Choice of protein standard important for assay of total protein in gelatin solution.

Gelatin is an important functional food biopolymer. When gelatin is extracted, or during studies of its chemical and rheological properties, the total protein count is often determined by using the Lowry or Biuret method, with commercial gelatin as the standard. However, gelatins can be obtained from different sources with different molecular weights so that the variation in protein color response may not be known.

The goal of scientists at Cornell University was to determine whether the choice of a standard affects total protein assays. They found that for the accurate determination of the total protein count in gelatin solution using the Lowry or Biuret method, the choice of standard is important. Gelatin from a similar source should be used.

In their studies, the researchers determined the color response at 650 nm for the Lowry and 540 nm for the Biuret methods. 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.

The investigators determined the molecular weight distribution by using SDS-PAGE. They determined hydroxyproline by using the Woessner method, and the thermal helix-random transformation temperature (Tt) by measuring the change of viscosity with temperature.

The standard curves obtained with the reference gelatins showed a significant variation. Using CSG as the standard led to a more accurate determination for cold-water fish gelatins, but it was less accurate for mammalian gelatins. Using PSG as the standard resulted in a less accurate determination for cold-water fish gelatins, but it was more accurate for mammalian gelatins. The difference in hydroxypoline content, which affected the Tt of the gelatins, correlated with the variation in color response. The difference in molecular weight did not significantly affect the color response.

Further information. Joe Regenstein, Department of Food Science, Cornell University, 8 Stocking Hall, Ithaca, NY 14853; phone: [phone]; fax: [phone]; email: [e-mail].