17E-25 |
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P. ZHOU and J. M. Regenstein. Dept. of Food Science, Cornell Univ., Stocking Hall, Ithaca, NY 14853-7201 Gelatin is an important functional food biopolymer. During gelatin extraction or during studies of its chemical and rheological properties, the total protein is often determined by the Lowry or Biuret method, with commercial gelatin as the standard. However, gelatins can be obtained from different sources with different molecular weight so that the variation in “protein color” response was unknown. Our objective was to determine whether the choice of standard affects both total protein assays. The color response was determined at 650 nm for Lowry or 540 nm for Biuret. Cod skin gelatin (CSG) and pork skin gelatin (PSG) were used as standards. The reference factor (RF) was calculated as a ratio of the concentration determined with standard curves to the concentration calibrated by dry weight. Molecular weight distribution was determined by SDS-PAGE; hydroxyproline by the Woessner method; the thermal helix-random transformation temperature (Tt) by measuring the change of viscosity with temperature. The standard curves obtained with the reference gelatins showed significant variation (P < 0.001). Using CSG as the standard resulted in a more accurate determination for cold-water fish gelatins (RF, 0.96~1 for both methods), but less accurate for mammalian gelatins (RF, Lowry: 0.61~0.63 and Biuret: 0.82~0.84). However, using PSG as the standard resulted in a less accurate determination for cold-water fish gelatins (RF, Lowry: 1.56~1.64 and Biuret: 1.20~1.23), but more accurate for mammalian gelatins (RF, 0.98~1.03 for both method). The difference in hydroxypoline content, which affected the Tt of gelatins, correlated with the variation in color response (R2 > 0.95). However, the difference in molecular weight did not significantly affect the color response. These results suggest that for the accurate determination of total protein in gelatin solution with the Lowry or Biuret method, the choice of standard is important, and gelatin from a similar source should be used.
Session 17E, Food Chemistry: Proteins
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