Assessment of the color modulation and stability of naturally copigmented anthocyanin-grape colorants with different levels of purification.

Grape skins or their by-products from wine production are rich sources of anthocyanins and various colorless phenolics, depending on the grape variety. Phenolics have strong antioxidant and anthocyanin stabilizing properties and help to produce functional anthocyanin colorants with improved stability. This study aimed to assess differences in color expression and stability of anthocyanin colorants from red grape varieties naturally copigmented and with different levels of purity and to compare them to synthetic FD&C Red No. 3. Model juice systems were prepared at pH 3.5 with anthocyanins and phenolic copigments extracted from four Vitis vinifera grape varieties ('Tempranillo', 'Syrah', 'C. Sauvignon', and 'Graciano') both crude and purified by C18 solid phase extraction. Attention was focused on differential colorimetry and phenolic composition related to the color. Degradation kinetics of total color were also studied during storage of 17 days in darkness at 25 °C. Grape variety significantly influenced pigment yield, proportion of acylation, and proportion of copigments:pigments ratios in crude extracts; purification modulated the copigment:pigment ratios. This proportion was related to perceptible color variability among colorants and to different stabilities. With the same pigment content, grape varieties richer in skin copigments and higher copigment/pigment ratios ('Syrah' and 'Tempranillo') produced more intensely colored crude extracts whose tonalities ranged from reddish ('Graciano') to red-bluish ('Syrah'), depending on the proportion of acylation. Increasing the purity of the pigments diminished the color variability due to variety, making them less vivid and visually more similar to one another and also to the synthetic colorant. Degradation kinetic studies showed that unpurified grape colorants had higher color stability over time, with the greatest stabilizing effects achieved with varieties richer in skin flavonols ('Tempranillo' and 'Syrah').