Effect of tannins on microwave-assisted extractability and color properties of sorghum 3-deoxyanthocyanins

Abstract

Sorghum derived 3-deoxyanthocyanins (3-DXA) are of growing interest as natural food colors due to their unique stability compared to anthocyanins, but are generally difficult to extract. Microwave-assisted extraction (MAE) can dramatically improve extraction efficiency of 3-DXA from sorghum tissue. However, condensed tannins common in some sorghums could impact MAE extractability and color properties of 3-DXA. The objective of this work was to determine how presence of condensed tannins affect MAE extractability, stability, and color properties of sorghum 3-DXA. Sorghums of varying 3-DXA profile and tannin content, as well as purified tannins, were subjected to MAE and pigment yield and profile, aqueous color properties and stability at pH 1 – 5 monitored over time using, UV–vis spectroscopy, colorimetry, and UPLC-MS. The relative yield of 3-DXA from tannin sorghums was higher (3 – 10-fold) after MAE than from non-tannin sorghum (2-fold). During MAE, condensed tannins underwent extensive oxidative depolymerization to anthocyanidins (cyanidin and 7-O-methylcyanidin), which caused the tannin-sorghum pigment extracts to have a redder hue (12–43H°) compared to the non-tannin pigment extract (58H°). The tannin-derived anthocyanidins transformed over time into xanthylium pigments, resulting in increased extract H°. Tannins enhanced both color intensity (pH 1) and stability (pH 3–5) of the 3-DXA over 14 days, indicating they acted as copigments. The presence of tannins in sorghum enhances MAE extractability of 3-DXA from sorghum tissue, and could also potentially enhance their functionality in aqueous food systems. However, the initial changes in extract hue properties due to tannin-derived anthocyanidins should be considered.