I fixed it adding the search bar in the title view, this way:
self.searchBar = [[UISearchBar alloc] initWithFrame:CGRectZero];
[self.searchBar setPlaceholder:@"Buscar Recetas"];
[self.searchBar sizeToFit];
[self.searchBar setDelegate:self];
[self.navigationItem setTitleView:self.searchBar];
Note that I'm not using a search display controller in this case. Search Display Controller already has a property where you can set the search bar in the navigation bar, like this:
self.searchdisplaycontroller.displaysSearchBarInNavigationBar = YES;