Canola phenolics, color and structural changes of sinapine and sinapic acid

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Cai, Rongxuan
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Dark color is one of the major problems that hinders canola protein utilization. In order to assess the role of canola phenolics in determining the visual attributes of canola protein as affected by the processing conditions, several studies investigating the effect of processing conditions on the color and structural changes of canola phenolics have been undertaken. Investigations were made using pure phenolic systems and phenolic-protein systems of canola seed and meal. Two typical processing conditions, autoclaving and pH adjustment, were used as treatments in the current research. Structural changes of the phenolics were followed by techniques including high performance liquid chromatography (HPLC), spectral analysis, thin layer chromatography (TLC), nuclear magnetic resonance (NMR) and mass spectroscopy (MS). Color properties were determined by spectral analysis for liquids and by a HunterLab Color/Difference Meter for solid materials. Phenolic content was determined by both a newly developed HPLC method and a conventional spectral colorimetric method (Folin-Ciocalteu's reagent method). Autoclaving was found to affect the visual properties of sinapic acid but not those of sinapine. The colorless sinapic acid solution turned yellow after a 15-minute autoclaving at 121C and 0.1 MPa. Filtering the solution through a 0.45-[mu]m filter resulted in a brown solid consisting of at least three undetermined colored substances (yellow, orange and purple) and a yellow liquid. A yellow substance, syringaldehyde, has been identified in the yellow liquid by NMR, mass spectroscopy and HPLC. The alkali induced air oxidation of sinapic acid at elevated pH values converted sinapic acid to thomasidioic acid by a first order process. The resulting thomasidioic acid further oxidized to form 2,6-dimethoxy-' p'-benzoquinone and 6-hydroxy-5,7-dimethoxy-2-naphthoic acid. Structural changes to sinapic acid during alkali induced air oxidation caused a darkening of the color for the system with the 2,6-dimethoxy-'p'-benzoquinone being one major color contributor. A similar study in the sinapine solution showed a more remarkable color darkening for the system, due to the structural changes induced by air oxidation under alkaline conditions. The movement of the research from pure phenolic systems to phenolic-protein systems required a rapid and reliable method for phenolic determination. A rapid HPLC method for sinapine and sinapic acid determination was developed, where sinapine and sinapic acid were separated using a gradient elution. Processing conditions have been shown to affect the color of canola flour and protein isolate. Alkaline extraction produced a darker protein isolate than did aqueous NaCl extraction. Autoclaving darkened the canola flours. While the total soluble phenolic content decreased the total insoluble-bound phenolic content increased after autoclaving. Sinapic acid increased the yellow intensity of canola protein during autoclaving while sinapine slightly decreased the lightness of canola protein. Autoclaving also decreased soluble phenolic content while increasing insoluble-bound phenolic content. Neither sinapic acid nor sinapine had any negative effect on the color of the canola protein during isolation using basic extraction with acidic precipitation.