1. Purple Cabbage Juice Turns Red, Purple, or Blue-Green Depending on the Liquid
Purple cabbage juice becomes reddish in acidic liquids, purplish in neutral liquids, and blue-greenish in alkaline liquids. An explanation from Imidas describes an example where juice with alkaline noodles turns blue-green, and adding vinegar to it turns it pink [2]. By simply adding everyday items like vinegar or alkaline noodles, the juice becomes a "color ruler."
However, the idea that "acidic means red, alkaline means blue" is only a rough guide. The actual colors change smoothly and continuously rather than switching abruptly [2]. The question remains: what causes this smooth shift in the juice?
2. Anthocyanin, the Natural Pigment That Gives Cabbage Juice Its Color
The true identity of this color is a natural pigment called anthocyanin. According to an explanation by Akio Tsukui, it is a pigment that has red, purple, blue, and intermediate colors. It is found in shiso (perilla), grapes, apples, strawberries, eggplants, and red cabbage [1]. While this pigment is also involved in autumn leaves turning red, here we focus only on why the juice color changes.
3. How Changes in Anthocyanin's Molecular Shape Change Its Color
The cause of the color change is that the shape of the anthocyanin molecule changes depending on the pH of the surrounding liquid. When the shape changes, the wavelength of light the molecule absorbs changes, which changes the color that reaches our eyes [3]. Tsukui’s explanation states that in strongly acidic liquids, it takes a red form; in weakly acidic to neutral liquids, a purple form; and in basic (alkaline) liquids, a blue form [1]. This description applies to representative anthocyanins, not just the specific values for purple cabbage pigment.
4. Why Anthocyanin's Color Can Disappear
The color of anthocyanin is not permanent. According to Tsukui, the purple form is unstable and tends to combine with water to become a colorless form [1]. It is said that if this combination with water is prevented, the pigment becomes stable. The fact that juice made from cabbage fades over time may be related to these changes. However, how the juice color changes specifically over time is not covered here.
5. Why Red Cabbage Pigment Is Stronger Than Other Anthocyanins
Anthocyanins are generally fragile pigments, but red cabbage pigment is considered highly stable among them. This is because many of its molecules have acyl groups attached to them (acylated molecules) [4]. Anthocyanins are also used to color foods. Tsukui’s explanation notes that in one year (Heisei 7), anthocyanins accounted for about 10% of domestic demand for natural pigments [1]. This is an old figure, so it is not the current percentage.
The National Agriculture and Food Research Organization (NARO) states that only polyacylated anthocyanins are exceptionally stable, and they support the blue color of morning glories [5].
6. How Anthocyanin Also Colors Morning Glories and Hydrangeas
According to NARO, the liquid inside the cells of the petals of the sky-blue morning glory (Sorairo Asagao) is about pH 6.6 (weakly acidic) when it is a bud. As the flower opens, this rises to pH 7.7 (weakly alkaline) when fully open. Along with this, the color changes from reddish-purple to blue [5]. However, flower blue is not determined by pH alone. NARO explains that the blue of cornflowers is due to complexes with metal ions like iron, magnesium, and calcium. It also explains that the difference between blue and red hydrangea flowers involves the amount of aluminum and coexisting substances. Soil pH does not directly determine the color [5].
7. How to Test Purple Cabbage Juice with Kitchen Items Yourself
Try adding things found in the kitchen to the purple cabbage juice and comparing the colors. Cut purple cabbage leaves finely and have an adult pour hot water over them. Once cool, take only the juice. Divide the juice into small cups and add vinegar, lemon juice, water, and water with baking soda dissolved, a little at a time. Since the color changes depending on the amount added, write the order and amounts in a notebook to make it easier to compare later. Do not drink the liquids used in the experiment. Be careful that detergents do not get into your eyes or mouth.
If you are growing morning glories, try comparing the flower colors in the morning and at noon. The original materials are listed under "Information Sources" at the bottom of the page.