Determination of fat in raw and processed milks by the Gerber method: Collaborative study

The Gerber method is used worldwide as a simple and rapid method for determ ining fat in raw and processed milks. However, the volume of the test porti on used in the method has not been internationally agreed upon. A collabora tive study was conducted to evaluate performance of the Gerber method using either a weighed test portion (11.13 g) or by a 10.77 mL test portion deli vered by pipet. For each method, laboratories received 10 test samples: 5 r aw and 5 pasteurized homogenized milks, 2 of which were blind duplicate pai rs. Eleven and 10 laboratories participated in the evaluation of aliquot ad dition by weight and pipet, respectively. Mojonnier ether extraction (Metho d 989.05) was used as the reference method. Interlaboratory study statistic s were similar between methods of test portion addition and between raw and processed materials; therefore, summary interlaboratory study statistics w ere pooled. The fat content of milk samples ranged rom 0.96 to 5.48%. Absol ute reproducibility and repeatability were not affected by fat level, and p ooled statistical performance (invalid and outlier ata removed) was (g fat/ 100 g milk) S-r = 0.026, S-R =.047, r = 0.074, and R = 0.132. Relative stan dard deviations increased with decreasing fat content, and were summarized by fat level: 1-2% fat milk, mean = 1.437, RSDr = 1.809%, RSDR = 3.271%; 2- 6% fat milk, mean = 4.156, RSDr = 0.626%, RSDR = 1.131%. Compared with ethe r extraction, test results by the Gerber method were slightly lower (0.02% fat) using a weighed test portion and significantly lower (0.06% fat) using a 10.77 mL volume addition by pipet. A trend toward underestimating fat co ntent at lower fat concentrations (1-2% fat) was observed with the weighed test portion but not when a pipet was used. The Associate Referee recommend s that the Gerber method using a weighed test portion be adopted as First A ction with applicability limited to whole milk.