COULD THE DUMAS METHOD REPLACE THE KJELDAHL DIGESTION FOR NITROGEN AND CRUDE PROTEIN DETERMINATIONS IN FOODS

Increased demand for determinations of nitrogen (N), and hence crude p rotein (CP), has led to wider use of the Dumas method in place of the traditional Kjeldahl methods. Although Kjeldahl N (KN) and Dumas N (DN ) represent different N fractions, published studies on infant formula , animal feed and meat products have indicated that DN could replace K N with little practical impact on the reliability of the N values obta ined. This study was conducted to establish whether DN determination c ould replace that of KN in a broader range of foods for CP calculation . Statistical analysis was performed on in-house assayed KN and DN val ues together with published KN and DN values for selected food product s. In the range 0.05-6.8% N, KN may be estimated from DN with the equa tion: KN=1.00((P<0.01)) X DN - 0.09((P=0.50)) (n = 101, R(2) = 0.98, P -regression < 0.01). Because N levels in individual groups of food did not span the entire range of N contents, KN : DN ratios were calculat ed for each food group. KN:DN ratios differed significantly (R(2) = 0. 25, P < 0.01) from group to group. Ratios of 1.01 for dairy, 1.00 for oilseeds, 0.99 for feed, 0.98 for infant formulas, 0.95 for cereals, 0 .94 for meats, 0.89 for vegetables, 0.80 for fish and 0.73 for fruits were valid for the estimation of KN and CP using DN data. CP was indep endently calculated as CP1 = H x KN or CP2 = H x KN:DN x DN, where H i s the nitrogen to protein conversion factor for the food group. Mean d ifferences between CP1 and CP2 values were 0% for dairy, oilseeds, fee d, infant formulas and baby foods, cereals, meat and meat products, ve getables and vegetable products and fruit, and 1% for fish. These resu lts suggest that DN may replace KN for the determination of N and CP i n selected food groups when appropriate coefficients are used.