I'm sure you're right about the "interaction" of borders and floats, I made differents tests that seem to confirm this. Unfortunately, placing the div image inside the div recette does not change anything. Thanks a lot anyway for the time you spent responding me Jellby !
As a solution, I put max-width of 60% for the ingredient div, which has the borders, and another max-width of 35% to the image div. It works quite well now.
Can I ask you what you are doing yourself ?