Identification of unique aldehyde dimers in sorghum wax recovered after fermentation in a commercial fuel ethanol plant

Authors: Nunez, Alberto; Moreau, Robert; SHARMA, MEGAN - Former ARS Employee; Mullen, Charles; Powell, Michael; Jones, Kerby; Harron, Andrew; CAFMEYER, JEFFREY - Battelle Memorial Institute

Submitted to: Journal of the American Oil Chemists' Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2020
Publication Date: 10/15/2020
Publication URL: https://handle.nal.usda.gov/10113/7224181
Citation: Nunez, A., Moreau, R.A., Sharma, M.E., Mullen, C.A., Powell, M.J., Jones, K.C., Harron, A.F., Cafmeyer, J.T. 2020. Identification of unique aldehyde dimers in sorghum wax recovered after fermentation in a commercial fuel ethanol plant. Journal of the American Oil Chemists' Society. https://doi.org/10.1002/aocs.12424.
DOI: https://doi.org/10.1002/aocs.12424

Interpretive Summary: Carnauba wax is used in a variety of products; including cosmetics, industrial polishes, food products, and paper products. The United States has no domestic source of carnauba wax, and imports 100% of its carnauba wax supply. Alternatively, sorghum wax has been demonstrated to have similar physical properties to carnauba wax, and could potentially be a viable substitute for carnauba wax. In recent years, some U.S. dry grind corn ethanol plants have fermented blends of corn and sorghum (milo). Our laboratory has obtained several samples of distillers oils from US plants that have fermented corn/milo blends. We found that significant levels of wax (2-10%) could be fractionated from distillers corn/milo oil, whereas only very low levels of wax (<1%) could be fractionated from distillers corn oil. We have analyzed the composition of the waxes fractionated from distillers corn/milo oils and we found that they contained some of the same components that we previously identified in the sorghum kernels surface waxes (fatty acids, fatty aldehydes, fatty alcohols and alkanes) but they also contained significant levels (5-50%) of heavy wax components, which were absent in sorghum kernel surface wax. These heavy components were identified as aldehyde dimers. This new research provides important information about the composition of sorghum wax from corn/milo distillers oil, information that is necessary to understand if this new wax (that contains unique chemical components not found in other waxes) is a safe replacement for carnauba wax, especially for edible applications.

Technical Abstract: Sorghum wax can be extracted from the surface of sorghum (Sorghum bicolor) kernels. It is composed mostly of a mixture of unsaturated C28 and C30 alkanes, fatty acids, fatty alcohols and fatty aldehydes. Like carnauba wax, sorghum wax is a hard wax with a high melting point and it has potential edible and industrial applications. The yield of sorghum wax from the surface of sorghum kernels is 0.2-0.5 grams of wax per 100 grams of kernels. Sorghum wax can also be recovered from the “distillers oil” which is obtained after fermentation of sorghum (milo) or sorghum/corn blends in dry grind fuel ethanol plants. This distillers sorghum wax can potentially be obtained in yields of up to 10% by chilling the distillers oil to precipitate the wax and then recovering it via centrifugation or filtration. Like sorghum kernel wax, distillers sorghum wax is mainly composed of C28 and C30 alkanes, alcohols and aldehydes in the molecular weight (MW) range of 350 to 450. However, we found that 5-50% w/w of distillers sorghum wax is composed of larger wax components with MW of 798 to 910. Analysis via high resolution atmospheric pressure chemical ionization mass spectrometry (MS-APCI) and gas chromatography with electron ionization mass spectrometry (GC/MS-EI) resulted in exact mass data and fragmentations patterns that suggested that these high MW compounds are monounsaturated fatty aldehyde dimers formed by aldol condensation. Further confirmation supporting the GC/MS data for the aldol reaction dimer was obtained by the synthesis of the C44 aldol product from the C22 corresponding aldehyde.