Comparison of Solid-Phase Microextraction and Dynamic Headspace Methods for the Gas Chromatographic--Mass Spectrometric Analysis of Light-Induced Lipid Oxidation Products in Milk

Abstract

A sensitive, rapid procedure for testing lipid oxidation products in milk is developed using solid-phase microextraction (SPME) and gas chromatography-mass spectrometry. SPME is as sensitive as dynamic headspace (DH) analysis for measuring the pentanal and hexanal produced in milk after exposure to light. Furthermore, compared with DH, SPME is less expensive and demonstrates better precision and accuracy. In addition, SPME does not exhibit carryover or septa artifact peaks. The linearity of calibration curves (based on the method of additions technique with an internal standard) is consistently better for SPME than for DH. Furthermore, replicate analyses of pentanal and hexanal spiked in skim milk and 2% milk at 2 ng/mL demonstrate significantly lower coefficients of variation using SPME. To further test the practicality of SPME for measuring light-induced chemical changes in milk, 2% milk and skim milk samples are exposed to fluorescent light (200 footcandles) for 0, 3, 6, 9, 12, 17, 24, and 48 h and analyzed by SPME and DH. Pentanal and hexanal in all samples are measured by SPME and DH. Correlation coefficients of resulting plots indicate that SPME is more accurate than DH in measuring the quantity of lipid oxidation products in milk. Autoxidation of unsaturated fatty acids involves a free radical reaction, forming fat hydroperoxides which degrade to various malodorous compounds (e.g., hexanal, the predominant lipid reaction byproduct in light-exposed milk in the case of linoleic acid) (3).