Graduate Thesis Or Dissertation
 

Biochemical and Physical Characterization of Fish Protein Isolate and Surimi for their Compatibility

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  • There are presently two successful methods used to refine fish muscle proteins: surimi and fish protein isolate (FPI). Both surimi and FPI have the ability to form an elastic gel upon comminution and heating. However, their gelation behaviors are different as they are refined in a biochemically opposite way based on the nature of protein denaturation. The focus of this study was to compare surimi and FPI under various processing conditions, such as rigor mortis, frozen storage, comminution conditions, and blending effect, and how these conditions can affect their functional properties including gel texture. The structural changes and rheological properties of tilapia protein prepared using FPI and surimi with pre- and post-rigor muscle were evaluated. No rigor effect was observed on the gel-forming ability of FPI, although higher storage modulus (G’) and better gel texture were obtained in surimi made from pre-rigor tilapia compared to surimi made from post-rigor tilapia. Results suggested pre-rigor processing may improve gel-formation properties of surimi, but not as much for the gelation of FPI. Storing fish in a freezer for extended periods of time can adversely affect the gel-forming ability of muscle proteins. The effect of frozen storage (0, 1, and 3 mo) on the biochemical and physical characterization of FPI and surimi made from tilapia was elucidated. The Ca²⁺ATPase activity of tilapia fillet continuously reduced throughout the frozen storage; however, the decline trend of its activity was slower than cold or temperate water species. As reported by storage modulus (G’), storing whole fish frozen for 3 mo did not affect the gelling ability of FPI and surimi. The results from surface hydrophobicity, surface reactive sulfhydryl (SRSH) content, and differential scanning calorimetry also corresponded to the results from storage modulus. Thus, frozen tilapia, if stored up to 3 mo, may be used like fresh fish in the processing of FPI and surimi and no negative effects on gel qualities. The uniqueness of tropical fish tilapia was thought due to its high thermal stability. The quality of surimi gels was affected more so under various rigor stages and frozen storage compared to FPI gels. Conversely, the addition of salt into FPI induced a higher degree of unfolding protein structure prior to gelation compared to surimi. In addition, comminution conditions affected the quality of FPI gel more than that of surimi gel. A significant increase in puncture gel texture was observed when FPI and surimi were chopped at 25°C for 18 min compared to samples chopped at 5°C for 6 min. The comparable results were detected as measured by storage modulus. FPI chopped with 3% salt at 5°C for 6 min showed the lowest gel texture among all treatments, possibly because protein structure was not disintegrated appropriately and formed larger protein aggregates and coarser gels demonstrated by microscopic analyses. Results suggested controlling chopping temperature and time, and the addition of salt, may be significant factors to enhance production of high quality gel in FPI and surimi. Moreover, the effect of various comminution conditions on structural changes were investigated using Fourier transform infrared (FT-IR) and Raman spectroscopy. Both procedures exhibited increasing chopping temperature and time, adding salt, promoted a higher degree of unfolding protein structure in FPI and surimi paste made from tilapia, when they were chopped at 25°C for 18 min compared to samples chopped at 5°C for 6 min. Also, FPI and surimi gels prepared after chopping at 25°C for 18 min revealed higher β-sheet contents and more chemical bonds such as hydrophobic interactions and disulfide bonds than those at 5°C for 6 min. Controlling comminution conditions may be one of the important factors to produce high quality gels from FPI and surimi using tropical fish like tilapia. Additionally, FT-IR and Raman spectroscopy are useful complementary tools, allowing a better interpretation of the structural changes in FPI and surimi under various comminution conditions. The gelation properties of blending two different fish proteins obtained from surimi and FPI at different ratios was evaluated. Effects of blending surimi and FPI on gel functionality (whiteness, hardness, and cohesiveness) demonstrated a linear pattern when the proportion of surimi is larger than or equal to that of FPI. Also, breaking force and penetration distance decreased significantly when the ratio of surimi to FPI decreased. Results indicated gels cooked in a water bath tended to exhibit a higher breaking force than gels cooked ohmically. On the other hand, a higher penetration distance was observed for gels cooked ohmically compared to gels cooked in a water bath. Blending surimi and FPI did not affect the inter-molecular interactions of protein in a linear pattern, like mixing various grades of surimi, but this might be feasible only when the proportion of FPI does not exceed 50%.
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