chi-Alg has the potential to be used in commercial feed formulations.