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FT-Raman Spectra of Unsoaked and NaOH-Soaked Wheat Kernels, Bran, and Ferulic Acid

¹Engineering Research Unit, USDA-ARS, Grain Marketing and Production Research Center, Manhattan, KS 66502. ²Corresponding author. Phone: 785-776-2761. Fax: 785-537-5534. E-mail: ramms@gmprc.ksu.edu. ³Grain Quality and Structure Research Unit, USDA-ARS, Grain Marketing and Production Research Center, Manhattan, KS 66502.

The sodium hydroxide (NaOH) test for determining wheat color class depends on the observation that on soaking in NaOH, red wheat turns a darker red and white wheat turns straw yellow. To understand the mechanism of this test, Raman spectra of wheat bran, wheat starch, ferulic acid, and whole kernels of wheat, before and after NaOH soak, were studied. The major observable components in the whole kernel were that of starch, protein, and ferulic acid, perhaps esterified to arabinoxylan and sterols. When kernels are soaked in NaOH, spectral bands due to ferulic acid shift to lower energy and show a slightly reduced intensity that is consistent with deprotonation of the phenolic group and extraction of a portion of the ferulic acid into solution. Other phenolic acids, alkyl resorcinols, and flavonoids observed in the NaOH extracts of wheat by HPLC were not observed in the Raman spectra. Wheat bran accounts for most of the ferulic acid in the whole kernel, as indicated by the increased intensity of the doublet at 1,631 and 1,600 cm^-1 in the bran. The intense starch band at 480 cm^-1 in whole kernel wheat was nearly absent in the wheat bran.