Myrtle pulp (Eugenia gracillima Kiaersk.) as an alternative natural colorant in a fermented dairy model system

Myrtle pulp (Eugenia gracillima Kiaersk.) is rich in anthocyanins and used in the processing of sweets, jams, and alcoholic or non-alcoholic beverages. However, its potential as a natural organic colorant is still underexplored. Therefore, this study aimed to evaluate myrtle pulp at different concentrations (0 mL, control; 1.5, 3.0, 4.5, 6.0, and 7.5 mL v/v) as a natural colorant for artisanal yogurts (80 mL), used as a fermented dairy model system. The stability of the model system was evaluated under storage at 6 ± 2 °C for 35 days. Physicochemical parameters, including pH, total acidity, H⁺ concentration, and water activity (a_w) were also assessed, as well as microscopic images, color behavior, colorimetric data (browning index and color index), and color degradation. The high H⁺ concentration of pulp favored preservation of the colored artisanal yogurt formulations through acidification. Significant increase (p < 0.05) in total acidity suggested a post-acidification process during refrigerated storage. The a_w ensured the viability of lactic acid bacteria, which can act as antagonist against spoilage or pathogenic bacteria. Fermented dairy products displayed a continuous, closed protein network before storage, with better chromatic homogeneity, consistent with good initial dispersion of myrtle pulp when added at the highest concentration. After 35 days of storage, crack-like channels, syneresis, and local color gradients were observed. Color shifted from the purple hue of pulp combined with the pale yellow of the artisanal yogurt to shades of soft reddish, light red, and red. These findings indicate that this natural colorant may represent a viable alternative to the use of synthetic colorants commonly used in fermented dairy products.