The default search engine of Safari doesn't want to change

This morning my computer decided that all my web-browsers (Safari, Firefox and Chrome) would use Yahoo as default search engine and as their homepage.

I could easily solve this issue for Firefox but not for Safari. In the Safari Preferences, I changed the hompage in the "general" tab and it worked fine. However, when I changed the search engine back to Google, there is no consequence. I tried to close and reopen safari and I restarted my computer but without success.