In Louis Pasteur's experiment to test the hypothesis of spontaneous generation, he primarily changed the shape of the flask. Pasteur used swan-necked flasks, which allowed air to enter while preventing dust and microorganisms from contaminating the broth. This design was crucial for demonstrating that sterilized broth remained free of microbial life when protected from contamination, thus challenging the idea of spontaneous generation.
While other variables like the broth used and the boiling time were controlled and kept consistent, the shape of the flask was a key component of his experimental design that differentiated it from previous experiments.