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

Abstract

An interlaboratory study was conducted to evaluate the reproducibility of a combustion method for determining crude protein in macaroni products according to the Japanese Agricultural Standard (JAS). Fourteen participating laboratories analyzed five test materials (five pairs of blind duplicates) of macaroni products by the combustion method. Six different types of instruments commonly used for the combustion method in Japan were included in this study. After removal of statistical outliers, the repeatability and reproducibility of this method were found to be 0.16 − 0.53 % and 0.89 − 1.1 %, respectively. The HorRat values of this method were 0.25 to 0.31. The results for the combustion method were compared with those for the Kjeldahl method used on the same test materials and reported previously. Differences of means between the two methods were 0.10 − 0.13 % in five test materials. The differences did not considerably affect judgment near the standard values 11 % and 12 % of the JAS.Keywords: interlaboratory study, macaroni, JAS, Kjeldahl, combustion *To whom correspondence should be addressed. E-mail: akiko_hakoda@nm.famic.go.jp IntroductionMacaroni products have been standardized according to the Japanese Agricultural Standard (JAS) (Ministry of Agriculture and Forestry of Japan, 1973;1994). The JAS, established by the Ministry of Agriculture, Forestry and Fisheries, is a standard of quality, such as composition, grading and performance. The JAS system refers to the certification system that attaches JAS marks to the products inspected in accordance with the JAS. This macaroni standard indicates that the minimum crude protein contents of macaroni products with and without eggs are 11 % and 12 %, respectively. In general, the crude protein content in macaroni products has been calculated from determined total nitrogen content. A detailed description regarding the determination of total nitrogen content is not provided in the JAS, except for the use of the Kjeldahl method. Since 2000, a regular revision process of the JAS has been implemented every five years to meet the changing social needs for foods listed in the JAS. 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. A thermal conductivity detector quantitatively measures the resulting freed nitrogen. Each operation is performed continuously in a special instrument with a furnace, isolation system, and detection system. This combustion method employs less hazardous chemicals, supports easier operation, and allows more rapid determination (e.g., in about five minutes) than the Kjeldahl method. For example, an operation to determine nitrogen content involves only the measurement of the weight of a test portion, which i...

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A Certified Urea Reference Material (NMIJ CRM 6006-a) as a Reliable Calibrant for the Elemental Analyses of Amino Acids and Food Samples

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