A SENSITIVE METHOD FOR DETERMINING INFANT MILK FORMULA LIPID OXIDATION

Milk is a complex system that contains polyunsaturated fatty acids (PUFA) prone to lipid oxidation. The oxidation of PUFAs in milk produces primary oxidation products known as oxylipins which can be measured by ultra-high pressure liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Because UPLC-MS/MS is highly sensitive, we tested the hypothesis that it would readily detect lipid oxidiation in infant milk formula compared to conventional methods such as 'Thiobarbituric Acid Reactive Substances' (TBARS) and hydroperoxide. A secondary aim was to test the impact of milk formula PUFA composition on lipid oxidation. In this study, lipid oxidation was measured in two infant milk formulas. Formula 1 contained canola oil and Formula 2 contained Canola Oil + 1% DHA Ethyl Ester as the fat source (each representing 4% of fat). The formulas (n=5) were sampled at baseline (timepoint 0) and then stored at 4 ºC for 21 days. Additional samples were collected on days 1,3,7,14 and 21. Oxylipins, hydroperoxide and TBARS concentrations were measured. Two-way analysis of variance showed that formula type but not storage time altered TBARs and hydroperoxide concentrations, whereas both storage time and formula compositon impacted oxylipins. TBARS concentrations, ranged from 0.29 to 0.38 and from 0.34 to 0.49 µg MDA.kg-1 for Formula 1 and 2, respectively, being significantly different at all time-points. Similarly, hydroperoxide of Formula 2 was significantly higher than Formula 1 values. PUFA-derived oxylipins increased with storage time for both formulas; however, those derived from long-chain omega-3 PUFAs only increased in Formula 2, whereas those derived from shorter chain PUFAs increased in Formula 1. These results support the hypothesis that free oxylipins can be used to detect early lipid oxidation in infant milk formula stored at 4 ºC compared to conventional methods. Storage-related changes in oxylipin species depend on the type of PUFAs present in milk formula.