Automated Modified Lowry Method for Protein Analysis of Milk

Abstract

Protein concentrations in milk were compared using a modified Lowry and the micro-Kjeldahl methods. Correlation coefficients for absorbance (Lowry) vs protein (Kjeldahl) were 0.925, 0.904 and 0.899 for individual Holstein, individual Guernsey and bulk raw milk samples, respectively, whereas, the RZ was 0.9998 for samples of 2.5-4.0% protein prepared from three brands of nonfat dry milk. Added lactose, up to 16 mg/ml milk, did not interfere in the test. Storage at 4°C for up to 6 days was without effect. However, whey proteins had a lower specific absorbance than did casein. The standard deviation of differences among triplicate Lowry tests was about one-third of that for duplicate micro-Kjeldahl tests.

Dry heat treatment of skim milk powder greatly improves the heat stability of recombined evaporated milk emulsions

“…Low-heat skim milk powder (SMP), characterized by a whey protein nitrogen index (WPNI) of 6.1 and a fat content of 0.8%, was obtained from Milcobel Dairy Corporation (Kallo, Beveren, Belgium). Protein and carbohydrate analysis by the methods described in previous research (Huang, Marshall, Anderson, & Charoen, 1976;Morais, Dweck, Valenzuela-Diaz, & Büchler, 2005) revealed that the SMP contained 37.7% of protein (7.6% whey protein, 30.1% casein, w/w) and 51.5% of lactose (w/w). Sodium caseinate and whey protein isolate (WPI) used as controls in SDS-PAGE were purchased from Sigma-Aldrich (Overijse, Belgium).…”