SIMPLIFIED GRAVIMETRIC-DETERMINATION OF TOTAL FAT IN FOOD COMPOSITES AFTER CHLOROFORM-METHANOL EXTRACTION

A modification of Association of Official Analytical Chemists (AOAC) m ethod 983.23 for the quantitative determination of total lipid in food composites was evaluated for the measurement of total fat. The proced ure is based on the Bligh and Dyer chloroform/methanol total lipid ext raction. Relative to AOAC 983.23, the proposed method is less labor-in tensive and is applicable to batch analysis of a larger number of samp les, thus reducing the cost of analysis and increasing sample throughp ut. Total lipid values from the proposed method are comparable to thos e from AOAC 983.23 and slightly higher than total fat determined by ac id hydrolysis (AOAC 954.02, 945.44, or 922.06). Recoveries of standard additions of different food-grade oils from a mixed food composite we re essentially quantitative, ranging from 96 to 101%. Total lipid meas ured in Total Diet Standard Reference Material 1548 (SRM 1548, Nationa l Institute of Standards and Technology) was 101% of the certified mea n total fat content and within the certified range. The method is to b e suitable for analysis of food composites with between 0.15 and 1.5 g total fat (3 to 30% by weight). More than 600 samples of a variety of total diet composites were collected and assayed as diet quality cont rol samples for two National Heart, Lung and Blood Institute-sponsored multicenter clinical feeding trials: DELTA (Dietary Effects on Lipopr oteins and Thrombogenic Activity) and DASH (Dietary Approaches to Stop Hypertension). The mean coefficient of variation was 1.2% for duplica te assays of these samples over the course of two years and multiple a nalysts. In addition, total lipid values for more than 200 samples of a diet composite quality control material, used in this laboratory ove r a two-year period, had a 3.99% coefficient: of variation. Although t he accuracy of all gravimetric total fat methods with respect to the U .S. Food and Drug Administration's Nutritional Labeling and Education Act (NLEA) definition of total fat as the sum of triglycerides remains to be determined, the reported modification of AOAC 983.23 yields a t otal fat content of acceptable accuracy relative to other gravimetric methods, and with proper quality control the method has excellent prec ision.