Pasteur disproved spontaneous generation by demonstrating that microorganisms did not grow in broth that was heated in swan-neck flasks and then allowed to sit undisturbed for several days. It was important that he placed the broth in control and experimental flasks before heating rather than transferring the broth to new flasks after heating. Why?

See the answer below

Explanation:

It was important for Pasteur to place the broth in the flasks before heating rather than transferring it to new flasks after heating because the new flasks might be a source of contaminants in the form of microorganisms, hence, leading to a biased outcome.

In order to prevent cross-contamination and preserve the integrity of the outcome of the experiment, the broth needed to be placed in the flasks before heating instead of transferring it after heating.