Edible fats and oils in their neat form are ideal candidates for Fourier transform infrared (FTIR) analysis, in either the attenuated total reflectance or the transmission mode. FTIR spectroscopy provides a simple and rapid means of following complex changes that take place as lipids oxidize. Safflower and cottonseed oils were oxidized under various conditions, and their spectral changes were recorded and interpreted. The critical absorption bands associated with common oxidation end products were identified by relating them to those of spectroscopically representative reference compounds. The power and utilty of FTIR spectroscopy to follow oxidative changes was demonstrated through the use of “real-time oxidation plots.” A quantitative approach is proposed in which standards are used that are spectroscopically representative of oxidative end products and by which the oxidative state of an oil can be defined in terms of percent hydroperoxides, percent alcohols and total carbonyl content. By using either relative absorption as a basis or calibrating on representative standards, FTIR analysis provides a rapid means of evaluating the oxidative state of an oil or of monitoring changes in oils undergoing thermal stress.

Monitoring the oxidation of edible oils by fourier transform infrared spectroscopy.

A sample of debromination methyl linoleate has been autoxidized to a peroxide value of 671 m.e./kg. at approximately 0°C. in the dark. An essentially pure concentrate of methyl octadecadienoate monohy-droperoxide was quantitatively separated; infrared and ultraviolet spectral studies were made on the peroxide concentrate and on the corresponding hydroxyl derivative obtained by reducing the peroxides with stannous chloride. The infrared data showed no conjugated peroxides having geometric configurations other than cis, trans; the same data also showed that the peroxide concentrate contained at least 90% conjugated cis,trans forms. Calculations based on ultraviolet spectrophotometric methods also indicated that the peroxides were at least 90% conjugated. The remaining 10% of the sample is most likely nonconjugated diene hydroperoxide. Since analogous cis, cis conjugated dienes have not been isolated and their infrared and ultraviolet properties are unknown, their presence here in small amounts is possible. Ultraviolet and infrared spectra of the reduced compounds conform closely to those of the peroxides except for reduction in the intensity of the OH bond at 2.88 μ. The infrared absorption spectra of the C−H structure and carbonyl groups of an essentially pure conjugated cis, trans methyl octadecadienoate monohydroperoxide were recorded, using a LiF prism. The infrared absorption spectra of the C−H strucfraction isolated from methyl linoleate autoxidized in the dark at 24°C. indicated that appreciable amounts of conjugated trans, trans hydroperoxides were present, in addition to those of the cis, trans type. It is possible that the conjugated cis, trans isomers were formed originally but were labile at the higher temperature and in the presence of catalysts (e.g., peroxides) were transformed to the thermodynamically more stable conjugated trans, trans isomer.

Structure of hydroperoxides obtained from autoxidized methyl linoleate

Spectroscopic evaluation of human tear lipids.

PURPOSE. To measure lipid compositional and structural changes in lenses as a result of hyperbaric oxygen (HBO) treatment in vivo. HBO treatment in vivo has been shown to produce increased lens nuclear light scattering. METHODS. Guinea pigs, approximately 650 days old at death, were given 30 and 50 HBO treatments over 10- and 17-week periods, respectively, and the lenses were sectioned into equatorial, cortical, and nuclear regions. Lipid oxidation, composition, and structure were measured using infrared spectroscopy. Phospholipid composition was measured using 31 P-NMR spectroscopy. Data were compared with those obtained from lenses of 29- and 644-day-old untreated guinea pigs. RESULTS. The percentage of sphingolipid approximately doubled with increasing age (29 ‐544 days old). Concomitant with an increase in sphingolipid was an increase in hydrocarbon chain saturation. The extent of normal lens lipid hydrocarbon chain order increased with age from the equatorial and cortical regions to the nucleus. These order data support the hypothesis that the degree of lipid hydrocarbon order is determined by the amount of lipid saturation, as regulated by the content of saturated sphingolipid. Products of lipid oxidation (including lipid hydroxyl, hydroperoxyl, and aldehydes) and lipid disorder increased only in the nuclear region of lenses after 30 HBO treatments, compared with control lenses. Enhanced oxidation correlated with the observed loss of transparency in the central region. HBO treatment in vivo appeared to accelerate age-related changes in lens lipid oxidation, particularly in the nucleus, which possesses less antioxidant capability. CONCLUSIONS. Oxidation could account for the lipid compositional changes that are observed to occur in the lens with age and cataract. Increased lipid oxidation and hydrocarbon chain disorder correlate with increased lens nuclear opacity in the in vivo HBO model. (Invest Ophthalmol Vis Sci. 2000;41:3061‐3073)

Impact of aging and hyperbaric oxygen in vivo on guinea pig lens lipids and nuclear light scatter.

UVA light in vivo reaches the nucleus of the guinea pig lens and produces deleterious, oxidative effects.

A study has been made of the infrared absorption spectra of autoxidized methyl linoleate in samples ranging from PV 1 to PV 940 m.e./kg. Principal changes occur in the frequency range 3400–3550 cm−1 where bonded−OH groups absorb and at 1650–1775 cm−1 where >c=0 groups absorb.

A. S. Henick

Papers

An infrared absorption study of autoxidized methyl linoleate

A chromatographic study of autoxidized methyl linoleate