Consumer trends are driving the food industry to replace artificial colorants with naturally-derived alternatives, however pigment stability is a limiting factor. Pyranoanthocyanins, pigments in red wine derived from grape anthocyanins during fermentation, are a promising option. Pyranoanthocyanins retain the vivid colors and antioxidant capacity of their anthocyanin precursors with improved pH and bleaching stability. As thermal treatments are prevalent in food processing, the objective of this experiment was to evaluate thermal stability of 10-catechyl pyranoanthocyanins in comparison to the corresponding anthocyanins. We hypothesized that pyranoanthocyanins would have improved thermal stability and different degradation products than anthocyanins.
Pelargonidin-3-glucoside, malvidin-3-glucoside, and cyanidin-3-glucoside were obtained from strawberries, Berberis boliviana, and elderberry, respectively. 10-catechyl pyranoanthocyanins were formed by incubating anthocyanin extracts with caffeic acid at 45°C in the dark. Pigments were isolated to high purity using semi-preparatory HPLC and mixed with pH 3.0 HCl water prior to heating. Solutions were standardized to equivalent absorbance (A=1) at their lambda max and heated at 90°C for 4.5 hours in a water bath. Throughout heating, absorbance and total pigment content were measured with a spectrophotometer, and pigment degradation and product formation were monitored through uHPLC-PDA-ESI-MS/MS analysis.
10-catechyl pyranoanthocyanins showed high stability with little or no changes in lambda max (lambda max 495-519nm) absorbance and total pigment content after heating for 4.5 hours. In contrast, anthocyanin absorbance and content decreased by 67-75% and 73-79% after 4.5 hours heating following first order kinetics (R 0.99). Cyanidin-3-glucoside was the most stable anthocyanin followed by pelargonidin and malvidin, however the impact of aglycone on pyranoanthocyanin stability could not be determined due to minimal change. Anthocyanin degradation compounds included phloroglucinaldehyde, a B-ring derivative (4-hydroxybenzoic acid (pelargonidin), protocatechuic acid (cyanidin), syringic acid (malvidin)), and a phloroglucinaldehyde-B-ring adduct. Trace amounts of the B-ring derivatives were identified in heated pyranoanthocyanin solutions in addition to a colored degradation compound (lambda max=478nm, m/z=315) which may contribute to their improved color stability.
10-catechyl pyranoanthocyanins are promising heat-stable naturally derived colorants, showing minimal change after intense heating. The identified degradation compounds increase understanding of pyranoanthocyanin degradation mechanism and may serve as indicators for thermal abuse or aging for future applications.