Characteristics and functional properties of sarcoplasmic proteins from ornate threadfin bream Nemipterus hexodon

Abstract

The general characteristics and functional properties of freeze-dried threadfin bream sarcoplasmic proteins (TBSP) were studied. The physicochemical properties, biochemical characteristics and functional properties of TBSP were examined for further information to use as a valuable food ingredient. The compositions of TBSP, extracted by using 0.1 M sodium phosphate buffer, were 44.20% crude protein, 0.20% carbohydrate, 1.98% lipid, 5.97% moisture and 47.65% ash. SDS-PAGE of TBSP showed major components with the molecular weights of 40-62 kDa (53%). Differential scanning calorimetry (DSC) produced three endothermic peaks with melting peak temperatures (Tm) at 56°C , 67 °C and 76 °C. The biochemical characteristics studies of TBSP showed significant proteolytic acitvity. The protease from TBSP (crude protease) was characterized. Azocasein was not a suitable substrate. In contrast, sodium caseinate or endogenous proteins provided effective assays for threadfin bream protease (s). The optimum temperature of crude protease activity from TBSP was 55°C and the optimum pH was 7. Proteolytic activity from TBSP was strongly inhibited by serine protease inhibitor, phenylmethylsulfonylfluoride. Crude protease from TBSP was strong activation by ca²⁺ and inhibition by Mg²⁺. The inhibitory pattern and effect of activators indicated that TBSP contained a mixture of serine, cysteine and metallo-protease. TBSP has no detectable transglutaminase activity. Preparation of TBSP from fresh frozen fish or frozen storage fish had no effect on the protease activity but freeze-drying reduced protease activity 43-44% TBSP powder was 42-46% soluble at pH 2-4 and pH 7-9 with minimum (16.09%) solubility at pH 5. The water lolding capacity of TBSP was 0.73±0.03 g water/g protein. The oil holding capacity of TBSP was 5.26±0.04 g oil/g protein. The foaming characteristics for TBSP were compared to beta-lactoglobulin or sodium caseinate reference proteins, at pH 2-9 and in the presence of a variety of anions or cations (0.2M). They had no effect on the foaming capacity of TBSP but foam stability increased in the presence of magnesium. These trends were similar to those obtained for sodium caseinat or beta-lactoglobulin. Emulsifying activity index (EAI) and emulsion stability index for TBSP were lower than that of beta-lactoglobulin. The combination of NaCl (100 mM) and CaCl₂ (5 mM) enhanced TBSP gelation more than either salt alone. Modification by succinylation and acetylation, and trypsin hydrolysis (trypsin-1 h) were studied. The order of surface hydrophobicity values was acetylated TBSP < native TBSP < succinylated TBSP < trypsin-1 h TBSP. From the pH-solubility profile for acylated TBSP, the lowest solubility was shifted from pH 5 to 4. TBSP foaming capacity was increased due to enzymatic modification. Acetylated TBSP was the highest foam stability. The EAI for succinylated TBSP was higher than unmodified TBSP. Values for the storage modulus (G') significantly increased with temperature ≥ 50degrees celsius, which showed TBSP was heat induced gelation