Stability comparison of conventional and foam-mat red and purple dried roselle calyces powder as a function of pH

This study evaluated the color stability, bioactive compounds, and antioxidant activity of roselle calyx powders dissolved at different pHs. The results showed that purple roselle calyx powders revealed better color than red roselle, and foam-mat dried powders were better than the conventionally dried powder. The best solution without pH adjustment was found in foam-mat dried purple calyces. pH values significantly affected color stability and quality, bioactive compounds, and antioxidant activity of roselle powder solutions. The lightness, redness, and yellowness of the solutions at different pH values ranged from 27.00 to 74.45, from 0.70 to 52.20, and from 3.9 to 47.50, respectively. The non-adjusted pH solution had a lightness of 33.80–49.05, redness of 19.15–42.45, and yellowness of 7.10–18.35. The color quality, indicated by color intensity at different pHs, ranged from 0.03 to 15.24; the non-adjusted pH solution had a color intensity of 1.37–33.48, revealing that pH decreased color quality. Retention of phenolics and flavonoids was 8.02%–99.69% and 0.00%–81.05%, respectively, indicating the instability of bioactive compounds. Roselle antioxidant activity was observed at different pHs, ranging from 0.00 to 1.11 mg Trolox equivalent (TE)/g for DPPH (2, 2-diphenyl-1-picrylhydrazyl) scavenging activity and 0.46–104.16 mg TE/g for the ABTS [2, 2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)] assay. Acidic pHs preserved better all dependent parameters. In practice, the pH value should be considered when using roselle calyx powders in food products. This study used a solution as a model for a watery food system. Suitable products to use roselle calyx powders are water-based products with an acidic pH, such as syrups, yogurts and yogurt-based products, fermented milk, and sour-taste beverages. Roselle calyx powders are suitable to use as coloring agents or as bioactive compounds to provide healthy functional properties. The use of foam-mat dried powders is suggested due to the better antioxidant activity and color preservation than conventionally dried powder.