Herrman, Dorothy Ann (2016-08). Improvement of Extractability and Aqueous Stability of Sorghum 3-Deoxyanthocyanins. Master's Thesis. Thesis uri icon

abstract

  • Consumer interest in natural ingredients has increased demand for natural food colorants. 3-Deoxyanthocyanin pigments from sorghum are more stable to food processing conditions than anthocyanin analogs. However, 3-deoxyanthocyanins self-associate in aqueous solution and are difficult to extract from sorghum tissue, limiting their application as natural food colorants. The goal of this research is to improve extractability and aqueous stability of sorghum 3-deoxyanthocyanins. Effect of gum arabic, sodium alginate (0.5g/L and 1.0g/L), and metal ions Fe3+ or Mg2+ (0.2mM) on aqueous stability of 3-deoxyanthocyanins was investigated at pH 3 and pH 5. Pigments were obtained from red sorghum leaf (predominantly luteolinidin derivatives) and sorghum leaf sheath (predominantly apigeninidin derivatives) via standard extraction procedure (1% HCl in methanol/2 h/20?C). Additionally, a black sorghum grain (luteolinidin/apigeninidin derivative mixture) was used to evaluate the effect of microwave irradiation on extractability of the 3-deoxyanthocyanins from black sorghum grain; this was investigated utilizing different power (300-1200 W) and time (0.25-40 min) settings. Blue maize and black cowpea, containing acylated and non-acylated anthocyanins pigments, respectively, were used to for comparison. Pigment yield and profile were determined using UV-Vis and UPLC-MS, respectively. Both gums effectively stabilized 3-deoxyanthocyanins. Gum arabic stabilized apigeninidin dominant (64-96% color retention) and luteolinidin/apigeninidin extracts (59-92% color retention) but not luteolinidin dominant extracts (0-11% color retention). This was likely caused by enhanced hydrophobic interaction with apigeninidin. The stabilizing effect of alginate was more effective at pH 5 (74% mean color retention) than pH 3 (64 mean % color retention). Thus alginate effect was likely due to ionic and hydrogen bonds. Metal ions exhibited generally negligible effect. However, luteolinidin in alginate and Fe3+ yielded a deep black color, possibly an Fe3+ mediated luteolinidin-alginate complex. Microwave-assisted extraction increased yield of 3-deoxyanthocyanins almost three-fold (2.44 mg/g) relative to control (0.88 mg/g). The 3-deoxyanthocyanin profile exhibited generally insignificant change; however, long exposure (30-40 min) at 600-1200 W produced cyanidin in extracts, possibly due to structural transformation caused by microwave irradiation. Anthocyanin profiles of maize and cowpea exhibited extensive hydrolysis of acyl esters and glycosides. Microwave-assisted extraction increased extractability of sorghum 3-deoxyanthocyanins, improving their potential application as natural food colorants.

publication date

  • August 2016