Submitted to: Journal of Agriculture and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 17, 2006
Publication Date: October 10, 2006
Citation: Moreau, R.A., Hicks, K.B. 2006. A reinvestigation of the effect of heat pretreatment of corn fiber and corn germ on the levels of extractable tocopherols and tocotrienols. Journal of Agriculture and Food Chemistry. 54, p.8093-8102. Interpretive Summary: Vitamin E (alpha-tocopherol) and other tocopherols and tocotrienols are phytonutrients that are valued because they are potent antioxidants and because they posses several other health-promoting properties. Previously, we reported that heating corn fiber at 150 degrees C for 1 hour caused a tenfold increase in the extractable levels of gamma-tocopherol, a vitamin E analog. In the current study, this previous heat pretreatment effect was reinvestigated using improved methods and we found that heat pretreatment did NOT cause an increase in the extractable levels of any of the tocopherols or tocotrienols. In fact, heating lowered the levels of gamma-tocopherol, gamma-tocotrienol, and delta-tocotrienol, and had no effect on the levels of delta-tocopherol and alpha-tocotrienol. Sensitive analytical methods indicated that in the previous study heating had probably created an oxidation product, which was incorrectly identified to be gamma-tocopherol. Similar heating experiments were also conducted with corn germ and corn oil and the same type of oxidation product was observed. Roasting is a process that is commonly used to treat nuts and grains to destroy harmful enzymes and to increase flavor and sometimes increase the availability of phytonutrients. This information will be important to corn processors, and food chemists, who will benefit from the knowledge that roasting corn fiber, corn germ, or corn oil, does not increase the levels of extractable tocopherols and tocotrienols, and it actually lowers them.
Technical Abstract: Previously, using HPLC with UV detection, we reported that heat pretreatment (150 degrees C, 1 h) of corn fiber cause up to a tenfold increase in the levels of extractable gamma-tocopherol. In the current study, this previous heat pretreatment effect was reinvestigated using improved methods (HPLC with fluorescence detection) for the quantitative analysis of tocopherols and tocotrienols. Using the new methodology, heat pretreatment did not cause an increase in the levels of any of the three tocopherols or two tocotrienols in corn fiber oil. In fact, heating lowered the levels of three of the tocols (gamma-tocopherol, gamma-tocotrienol, and delta-tocotrienol) and had no effect on the levels of the other two (delta-tocopherol and alpha-tocotrienol). Heat pretreatment of corn germ also decreased the levels of four of its five tocols. Using the HPLC-UV method from our previous report, we heat pretreated corn fiber, corn germ, and corn fiber oil and in each case, heating generated an unknown UV-absorbing peak which had an identical retention time to gamma-tocotrienol. Using LC DOD and LC-MS-APCI, we obtained UV spectra and mass spectra of the unknown peak. The evidence indicates that the peak which we had identified as gamma-tocopherol in our previous report was mainly a triacylglycerol hydroperoxide (oleoyl-linoleyl-linolenyl-OOH), and the peak also contained smaller amounts of other molecular species of triacylglycerol hydroperoxides. This same unknown peak that co-chromatographed with gamma-tocopherol was generated by heating corn fiber, corn germ, and corn fiber oil.