Chemical and physical properties of vitamin E

Vitamin E is the major lipid-soluble component in the cell antioxidant defence system and is exclusively obtained from the diet. The tocopherols are viscous oils at room temperature, insoluble in water but soluble in ethanol and aprotic solvents. Vitamin E is a slightly yellow to amber, practically odorless and tasteless, clear, viscous oil, which darkens on exposure to air or light by oxidation.

Vitamin E is a generic term denoting eight different isomers among which α-tocopherol is the most important and most active.

The melting point of RRR-α-tocopherol is 3 °C. The optical rotations of tocopherols are very small and depend on the nature of the solvent. RRR-α-tocopherol (RRR-α-T), is known to be more bioactive than all-rac-α-tocopherol (all-rac-α-T), a synthetic racemic mixture of 8 stereoisomers.

Tocopherols are easily oxidized and can be destroyed by peroxides ozone and permanganate in a process catalyzed by light and accelerated by polyunsaturated fatty acids and metal salts.

The ultraviolet absorption spectra of tocopherols and tocotrienols in ethanol show an absorption maximum at 292–298 nm, while the infrared spectra show OH (2.8 ± 3.0 µm) and CH (3.4 ± 3.5 µm) stretching and a characteristic band at 8.6 µm.

Vitamin E is a fat-soluble vitamin that consists of a group of tocols and tocotrienols with hydrophobic character, but possessing a hydroxyl substituent that confers an amphipathic character on them.

The interactions between vitamin E and water are hydrophobic in nature; hence, vitamin E is found to be insoluble in water since vitamin E is non-polar and water is polar in nature. Solubility is reported for α-tocopherol in aqueous mixtures containing ethanol at 33 °C. It has very low solubility in pure water owing to hydrophobic repulsion.

α-Tocopherol is fluorescent with an emission maximum about 325 nm in a hydrophobic solution. The bond dissociation energy of α-tocopherol's O–H bond is 77.1 kcal mol−1.31 The pKa values for α-, β-, γ-, and δ-tocopherol in a micellar solution were reported as 13.1, 12.8, 12.7, and 12.6 respectively.

The α-tocopherol are unstable to air and light, particularly when in alkaline media. α-tocopherol acid succinate is insoluble in water, slightly soluble in alkaline solutions, soluble in alcohol, in ether, in acetone and in vegetable oils; very soluble in chloroform.

The partition coefficient of α-tocopherol is 12.2, while that for ascorbic acid is −1.85,30 showing their lipophilic and hydrophilic characters, respectively.
Chemical and physical properties of vitamin E

Tocopherol losses in food

Naturally occurring vitamin E comprises tocopherols and tocotrienols. Processing and storage of foods can result in substantial tocopherol losses. Cooking of porridges of rolled oats and rye meal implies only minor effects on their tocopherol and tocotrienols contents.

Studies of alpha-tocopherol in UHT processed milk suggest that the processing conditions affect the subsequent losses, but that in all cases increased storage temperature led to increased rate of loss.

There was a loss of tocopherol reported in the study of potato chips. After only two weeks’ storage of the chips at room temperature, nearly half of the tocopherol was lost.

The losses were only slightly smaller during storage at freezer temperature.

Deep fat frying of fresh vegetable oil causes losses of about 10% but storage of fried foods, even at low temperature, may cause large losses.  The low content of alpha-tocopherol in frozen foods is surprising and indicates serious degradation even at 12 °C.

Boling of vegetables in water for up 30 minutes results in only minor losses of tocopherol.

Baking of white bread (200 °C, 30 minutes) destroyed of about 5% of the tocopherol in the crumb.

Tocopherol losses during microwave cooking are mainly caused by the effect of high temperature and not by microwaves as such. When sunflower oil was subjected to microwaves discontinuously for 120 min at two constant temperatures: namely 170 °C and less than 40 °C, tocopherol losses were 72% and 21%, respectively.
Tocopherol losses in food