Quantification of hexanal as an index of lipid oxidation in human milk and association with antioxidant components

Abstract

Hexanal, a secondary product of lipid oxidation, was identified as the major volatile aldehyde generated from lipid peroxidation in human milk. Hexanal was quantified in human milk using solid phase microextraction-gas chromatography/flame ionization detection that required correction for recovery based on the fat content of human milk. Alpha-tocopherol was the only tocopherol isomer in human milk found to be significantly correlated with hexanal (R = -0.374, p<0.05) and the total antioxidant capacity of human milk (ORAC(Fl) (R = 0.408, p<0.01)). Ascorbic acid content was negatively correlated (R = -0.403, p<0.05) with hexanal, but not to ORAC(Fl) in human milk. The effect of Holder pasteurization on oxidative status of human milk was determined using multiple parameters that included, hexanal level and malondialdehyde as markers of lipid oxidation, vitamins C and E content and antioxidant capacity (e.g. ORAC(Fl)). Pasteurization did not affect the oxidative status of milk as measured by hexanal level, ORAC(Fl) and malondialdehyde content. We conclude that hexanal is a sensitive and useful chemical indicator for assessing peroxidation reactions in human milk and that alpha tocopherol and ascorbic acid are two key antioxidant components in milk that contribute to protection against oxidation of milk lipids.

Keywords: SPME; antioxidant capacity; hexanal; human milk; pasteurization.

Fig. 1

Volatiles identified from milk samples by SPME-GC/MS. Compounds identified are 1. CO₂; 2. 2-methyl-2-propanol; 3. Cyclohexane; 4. Hexane; 5. 2-ethoxy-2-methyl-propane; 6. 4-methyl-1-heptanol; 7. Pentanal; 8. 1-chloropentane; 9. 4-methyl-heptane; 10. Hexanal; 11. 2,4-dimethyl-heptane; 12. 2,4-diethyl-1-heptene; 13. Heptanal; 14. Hexanoic acid; 15. Octanal; 16. Dichlorobenzene; 17. 3-ethyl-2-methyl-1,3-hexadiene; 18. 2-octenal; 19. Nonanal; 20. Octanoic acid; 21. 1,4-di-tert-butyl-benzene; 22. Decanoic acid; 23. Dodecanal.

Fig. 2

Results of 4-heptanone (internal standard) recovered from representative milk samples chosen with distinct fat contents. Milk samples were spiked with 0.2 ppm internal standard.

Fig. 3

Regression analysis of the recovery of hexanal from human milk affected by fat content. Milk samples were spiked with 0.2 ppm hexanal for recovery purposes.

Fig. 4

Recovery of pentanal, hexanal and 4-Heptanone (internal standard) in milk samples of varying fat content as determined using GC-FID. The peaks are obtained by subtracting the chromatogram obtained for milk spiked with 0.2 ppm of internal standard with the chromatogram of unspiked milk.

Fig. 5

Hexanal and pentanal generation by treatment of milk at 50°C for 24 h as detected using static headspace gas chromatography.

Fig. 6

The aldehyde profile of human milk containing low vitamin C content (A) and high vitamin C content (B).