Determination of major minerals in dairy products digested in closed vessels using microwave heating

A method is described for digestion of a variety of solid dairy products (cheeses, caseinates, and milk powders) for subsequent determination of Ca, Mg, Na, and K by flame atomic spectrophotometry. Sample preparation included a microwave digestion system in closed vessels. The procedure has been validated for skim milk powder employing various reaction mixtures that contain HNO3, HClO4, and H2O2. Accurate results were achieved for Na and K with mixtures of HNO3 and H2O2, but values for Ca and Mg were low. Although the results of these elements were improved when HClO4 was used instead of HNO3, greatest accuracy was obtained by measurement, using standard addition, of samples digested with mixtures of HNO3 and H2O2. The assays exhibited varying degrees of precision when used for caseinates and cheese. The coefficients of variation for replicate analyses of caseinates and cheeses were lowest for Mg(1.34 to 2.46%) and Ca (1.15 to 2.69%) and poorest for Na (3.35 to 9.02%). However, accuracy was greater for Na and K analyses; recoveries of these elements were very close to 100% (Na, 96.3 to 99.2%; K, 97.8 to 99.8%) but recoveries of Ca and Mg were lower (Ca, 91.6 to 98.8%; Mg, 94.0 to 98.4%).