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Effect of degree of pectin methoxylation on aqueous stability of sorghum 3-deoxyanthocyanins
J. BRANTSEN (1), J. Awika (2) (1) Texas A&M University, College Station, TX, U.S.A.; (2) Texas A&M University, , U.S.A..

Sorghum 3-deoxyanthocyanin pigments are more stable than anthocyanins but readily associate and precipitate in aqueous systems due to reduced hydrophilicity. The objective of this study was to determine the effect of degree of methoxylation (DM) of pectin on the aqueous stability of sorghum 3-deoxyanthocyanins. Pigments were extracted from sorghum tissues (leaves, sheath, and grain) with differing 3-deoxyanthocyanin compositions and solubilized with pectin (DM 6.7-85%) in pH 1-5 buffers for 14 days. Absorbance spectra were recorded with an UV-Vis spectrophotometer and pigment compositions were analyzed using HPLC-DAD. The 3-deoxyanthocyanin pigments were most stable in pH 1 buffer and tendency to aggregate increased with increasing pH. Pectin at a 0.1% generally prevented association of pigments at all pH but the stability depended on extract composition. At pH 5, 3-deoxyanthocyanins from tannin-containing sorghum grain showed 2.4-4% color loss compared to 3-deoxyanthocyanins from non-tannin sorghum grain (23.6-27% loss) and the controls (72-75% loss). As DM increased, the aqueous stability decreased with extracts higher in hydrophilic 3-deoxyanthocyanins; e.g., at pH 5, there was 61% color loss with high DM (85%) pectin compared to 29% color loss with low DM (6.7%) pectin. Increased apigeninidin content of the pigment extracts correlated with better stability in pectin, and the presence of tannins appears to enhance the effectiveness of pectin.