Most proteins have amino acids with aromatic ring, so they have yellow reaction. Phenylalanine is not easy to nitrify, so it is necessary to add in a little vitriol oil.

• 多数蛋白质分子含有带苯环的氨基酸，所以都会发生黄色反应。苯丙氨酸不易硝化，需加少量浓硫酸后才能够发生黄色反应。