Comparative Study of Washing Treatments and Alkali Extraction on Gelation Characteristics of Striped Catfish (Pangasius hypophthalmus) Muscle Protein

Protein Recovery of Tra Catfish (Pangasius hypophthalmus) Protein-Rich Side Streams by the pH-Shift Method

Increasing protein demand has led to growing attention being given to the full utilization of proteins from side streams in industrial fish processing. In this study, proteins were recovered from three protein-rich side streams during Tra catfish (Pangasius hypophthalamus) processing (dark muscle; head-backbone; and abdominal cut-offs) by an optimized pH-shift process. Physicochemical characteristics of the resulting fish protein isolates (FPIs) were compared to industrial surimi from the same raw material batch. The pH had a significant influence on protein extraction, while extraction time and the ratio of the extraction solution to raw material had little effect on the protein and dry matter recoveries. Optimal protein extraction conditions were obtained at pH 12, a solvent to raw material ratio of 8, and an extraction duration of 150 min. The resulting FPI contained <10% of the fat and <15% of the ash of the raw material, while the FPI protein recovery was 83.0−88.9%, including a good amino acid profile. All FPIs had significantly higher protein content and lower lipid content than the surimi, indicating the high efficiency of using the pH-shift method to recover proteins from industrial Tra catfish side streams. The FPI made from abdominal cut-offs had high whiteness, increasing its potential for the development of a high-value product.

Protein extraction from catfish byproducts and physicochemical properties of the protein isolates

In order to optimize protein recovery from catfish byproducts by alkaline extraction, the effects of different factors, including particle size, mince-to-water ratio, pH, and extraction time were investigated. It was found that a protein recovery of about 30% could be achieved. Increases in pH (pH 10.5, 11, and 11.5) not only improved protein recovery, but also increased protein denaturation evidenced by decreased solubility, decreased α-helix, increased β-sheet, and increased random coil. The color and texture of gels made from protein isolate were greatly affected by the pH values used for protein extraction. For the gels made from fillet mince, and protein isolates extracted at pH 10.5, 11, and 11.5, the "L" values were 78.96, 60.38, 57.74, and 54.39, the breaking forces were 205, 492, 585, and 458 g, and deformation values were 10.59, 8.07, 6.73, and 5.04 mm, respectively. Electrophoresis revealed protein degradation during alkali-aided extraction with MHC, the most predominant band, showing about 50% decrease in comparison with fillet mince. It also demonstrated that gelation not only caused cross-linking, but also autolysis with 53%, 56%, 59%, and 81% decrease in MHC intensity for fillet mince, protein isolates extracted at pH 10.5, 11, and 11.5, respectively. Fillet mince and protein isolates exhibited different storage modulus patterns during temperature sweep, implying different gelation mechanisms. This study proved the protein extracted from catfish byproducts was potential to be utilized as edible food components especially in gel making. PRACTICAL APPLICATION: Catfish byproducts, which account for 70% of total weight and 50% of total protein of catfish, are normally used as animal feed, fertilizer, or even waste. This study demonstrated the potential of the utilization of catfish wastes to develop edible food components. This could reduce the total processing waste being discarded into the environment and nutrient loss, therefore increasing profitability of catfish industry.

Changes in salt solubility and microstructure of proteins from herring (Clupea harengus) after pH-shift processing

Salt solubility of pH-shift isolated herring (Clupea harengus) muscle proteins was studied in relation to pH exposure and microstructure using transmission electron microscopy (TEM). Using protein solubilization at pH 11.2 with subsequent precipitation at pH 5.5, salt solubility of the proteins decreased from 78 to 17%. By precipitating the alkali-solubilized proteins at the pH of native herring muscle, 6.5, salt solubility only decreased to 59%, proving that pH values between 6.5 and 5.5 affected protein salt solubility more than the pH cycle 6.5 → 11.2 → 6.5. Precipitation at pH 5.5 resulted in hydrogen bonds, hydrophobic interactions, and S-S bridges, whereas precipitation at pH 6.5 resulted only in the formation of hydrophobic interactions. The alkaline pH-shift isolation process severely rearranged the protein microstructure, with precipitation at pH 6.5 forming a finer, more homogeneous network than precipitation at pH 5.5. The former protein isolate also contained less lipid oxidation products and formed more deformable gels, without affecting protein yield.

Structural changes and dynamic rheological properties of sarcoplasmic proteins subjected to pH-shift method

Structural changes and dynamic rheological properties of sarcoplasmic proteins from striped catfish ( Pangasius hypophthalmus ) treated by various pH-shift processes were investigated. Isoelectric precipitation of acid-extracted sarcoplasmic proteins led to the lowest solubility in water. Sarcoplasmic proteins were unfolded after extremely acidic and alkaline extraction, exposing tryptophan and aliphatic residues. The alpha-helical structure was converted to beta-sheet following acidic extraction, whereas alkaline treatment did not disturb the alpha-helical structure of sarcoplasmic proteins. Disulfide formation, hydrogen bonding via tyrosine residues, and hydrophobic interactions occurred under extreme pH extraction. Acidic extraction induced denaturation and aggregation of sarcoplasmic proteins to a greater extent than did alkaline treatment. Hydrophobic interactions via aliphatic and aromatic residues were formed during isoelectric precipitation. Sarcoplasmic proteins were partially refolded after isoelectric precipitation followed by neutralization. Sarcoplasmic proteins prepared from an alkaline pH-shift process readily aggregated to form a gel at 45.10 degrees C, whereas higher thermal denaturation temperatures (>80 degrees C) and gel points ( approximately 78 degrees C) were observed in acid-treated sarcoplasmic proteins. The pH condition used for extraction, precipitation, and neutralization greatly affected structural changes of sarcoplasmic proteins, leading to different thermal and dynamic rheological properties.