I was in a hurry. I got the kettle. Poured in about two cups of water. Placed it over the highest flame setting. Then prepared a cup of black coffee for my ROTC supervisor. I thought I did a good thing. He looked a me. His eyes became as round as a marble and his cheeks became rosy red. It was obvious that he was very angry. He loudly said that I never boiled the water well. It never reached the 100ºC boiling point. The foamy coffee surface proved my mistake.
The myth: Not boiling the water well makes the coffee bubbly or foamy. I never know what’s wrong with that and why he yelled at me.
A simple test.
I placed instant coffee powder in mug and set it aside. I brought a cup of water to a near boil. Near boil – formation of numerous tiny bubbles on side of cooking vessel and some small bubbles can be seen rushing to the surface. Then poured it immediately in mug with instant coffee powder. The result was a foamy coffee.
I made a second trial. I brought the water to boiling this time – large and numerous bubbles are rushing to the surface and fewer tiny bubbles can be seen clinging on cooking vessel surface. Then immediately poured it to mug with instant coffee. The resulting mixture was slightly foamy.
Foam disappeared faster than first trial.
Third trial. I let the recently boiled water to stand for few minutes and poured it to mug with instant coffee. No bubbles appeared.
The myth is not true. Bringing the water to near boil makes the coffee bubbly / foamy cause of the numerous tiny bubbles produced on side of casserole. The numerous bubbles are being trapped by coffee to give a foamy surface for short period. Boiling water has less tiny bubbles so the foaming effect is less intense. If the boiled or near-boiled water is set aside for few minutes, all the bubbles will rise to surface and pop out and will not make a cup of coffee foamy.