Changes in Sulfhydryl Content of Egg White Proteins Due to Heat and Pressure Treatment

• Published in: Journal of agricultural and food chemistry Vol. 53; no. 14; pp. 5726 - 5733
• Main Authors: Van der Plancken, Iesel, Van Loey, Ann, Hendrickx, Marc E. G
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 13.07.2005
• Subjects: Biological and medical sciences; Dithionitrobenzoic Acid; egg albumen; Egg and egg product industries; egg proteins; Egg Proteins - chemistry; elasticity (mechanics); Food industries; Fundamental and applied biological sciences. Psychology; gels; heat treatment; high pressure treatment; Hot Temperature; Hydrogen-Ion Concentration; Pressure; Protein Denaturation; solubility; Sulfhydryl Compounds - analysis; sulfhydryl groups; Sulfhydryl Reagents; sulfhydryl content; Heat treatment; Physical dressing; Solubility; Egg product; Egg white; high-pressure treatment; Pressure; Protein content; High pressure
• ISSN: 0021-8561; 1520-5118
• DOI: 10.1021/jf050289+

The sulfhydryl (SH) content of egg white proteins (10% v/v or 9.64 mg of protein/mL) after heat (50−85 °C) and combined heat- and high-pressure treatments (100−700 MPa, 10−60 °C) was determined using 5‘,5-dithiobis (2-nitrobenzoic acid) (DTNB), both for the soluble fraction and the total protein fraction. Only irreversible changes were taken into account. Both physical treatments were performed at two pH levels:  pH 7.6, corresponding to the pH of fresh egg white, and pH 8.8, corresponding to that of aged egg white. Both heat and combined heat- and high-pressure treatment resulted in an exposure of buried SH groups. These exposed SH groups were involved in the formation of disulfide bond stabilized protein aggregates, as shown by gel electrophoresis. Under severe processing conditions (above 70 °C at atmospheric pressure or above 500−600 MPa, depending on the temperature applied), a decrease in total SH content could be observed, probably due to the formation of disulfide bonds by oxidation, especially at alkaline pH when the thiolate anion was more reactive. The high degree of exposure of sulfhydryl groups, and subsequent oxidation and sulfhydryl−disulfide bond exchange reactions resulting in soluble aggregates, can explain why pressure-induced egg white gels are softer and more elastic than heat-induced ones. When pressure treatment was performed at low temperatures (e.g., 10 °C), a lower pressure was required to induce similar changes in the sulfhydryl content, as compared to higher temperatures (e.g., 25 °C), indicating an antagonistic effect between pressure and temperature in the domain studied (10−60 °C, 100−700 MPa). Treatment conditions resulting in extensive protein insolubilization were accompanied by a transfer of free sulfhydryl groups from the soluble to the insoluble protein fraction. These SH groups were mainly accessible to DTNB. Keywords: Egg white; sulfhydryl content; solubility; heat treatment; high-pressure treatment