Why vegetable stains turns reddish brown when washed with a soap solution?

When washing vegetable-stained clothing with soap, several factors contribute to the stains turning reddish-brown. Here's an explanation of the underlying mechanisms:

1. Alkaline Nature of Soap: Most soaps are alkaline with a pH level higher than 7. When such a soap solution comes into contact with the pigments responsible for vegetable stains, it can alter their chemical structure. The soap's alkalinity may cause certain pigment molecules to transform, resulting in a shift in their color toward a reddish-brown hue.

2. Oxygen Release from Soap: Many laundry soaps contain ingredients that release oxygen or oxidizing agents when they dissolve in water. These substances, often referred to as "bleach activators," can produce nascent oxygen, which acts as a bleaching agent. However, on some vegetable pigments, the interaction with oxygen can lead to discoloration, including a shift towards reddish-brown shades.

3. Complexation Reactions: Vegetable stains often contain compounds called polyphenols, which are complex organic molecules with antioxidant properties. Polyphenols can react with certain metal ions, like iron and copper, present in water or soap ingredients, forming complexes called "metal-polyphenol complexes." These complexes can exhibit reddish-brown or yellowish-brown colors, contributing to the stain's changed appearance after washing.

4. Chemical Reactions with Soap Additives: Some laundry soaps contain additives like enzymes, brighteners, and fragrances. These substances can interact with the pigments of vegetable stains, sometimes leading to color changes or even further browning.

5. Heat Effect: The temperature of the washing solution also plays a role. Higher water temperatures can accelerate chemical reactions and pigment transformations, potentially intensifying the reddish-brown discoloration of vegetable stains.

To avoid or minimize reddish-brown discoloration of vegetable stains, you can take certain precautions when washing:

- Pre-treat the stains with appropriate stain removers or solutions before washing.

- Use a neutral or pH-neutral detergent instead of an alkaline soap.

- Avoid using soaps containing harsh bleaches or oxidizing agents on delicate fabrics or when dealing with certain vegetable stains.

- Follow the garment's care instructions and use the recommended water temperature.

- Rinse thoroughly after washing to remove any residual soap that might contribute to further color changes.