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Chemistry of Color Fixation in Crude, Physically Refined and Chemically Refined Rice Bran Oils upon Heating

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Journal of the American Oil Chemists' Society

Abstract

Compared to other vegetable oils, rice bran oil (RBO) has a characteristic dark color which further deepens upon heating or frying of foods in the oil. Darkening of the oil during heating has been studied. The dark color-causing material in crude, chemically refined and physically refined rice bran oils was separated using a silica gel column for a hexane-eluted oil fraction and a methanol eluted fraction. The methanol eluted fraction for all the above three types of RBO produced a dark color upon heating, hence the physically refined RBO methanol fraction was investigated further and contained monoglycerides (23.4 %) and diglycerides (67.4 %) of linoleic + linolenic acids in its methanol fraction as analyzed by column chromatography and HPLC which decreased in concentration after heating. The linoleic acid level of 37.7 % in the methanol fraction was reduced significantly to 18 % after heating (52.3 % reduction). The IR and NMR spectra were similar to those of a monoglyceride/diglyceride with NMR spectra indicating a lower amount of olefinic protons for the heated sample. These results showed that the darkening of RBO was due to the oxidation and polymerization of monoglycerides/diglycerides containing linoleic acid/linolenic acid.

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Acknowledgments

The authors are thankful to Dr. G. Venkateshwara Rao, Director, CSIR-CFTRI, Mysore for providing infrastructure facilities and encouragement to carry out this research work, Dr. P. Srinivas, Head, Plantation Products, Spices & Flavour Technology Department, CSIR-CFTRI for useful discussions, and Mr. M. R. Manjunatha for recording IR and NMR spectra. The authors thank M/s AP Organics Ltd., Dhuri for a generous supply of rice bran oil samples as and when required for the study.

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Authors and Affiliations

  1. Department of Lipid Science and Traditional Foods, CSIR-Central Food Technological Research Institute (CSIR-CFTRI), Mysore, 570020, India

    Basavalingappa Rekha, Belur Rangaswamy Lokesh & Ambale Gundappa Gopala Krishna

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  1. Basavalingappa Rekha
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  2. Belur Rangaswamy Lokesh
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  3. Ambale Gundappa Gopala Krishna
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Correspondence to Ambale Gundappa Gopala Krishna.

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Rekha, B., Lokesh, B.R. & Gopala Krishna, A.G. Chemistry of Color Fixation in Crude, Physically Refined and Chemically Refined Rice Bran Oils upon Heating. J Am Oil Chem Soc 91, 1665–1676 (2014). https://doi.org/10.1007/s11746-014-2520-4

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  • DOI: https://doi.org/10.1007/s11746-014-2520-4

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