Why are fermentation tubes evaluated in 24 to 48 hours?

Answer 1

Yeast fermentation is a two step process. Initially the yeast (usually aerobically) buds and increases in numbers, followed by the anaerobic fermentation step.

One to two days is usually required to really "get things started."

Bacterial fermentation typically is just as slow. Often little is seen at the 24 hours evaluation and in some cases 72 hours is required.