Abstract: The effects of steam explosion Steam explosion pretreatment is widely used at industrial scales.In this study, we mainly focused on the effects of steam explosion treatment (SET) on the phenolic acid composition and antioxidant activities of wheat bran, to provide theoretical support for the wheat industry development.The SET was performed at different pressures(0.5, 1.5, 2.5 MPa) and different residence times(30 s, 90 s).It was prior to rapid decompression(explosion) brought about by opening the ball valve.The exploded materials were collected, dried under vacuum and powdered.The phenolic compounds in wheat bran were divided into 2 fractions by methanol extraction and alkaline hydrolysis, the free phenolic acids and the bound phenolic acids.Different fractions contained different phenolic acid profiles, so High performance liquid chromatography(HPLC) was used for analysis of main phenolic acids(vanillic acid, syringic acid, 4 coumaric acid, ferulic acid) in 280 nm and 320 nm.The total phenolic content of the sample was discussed before and after SET.Several authoritative methods of antioxidant capacity, like 2,2 Diphenyl 1 (2,4,6 trinitrophenyl) hydrazyl(DPPH) radical scavenging capacity, reducing capacity and 2,2′ Azino bis(3 ethylbenzothiazoline 6 sulfonic acid) diammonium salt(ABTS) radical scavenging assay, antioxidant assay in a linoleic acid system were also determined in this paper.The results indicated that the contents of main phenolic acids increased with SET.In the free phenolic acids, the content of vanillic acid was increased about 50% after steam explosion.The significant increasing in the contents of syringic acid, 4 coumaric acid and ferulic acids could be observed, at 2.5MPa 30s, the increasing of content was separately 10, 36, 11 fold higher than that of the untreated one.Also, when the wheat bran was treated at 2.5 MPa with 30 s, the highest contents of these free phenolic acids was obtained except vanillic acid, and the further increase of the treatment pressure or time could decrease the content of the phenolic acids in wheat bran.In the bound phenolic contents, it demonstrated that the contents of syringic acid, 4 coumaric acid, ferulic acids increased with increasing SET to reach a peak at 2.5 MPa with 30 s, and then decreased.The highest contents were 36.561±0.544, 39.025±0.202, 1 442.474±42.990 μg/g separately at 2.5 MPa with 30 s.The content of ferulic acids ranged from 59.060±4.310 to 1 442.474±42.990 μg/g, nearly increased 25 times higher than the untreated one, which indicated that the SET is an effect method to free the phenolic acids in wheat bran.We also found that the bound phenolic contents of vanilic acid and ferulic acid were significantly higher than the free phenolic contents, indicating that these two major phenolic acids in wheat bran were not extractable by aqueous methanol but released upon alkaline hydrolysis.Especially for ferulic acid, the content in bound part was nearly 4.4 times higher than that in free part.The total phenolic content of the sample was detected by the Folin Ciocalteu method.There was no difference between the untreated sample and steam exploded samples at 0.5 MPa.Then, with the increase of treatment time and pressure of steam explosion, the total phenolic contents increased with increasing SET to reach a peak at 2.5 MPa, 30 s, and then decreased.The total phenolic content of the sample treated at 2.5 MPa, 30 s was 28 mg/g, nearly 9 times higher than the untreated one.Several authoritative methods of antioxidant capacity gave the consistent conclusion, compared with the untreated sample, the steam exploded wheat bran showed the higher DPPH and ABTS radical scavenging activities, antioxidant activity in a linoleic acid system and reducing power.SET is an efficient pretreatment method to release the insoluble bound phenolic acids and enhance the antioxidant capacity of wheat bran.The analysis indicates that when the wheat bran is treated by 2.5 MPa with 30 s, the release content of phenolic acids reach is the best and the antioxidant activity reach is the highest in the treatments.