pH RESPONSE AND OPTICAL PROPERTIES OF INDICATOR FILMS BASED ON ANTHOCYANINS FROM BORDÔ GRAPES (Vitis labrusca L.)

Grape peel flour has potential for technological applications due to the presence of bioactive compounds, particularly anthocyanins, natural pigments whose color varies depending on the pH of the medium. This property enables their use in the development of indicator films capable of signaling pH changes. The aim of this study was to determine the anthocyanin content of Bordô grape peel flour extract (Vitis labrusca L.) used in film formulations containing 0%, 0.5%, and 1% extract, and to evaluate the resulting films in terms of their colorimetric response in acidic, neutral, and basic media (pH 1 – 10), transmittance, and opacity. Anthocyanins were extracted using ethanol/water (75/25%). Their content was determined spectrophotometrically at 520 and 700 nm using the pH differential method (pH 1.0 and 4.5). Films were prepared in triplicate by casting. PVA (5.0 g in 70 mL of extract) and sodium alginate (2.0 g in 100 mL of extract) solutions were heated in a water bath for 60 and 30 min, respectively, combined with 1.5 g of glycerol, sieved, cooled, and distributed into Petri dishes at 0.34 g/cm². Drying was performed at 40 °C for 12 h, followed by storage in a desiccator for 48 h. The colorimetric response of the films was visually evaluated using buffer solutions over a pH range of 1–10. Film transmittance and opacity were assessed by spectrophotometry. Transmittance was evaluated by measuring absorbance over the wavelength range of 200–800 nm, while opacity was determined from absorbance at 600 nm. Anthocyanin contents ranged from 0 to 152.02 mg/L, increasing with extract concentration, as expected, since higher proportions of grape peel flour contributed to a greater presence of these pigments. The films exhibited visible color changes from pinkish to greenish tones across the pH range of 1–10. These changes result from pH-dependent structural transformations of anthocyanins, associated with their ionic nature, which influences pigment coloration. Film transmittance ranged from 0.6% to 86.49% over the wavelength range of 200–800 nm for formulations containing 0%, 0.5%, and 1% grape peel flour. Increasing the extract concentration reduced transmittance, indicating lower light transmission. This behavior may be related to the presence of anthocyanins and other phenolic compounds in grape peel flour, which absorb ultraviolet and visible radiation. The opacity of the films increased with extract concentration, ranging from 58.94 to 200.70, indicating greater light-blocking capacity in formulations with higher extract concentrations. This characteristic may be advantageous for packaging light-sensitive foods, as reduced light exposure may help minimize photoinduced deterioration reactions. The results demonstrate the potential of these films as colorimetric pH indicators, with optical properties influenced by extract concentration.

Acknowledgments: PAPq/UEMG (Research Grantee of the Institutional Research Support Program – PAPq/UEMG )