Interlaboratory Study on the Determination of Crude Protein in Macaroni Products on JAS by Kjeldahl Method Using Copper Catalysts

Abstract

The Kjeldahl method using copper catalysts for digestion was established as an analytical method for the determination of crude protein in macaroni products in the Japanese Agricultural Standard (JAS). An interlaboratory study was performed to evaluate this Kjeldahl method. Eleven participating laboratories independently analyzed five test materials (five pairs of blind duplicates) of macaroni products. After removal of statistical outliers, the repeatability (RSD r ) and reproducibility (RSD R ) of the evaluated methods were 0.25-0.59 % and 1.2-1.5 %, respectively. The HorRat values (RSD R (%)/predicted RSD R (%)) of the total nitrogen content were calculated to be at 0.32 to 0.43. This method was shown to have acceptable precision in this study.

Determination of Crude Protein in Macaroni Products by the Combustion Method and Comparison with the Kjeldahl Method: Interlaboratory Study

“…For the interlaboratory study, each 200 g of four kinds of commercially available macaroni products made from durum semolina with or without egg, tomato, and spinach were ground in the mill. The ground products, ranging in size from 500 μm to 850 μm (Hakoda et al, 2008;2009) were selected using two sieves with pore sizes of 500 μm and 850 μm. Five test materials with different crude protein contents were prepared from durum semolina and egg (material 1), durum semolina (material 2 and 3), durum semolina, tomato, and spinach (material 5), durum semolina, egg, tomato, and spinach, resulting from the mixture of materials 1 and 5 (material 4).…”

“…Fourteen laboratories analyzed five test materials (five pairs of blind duplicates) of macaroni products. The results for the combustion method were also compared with those for the Kjeldahl method on the same five test materials reported previously (Hakoda et al, 2009). …”

“…To review the standard for macaroni products, validated analytical methods were required. The Kjeldahl method has been validated in a previous study (Hakoda et al, 2009).The combustion method is another method for the determination of total nitrogen content. An organic substance is oxidized to inorganic gases such as NO 2 and CO 2 by furnace heating at an elevated temperature, in pure oxygen.…”

“…Five test materials with different crude protein contents were prepared from durum semolina and egg (material 1), durum semolina (material 2 and 3), durum semolina, tomato, and spinach (material 5), durum semolina, egg, tomato, and spinach, resulting from the mixture of materials 1 and 5 (material 4). Each test material for the interlaboratory study was divided into 72 light-proof bags (28 bags for the laboratory of the combustion method, 20 bags for the laboratory of the Kjeldahl method in the previous study (Hakoda et al, 2009), 10 bags for homogeneity test and 14 bags for spare samples), at a weight of 2 g per bag, purged with nitrogen and sealed. These test materials were identical to the ones in the previously reported interlaboratory study using the Kjeldahl method (Hakoda et al, 2009).…”

“…Each test material for the interlaboratory study was divided into 72 light-proof bags (28 bags for the laboratory of the combustion method, 20 bags for the laboratory of the Kjeldahl method in the previous study (Hakoda et al, 2009), 10 bags for homogeneity test and 14 bags for spare samples), at a weight of 2 g per bag, purged with nitrogen and sealed. These test materials were identical to the ones in the previously reported interlaboratory study using the Kjeldahl method (Hakoda et al, 2009).Although collaborative studies were previously conducted to compare the combustion methods with Kjeldahl methods for the determination of crude protein contents in cereal grains and oilseeds, animal feed and fishmeal (Bicsak, 1993;Sweeney, 1989;Miller et al, 2007), those on macaroni products were not found. Moreover, these collaborative studies did not include various types of instruments commonly used for the combustion method in Japan.…”