Classification of Food Colours
Anthocyanins appear blue in which condition?
Anthocyanins appear blue in which condition?
Try to answer before revealing the correct option and explanation.
Correct Answer
✅ C. Alkaline
Explanation
Anthocyanins are water-soluble flavonoid pigments responsible for the red, purple, and blue colors of many fruits, vegetables, and flowers. Their color depends mainly on the pH of the surrounding medium because changes in pH alter the chemical structure of the anthocyanin molecule.
- Acidic medium (pH 1–3): Anthocyanins exist mainly as the flavylium cation, which gives a bright red color.
- Slightly acidic to neutral (pH 4–6): The red pigment gradually changes to purple as different molecular forms are produced.
- Alkaline medium (pH 7–9 or higher): Anthocyanins convert to the quinonoidal base, producing a blue color.
- Strongly alkaline conditions: The pigment becomes unstable and eventually degrades, resulting in a loss of color or formation of brown compounds.
Key Concept
Anthocyanins are natural pH indicators. They appear red in acidic conditions, purple near neutral pH, and blue in alkaline conditions because changes in pH alter the molecular structure of the pigment. However, prolonged alkaline conditions also accelerate anthocyanin degradation.
Related Sub-categories
References
- Damodaran, S., Parkin, K. L., & Fennema, O. R. (2017). Fennema's Food Chemistry (5th ed.). CRC Press.
- Discusses anthocyanin chemistry, pH-dependent structural transformations, and color changes.
- Belitz, H. D., Grosch, W., & Schieberle, P. (2009). Food Chemistry (4th ed.). Springer.
- Explains the effect of pH on anthocyanin structure, stability, and color.
- Coultate, T. P. (2016). Food: The Chemistry of Its Components (6th ed.). Royal Society of Chemistry.
- Describes food pigments, including anthocyanins and their behavior under different pH conditions.
- Wrolstad, R. E., Durst, R. W., & Lee, J. (2005). Tracking color and pigment changes in anthocyanin products. Trends in Food Science & Technology, 16(9), 423–428.
- A widely cited review on anthocyanin color chemistry and stability.
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