AOAC Methods in Codex STAN 234 (Preliminary Methods Review)

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BARBANO ET At.: J. ASSOC. OFF ANAi.. CHEM. (VOi., 73. NO. 6, 1990)

D. H. Kleyn and L. Lengyel, Rutgers University, New Brunswick, NJ R. K. Stuckey, T. Cronau, J. Siebodnik, and J. Scott, Indiana State Board of Health, Indianapolis, lN T. Way, C. Mayfield, and C. Strayer, Applied Microbiolo– gy Services, Inc., College Station, TX H. M. Wehr and B. Burwell, Oregon Dept of Agriculture, Salem, OR REFERENCES (I) Kay, H. D., & Graham, W. R. (1935)}. Dairy Res. 6, 191-203 (2) Official Methods ofAnalysis ( 1984) 14th Ed. ancl { 1990) 15th Ed., AOAC, Arlington, VA (3) Rocco,R.M. ( 1990)J.FoodProt 53, 588- 591

be conducted to assess the suitability ofthis metbod for other dairy products including cheese, whey, and cream.

Acknowledgment• The. cooperationof the following collaborators is gratefully acknowledged: R. L. Bradley, Jr, M. Lundberg, and J. Beyer, University llf Wisconsin, Madison, WI D. Elliott and B. Thornhill, Florida Dept of Agriculture, Winter Haven, FL L Hensel, Mid-America Dairymen, Inc., Winsted, MN J. Jaworski, R. Cyr, and L. Justis, Vermont Dept of Agri– culture, Montpelier. VT

Kjeldahl Method for Determination of Total Nitrogen Content of Milk: Collaborative Study

DAVfD M. BARBANO and JENNY L. CLARK Cornell University, Department ofFood Science, Ithaca, NY 14853 CHAPMAN E. DUNHAM and .J. RICHARD FLEMING' Texas Milk Markel, PO Box I 10939, Carrolflon, TX 75011

Collaborating Laboratories: Cornell University and Northeast Dairy Herd Improvement Cooperative; and laboratories operated by or under contract to the following Federal Milk Markets: Chicago Regional; Eastern Ohio/Western Pennsylvania; Greater Kansas City; New England; Texas; Upper Midwest

protein determination that accurately mellsures protein . ln· frared milk analyzers, now in commercial use by the dairy industry, can be calibrated to predict the protein content of milk [972.16, 15th Ed. (l)] based on infrared light absor– bance at 6.465 ,um wavelength by the N-H bonds within the protein. Data from an accurate reference method for milk protein determinlltion is necessary for proper calibration of infrared milk anaJyzers. The Kjeldahl method measures nitrogen and from the nitrogen content of a sample the protein content can be estimated. The Kjeldabl method has been widely studied (2- 14). Many researchers have attempted to substitute reagents (3- 7, IO, 11), vary reagent quantities (5, 8, 9, 12, 13), and optimize digestion parameters (8, 9, 12-14) to improve the test accuracy, decrease testing time, and eliminate hazardous chemicals (e.g., mercury) that have a detrimental impact on the environment. The Kjeldabl method uses an acid digestion to release bound organic nitrogen and retain it as ammonium sulfate Received for publication February 27, 1990. This report was prcsen1ed nt lhc 103rd AOAC Annual International Meet– ing, September 25-21!, 1989. a1 St. Loui~, MO. The report has been evaluated by the General Referee and the Cu111mit1ce Statistician and reviewed by the Committee on Foods I. The method was approved interim official first action by the Chairman of the Official Methods Board a nd was adopted official first action at the 104th AOAC Annual International Meeting, September 9-13. 1990, a1 New Orlcuns, LA. Assoc iation actions will be published in " Change, in Offi-.;ial Methods of Analysis'' ( 1991) J. Assoc. Off Anal. Chm,. 74, January/ February issue. 1 J. Richard Fleming ii, Chairman. Test Procedures Commi\tec of the Feder– al Milk Marketing Orders.

Codex Trial Method Review Dairy farmers in some regions of the United States are paid on the basis of both the fat and protein contents of their milk or receive bonus payments for high milk protein content. Thus, it is very important to have a reference method for milk Amacro-KJeldahl ptocedure using a copper catalyst tor de– tentllnatlon of milk total nitrogen wa1 developed for both tradltlonal and block dlgeatot/steam dl1llller equipment, and the performance was evaluated by collaboratlve study. In the fll'1t trlal of the collaborative study, 9 pairs Of bllnd duplicate mllk 1amplee wet• analyzed for total nitrogen and total nHrogen was conver1,ed to "protein" by using a factor of 6.38. Prot•ln content of mllk samples ranged from 3.088 lo 3.810%. In the first trlal, It\ and R values for the block dlgeators were Influenced slgnlftcantty by protein concentra– tion; 1 11 and R values were not Influenced by protein concen· tratlon tot traditional equipment. H was hypothesized that total digestion time for some block dlgeafors In the first trlal waa not sufficient tor high ptoteln mllk samplee. Thut, a NCond trlal was undertaken wflh bolling time after clearing lnereased by 0.5 h. In the second trial, none of the parame- • lera for reproduclblllty with either type of equipment were · ll!fluenced by protern concentration. It waa concluded that laboratory-to-laboratory dltrerences In tine voltage mar re· quire different total digestion times In different laboratorlea, particularly those using bloekdJge1tor1. The i