Fluorometric determination of sulfhydrl plus disulfide as an index to heat treatment of half-and-half

A fluorometric technique was developed for measuring -SH + -SS- groups in acid whey from half-and-half at pH 4.6. Measurements were based on proportional decreases (quenching) by these groups of the fluorescence of fluorescein mercuric acetate. Percent quenching was determined with a fluorometer at exciting wavelengths of 415 to 436 nm and 515 nm for measurement. Calculations were made according to the formula: µmoles of -SH + -SS- (X) = percent quenching (Y)/10,256 + 0.00068. Heat treatments of half-and-half between 65.6°C and 82.2°C for 30 min caused progressive decreases in the -SH + -SS- content of the acid whey. A linear regression analysis relating µmoles of -SH + -SS- per ml of whey (Y) on processing temperatures (X) was: Y = 7.69 - 0.089X. There were no additional decreases in these groups by heat treatments between 82.2°C and 93.3°C. After removal of the heat denaturable whey proteins, the acid whey contained 0.44 + 0.04 µmoles of -SH + -SS- per ml of whey or 19.5% of the total in unheated acid whey.

Percentage decreases in whey protein nitrogen were closely related to the decreases in -SH + -SS-. The correlation coefficient was 0.99. Fluorometric measurement of -SH + -SS-, however, reflected a wider range of heat treatments than semimicro-Kjeldahl measurement of whey protein nitrogen.