Add vinegar when dyeing egg?

Hi, have you wondered why some people add vinegar to the dye when dyeing egg? Let me explain to you. The egg shell is makeup of calcium carbonate. Covering the surface is a layer of cuticle which is a kind of protein. Protein contain amino group (NH2) which will bind to the carboxylate (-COO=) or the sulphonate (-SO3H) group that is present in the food dye and form a salt linkages. This enable the dye to attach to the egg shell.

Based on the above theory, vinegar which is acidic in nature provides hydrogen ions (H+) which combine with the amino group (NH2) to form NH3. This create more reactive sites for salt linkage. Whereas if the dye is alkaline,less dye will be adhered to the egg.

If you do not believe, try this at home. Add 100ml of vinegar, 10ml of dye and top up with water to 500ml. Make another pot of dye using only 10ml of dye and top up to 500ml of water and the last one by adding 5g of baking soda with 10ml of dye and top up to 500ml of water. This experiment is to test whether the pH of the dye will affect the colour of the egg. You should get a darker egg when it is soaked in the pot with vinegar and the lightest egg will be the one that is soaked in the dye that contain baking soda.