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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 protein (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 KN with little practical impact on the reliability of the N values obtained. This study was conducted to establish whether DN determination could replace that of KN in a broader range of foods for CP calculation. Statistical analysis was performed on in-house assayed KN and DN values together with published KN and DN values for selected food products. In the range 0.05-6.8% N, KN may be estimated from DN with the equation : 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 calculated 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 independently calculated as CP1 = H x KN or CP2 = H x KN : DN x DN, where H is the nitrogen to protein conversion factor for the food group. Mean differences between CPI and CP2 values were 0% for dairy, oilseeds, feed, infant formulas and baby foods, cereals, meat and meat products, vegetables and vegetable products and fruit, and 1% for fish. These results suggest that DN may replace KN for the determination of N and CP in selected food groups when appropriate coefficients are used.