Physical characteristics of muscle protein extracts prepared using low ionic strength, acid solubilization/precipitation
Solubilization of fish myofibrillar proteins at low ionic strength, neutral pH was found to be species dependent and possibly linked to pH of the intact muscle. Two distinct groups formed, one soluble, the other non-soluble, depending upon their initial muscle pH. Incubation of Atlantic cod muscle, whose proteins were initially soluble, in low pH (5.8–6.2) adjusted media resulted in a reversible, time-related insolubilization. Adjustment of pH to slightly greater than neutral (7.2) in Atlantic mackerel light muscle, a nonsolubilizing specie resulted in the almost complete solubilization of its myofibrillar proteins.
Most species investigated achieved complete solubilization of their myofibrillar proteins at low ionic strength, pH 3.0. Gadoid fish that underwent abusive storage were insoluble and had moderately high levels of dimethylamine possibly indicative of protein crosslinking. Centrifugation of acid soluble proteins removed a substantial portion of membrane lipids as determined by phosphorus detection. Recovery of acid solubilized proteins using isoelectric precipitation resulted in yields around 85%. High yields were due to the recovery of sarcoplasmic proteins in addition to myofibrillar proteins. Protein isolates from fish and chicken breast muscle were found to retain functionality, producing gels at neutral pH with high stress and strain values. Dynamic rheological testing found that acid solubilized gels begin formation at a lower temperature than gels that consistently stayed at neutral pH. At low protein concentration, membrane removal decreased the storage modulus of the gel.
Membrane removal was found to improve stability to oxidation in Atlantic mackerel, however, long term stabilization may require an antioxidant system. Freeze-dried protein isolates were prepared from acid solubilized fish proteins, when re-hydrated, produced gels of good color and acceptable strain and stress.