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Why did we measure the breakdown of ONPG instead of lactose?
Lactose and its products (galactose and glucose) are colorless. ONPG is also colorless. However, o-nitrophenol, produced upon the breakdown of ONPG, creates a yellow color. The more ONPG is broken down, the darker yellow the solution becomes. Therefore, we can measure the reaction's progression with a visible spectrophotometer, which relies on color wavelengths to work properly.