Introduction. In order to utilize catfish by-products and improve the profitability of catfish farming industry, alkaline protein extraction from catfish by-products has proven economically feasible. However, the gels made from alkaline extracted fish protein was of low elasticity. The study’s objective was to improve the texture quality of the gel made from catfish protein isolate with microbial transglutaminase (MTGase).
Methods. Protein isolate was extracted at pH 11 from catfish by-products including frames and heads. Microbial transglutaminase was applied at different levels (0, 0.1, 0.2, 0.3, 0.4, 0.5 and 0.6 unit/g) to make gels. Gels was also prepared with catfish fillet mince as control. Texture profile analysis, solubility (non-reducing and reducing), SDS-PAGE, and dynamic rheological analysis were conducted to determine the effects of disulfide bonds and MTGase induced cross-linking on the gel formation of protein isolate.
Results. All texture parameters were improved with the increase of MTGase added up to 0.5 unit/g. Springiness increased from 79 to 88%, deformation increased from 6.51 to 11.13 mm, and chewiness increased from 2200 to 12000 g. MTGase increased the elasticity of gels prepared from protein isolate and made them comparable to those made from fillet mince. The solubility of gels in both non-reducing and reducing buffers decreased with the increase of MTGase added, but the solubility in reducing buffer was at least 50% higher than that in non-reducing buffer, indicating both disulfide bonds and MTGase induced cross-linking played an important role in the gelling process. SDS-PAGE showed those MHC bands which were not observed in non-reducing gel showed up clearly in reducing gel, indicating the denatured proteins after alkaline extraction were prone to disulfide formation due to the exposure of sulfhydryl groups. With MTGase added, less low-molecular-weight bands showed up, which suggested that MTGase could inhibit proteolysis induced by endogenous enzymes to some extent. The temperature sweep showed protein isolate and fillet mince had different sol-gel transition patterns. Addition of MTGase did not change the color of the gels.
Significance. This study solved the textural quality issues of gels made from recovered catfish protein and elucidated the molecular mechanism during gelation.

Sam Chang

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