Automatic microgravimetric determination of fats in milk products by use of supercritical fluid extraction with on-line piezoelectric detection

An on-line supercritical fluid extraction-piezoelectric detection system was developed and applied to the quantitative gravimetric determination of total fat in food samples (skimmed milk and cocoa). The proposed assembly provides all the advantages of an on-line system as regards automation, in addition to acceptable sensitivity and precision. Its strength lies in the design of the interface between the supercritical fluid extractor and the piezoelectric detector. Samples of skimmed milk and cocoa are weighed in the extraction thimble, previously loaded with 1 g of diatomaceous earth. A temperature of 100°C and a CO2 fluid density of 0.60 mg/ml are used for extraction. The linear calibration range thus achieved is 0.005–0.07% w/w total fat, and the relative standard deviation is ±2.3% (n=11; P=0.05). The throughput is six samples h−1 (for the overall process). The proposed method was used to determine the total fat in food samples (milk, cocoa), the results being competitive with those of the Soxhlet methods for the same purpose.