Episesamin is a stereoisomer of sesamin. To be more specific, sesamin is an optically active compound having the structure represented by formula I:

and episesamin, an isomer of sesamin, is an optically active compound having the structure represented by formula II:

As is clear from the two formulas, sesamin has a symmetrical structure in the plane whereas episesamin has an asymmetric structure.
Sesamin is one of the principal lignan compounds in sesame and is contained in sesame seeds in amounts of 0.1-0.5%. In contrast, episesamin does not naturally occur in sesame seeds but when sesame oil obtained by pressing is passed through the step of refining to salad oil of higher purity and the like, sesamin undergoes epimerization to give episesamin as a by-product (non-patent reference 1), and sesamins refined from the refined sesame oil are known to contain sesamin and episesamin in proportions of nearly 1:1 by weight ratio (non-patent reference 2).
Experiments with mixtures of sesamin and episesamin (at ca. 1:1) have revealed various physiological activities of sesamins including, for example, the action of inhibiting Δ5-unsauration enzymes (non-patent references 3 and 4, as well as patent reference 1), anti-oxidizing action against lipids (patent references 2 and 3), antihypertensive action (patent reference 4), the action of improving hepatic functions (patent reference 5), the action of scavenging active oxygen (patent reference 6), the blood cholesterol lowering action and/or cholesterol lowering action (patent references 7 and 8), the action of in vivo stabilization of highly unsaturated fatty acids (patent reference 9), and the action of preventing sickness from drinking (patent reference 10).
Recent studies have also revealed the differences between the physiological activities of sesamin and episesamin. For example, rats administered orally with a mixture of sesamin and episesamin (ca. 1:1) were shown to have such an in vivo distribution that the transfer of episesamin into organs was at least twice as much as that of sesamin (non-patent reference 5). In addition, experiments where rats were separately administered orally with sesamin and episesamin yielded a report showing that episesamin markedly increased the gene expression and enzymatic activity of β-oxidation enzymes in the liver as compared with sesamin and that there was no difference between sesamin and episesamin in terms of activity for inhibiting fatty acid synthases (non-patent reference 6). These are a few of the reports that have recently been made of the beneficiary effects of episesamin.
Methods so far proposed for sesame lignan production include extracting sesame oil with organic solvents such as alcohol (e.g. methanol), acetone, petroleum ether and acetonitrile, or mixtures of these solvents with water, as well as subjecting sesame oil to molecular distillation (patent reference 11).    [Patent reference 1] the official gazette of JP 3-27319 A    [Patent reference 2] the official gazette of JP 5-051388 A    [Patent reference 3] the official gazette of JP 2001-139579 A    [Patent reference 4] the official gazette of JP 8-268887 A    [Patent reference 5] the official gazette of JP 4-099331 A    [Patent reference 6] the official gazette of JP 6-227977 A    [Patent reference 7] the official gazette of Japanese Patent No. 3001589    [Patent reference 8] the official gazette of JP 4-159221 A    [Patent reference 9] the official gazette of JP 11-269456 A    [Patent reference 10] the official gazette of Japanese Patent No. 3124062    [Patent reference 11] the official gazette of JP 10-120695 A    [Non-patent reference 1] Namiki et al., “Goma—Sono Kagaku to Kinousei (Sesame—Its Science and Functions)”, Maruzen Planet Co., Ltd. (1998)    [Non-patent reference 2] Fukuda, Y. et al., J. Am. Oil Chem. Soc., 63, 1027-1031 (1986)    [Non-patent reference 3] S. Shimizu et al., J. Am. Oil Chem. Soc., 66, 237-241 (1989)    [Non-patent reference 4] S. Shimizu et al., Lipid, 26, 512 (1991)    [Non-patent reference 5] Sawada, R. et al., Lipids, 34, 633 (1999)    [Non-patent reference 6] Kushiro, M. et al., J. Nutr. Biochem., 13, 289-295 (2002)